SEI CERT C Coding Standard

SEI CERT 

C Coding Standard 

Rules for Developing Safe, Reliable, and Secure Systems 

2016 Edition

Copyright 2016 Carnegie Mellon University 

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v2016-06-29-1140

Table of Contents 

1 Introduction 1 

1.1 Scope 2 

1.2 Audience 3 

1.3 History 4 

1.4 ISO/IEC TS 17961 C Secure Coding Rules 5 

1.5 Tool Selection and Validation 7 

1.6 Taint Analysis 9 

1.7 Rules Versus Recommendations 10 

1.8 Conformance Testing 11 

1.9 Development Process 12 

1.10 Usage 13 

1.11 System Qualities 13 

1.12 Vulnerability Metric 13 

1.13 How This Coding Standard Is Organized 14 

1.14 Automatically Generated Code 18 

1.15 Government Regulations 19 

1.16 Acknowledgments 20 

2 Preprocessor (PRE) 23 

2.1 PRE30-C. Do not create a universal character name through concatenation 23 

2.2 PRE31-C. Avoid side effects in arguments to unsafe macros 25 

2.3 PRE32-C. Do not use preprocessor directives in invocations of function-like macros 30 

3 Declarations and Initialization (DCL) 32 

3.1 DCL30-C. Declare objects with appropriate storage durations 32 

3.2 DCL31-C. Declare identifiers before using them 36 

3.3 DCL36-C. Do not declare an identifier with conflicting linkage classifications 40 

3.4 DCL37-C. Do not declare or define a reserved identifier 43 

3.5 DCL38-C. Use the correct syntax when declaring a flexible array member 50 

3.6 DCL39-C. Avoid information leakage when passing a structure across a trust boundary 53 

3.7 DCL40-C. Do not create incompatible declarations of the same function or object 60 

3.8 DCL41-C. Do not declare variables inside a switch statement before the first case label 66 

4 Expressions (EXP) 68 

4.1 EXP30-C. Do not depend on the order of evaluation for side effects 68 

4.2 EXP32-C. Do not access a volatile object through a nonvolatile reference 74 

4.3 EXP33-C. Do not read uninitialized memory 76 

4.4 EXP34-C. Do not dereference null pointers 85 

4.5 EXP35-C. Do not modify objects with temporary lifetime 90 

4.6 EXP36-C. Do not cast pointers into more strictly aligned pointer types 93 

4.7 EXP37-C. Call functions with the correct number and type of arguments 98 

4.8 EXP39-C. Do not access a variable through a pointer of an incompatible type 103 

4.9 EXP40-C. Do not modify constant objects 109 

4.10 EXP42-C. Do not compare padding data 111 

4.11 EXP43-C. Avoid undefined behavior when using restrict-qualified pointers 114 

SEI CERT C Coding Standard: Rules for Developing Safe, Reliable, and Secure Systems 

Software Engineering Institute | Carnegie Mellon University 

[DISTRIBUTION STATEMENT A] Approved for public release and unlimited distribution. 

i

4.12 EXP44-C. Do not rely on side effects in operands to sizeof, _Alignof, or _Generic 122 

4.13 EXP45-C. Do not perform assignments in selection statements 126 

4.14 EXP46-C. Do not use a bitwise operator with a Boolean-like operand 131 

5 Integers (INT) 132 

5.1 INT30-C. Ensure that unsigned integer operations do not wrap 132 

5.2 INT31-C. Ensure that integer conversions do not result in lost or misinterpreted data 138 

5.3 INT32-C. Ensure that operations on signed integers do not result in overflow 147 

5.4 INT33-C. Ensure that division and remainder operations do not result in divide-by-zero 

errors 157 

5.5 INT34-C. Do not shift an expression by a negative number of bits or by greater than or 

equal to the number of bits that exist in the operand 160 

5.6 INT35-C. Use correct integer precisions 166 

5.7 INT36-C. Converting a pointer to integer or integer to pointer 169 

6 Floating Point (FLP) 173 

6.1 FLP30-C. Do not use floating-point variables as loop counters 173 

6.2 FLP32-C. Prevent or detect domain and range errors in math functions 176 

6.3 FLP34-C. Ensure that floating-point conversions are within range of the new type 185 

6.4 FLP36-C. Preserve precision when converting integral values to floating-point type 189 

6.5 FLP37-C. Do not use object representations to compare floating-point values 191 

7 Array (ARR) 193 

7.1 ARR30-C. Do not form or use out-of-bounds pointers or array subscripts 193 

7.2 ARR32-C. Ensure size arguments for variable length arrays are in a valid range 203 

7.3 ARR36-C. Do not subtract or compare two pointers that do not refer to the same array 207 

7.4 ARR37-C. Do not add or subtract an integer to a pointer to a non-array object 209 

7.5 ARR38-C. Guarantee that library functions do not form invalid pointers 212 

7.6 ARR39-C. Do not add or subtract a scaled integer to a pointer 222 

8 Characters and Strings (STR) 226 

8.1 STR30-C. Do not attempt to modify string literals 226 

8.2 STR31-C. Guarantee that storage for strings has sufficient space for character 

data and the null terminator 230 

8.3 STR32-C. Do not pass a non-null-terminated character sequence to a library function 

that expects a string 242 

8.4 STR34-C. Cast characters to unsigned char before converting to larger integer sizes 247 

8.5 STR37-C. Arguments to character-handling functions must be representable as an 

unsigned char 251 

8.6 STR38-C. Do not confuse narrow and wide character strings and functions 253 

9 Memory Management (MEM) 256 

9.1 MEM30-C. Do not access freed memory 256 

9.2 MEM31-C. Free dynamically allocated memory when no longer needed 262 

9.3 MEM33-C. Allocate and copy structures containing a flexible array member 

dynamically 264 

9.4 MEM34-C. Only free memory allocated dynamically 269 

9.5 MEM35-C. Allocate sufficient memory for an object 273 

9.6 MEM36-C. Do not modify the alignment of objects by calling realloc() 277 




ii

10 Input/Output (FIO) 281 

10.1 FIO30-C. Exclude user input from format strings 281 

10.2 FIO32-C. Do not perform operations on devices that are only appropriate for files 285 

10.3 FIO34-C. Distinguish between characters read from a file and EOF or WEOF 291 

10.4 FIO37-C. Do not assume that fgets() or fgetws() returns a nonempty string when 

successful 296 

10.5 FIO38-C. Do not copy a FILE object 299 

10.6 FIO39-C. Do not alternately input and output from a stream without an intervening 

flush or positioning call 301 

10.7 FIO40-C. Reset strings on fgets() or fgetws() failure 304 

10.8 FIO41-C. Do not call getc(), putc(), getwc(), or putwc() with a stream argument that 

has side effects 306 

10.9 FIO42-C. Close files when they are no longer needed 309 

10.10 FIO44-C. Only use values for fsetpos() that are returned from fgetpos() 313 

10.11 FIO45-C. Avoid TOCTOU race conditions while accessing files 315 

10.12 FIO46-C. Do not access a closed file 319 

10.13 FIO47-C. Use valid format strings 321 

11 Environment (ENV) 326 

11.1 ENV30-C. Do not modify the object referenced by the return value of certain functions 326 

11.2 ENV31-C. Do not rely on an environment pointer following an operation that may 

invalidate it 331 

11.3 ENV32-C. All exit handlers must return normally 336 

11.4 ENV33-C. Do not call system() 340 

11.5 ENV34-C. Do not store pointers returned by certain functions 347 

12 Signals (SIG) 353 

12.1 SIG30-C. Call only asynchronous-safe functions within signal handlers 353 

12.2 SIG31-C. Do not access shared objects in signal handlers 363 

12.3 SIG34-C. Do not call signal() from within interruptible signal handlers 367 

12.4 SIG35-C. Do not return from a computational exception signal handler 371 

13 Error Handling (ERR) 374 

13.1 ERR30-C. Set errno to zero before calling a library function known to set errno, 

and check errno only after the function returns a value indicating failure 374 

13.2 ERR32-C. Do not rely on indeterminate values of errno 381 

13.3 ERR33-C. Detect and handle standard library errors 386 

14 Concurrency (CON) 403 

14.1 CON30-C. Clean up thread-specific storage 403 

14.2 CON31-C. Do not destroy a mutex while it is locked 407 

14.3 CON32-C. Prevent data races when accessing bit-fields from multiple threads 410 

14.4 CON33-C. Avoid race conditions when using library functions 414 

14.5 CON34-C. Declare objects shared between threads with appropriate storage durations 418 

14.6 CON35-C. Avoid deadlock by locking in a predefined order 426 

14.7 CON36-C. Wrap functions that can spuriously wake up in a loop 431 

14.8 CON37-C. Do not call signal() in a multithreaded program 435 

14.9 CON38-C. Preserve thread safety and liveness when using condition variables 437 

14.10 CON39-C. Do not join or detach a thread that was previously joined or detached 445 




iii

14.11 CON40-C. Do not refer to an atomic variable twice in an expression 447 

14.12 CON41-C. Wrap functions that can fail spuriously in a loop 451 

15 Miscellaneous (MSC) 455 

15.1 MSC30-C. Do not use the rand() function for generating pseudorandom numbers 455 

15.2 MSC32-C. Properly seed pseudorandom number generators 459 

15.3 MSC33-C. Do not pass invalid data to the asctime() function 463 

15.4 MSC37-C. Ensure that control never reaches the end of a non-void function 466 

15.5 MSC38-C. Do not treat a predefined identifier as an object if it might only 

be implemented as a macro 470 

15.6 MSC39-C. Do not call va_arg() on a va_list that has an indeterminate value 473 

15.7 MSC40-C. Do not violate constraints 476 

Appendix A: Bibliography 481 

Appendix B: Definitions 501 

Appendix C: Undefined Behavior 510 

Appendix D: Unspecified Behavior 525 




iv

Introduction - Scope 

1 Introduction 

The SEI CERT C Coding Standard, 2016 Edition provides rules for secure coding in the C programming 

language. The goal of these rules and recommendations is to develop safe, reliable, and 

secure systems, for example by eliminating undefined behaviors that can lead to undefined program 

behaviors and exploitable vulnerabilities. Conformance to the coding rules defined in this 

standard are necessary (but not sufficient) to ensure the safety, reliability, and security of software 

systems developed in the C programming language. It is also necessary, for example, to have a 

safe and secure design. Safety-critical systems typically have stricter requirements than are imposed 

by this coding standard, for example requiring that all memory be statically allocated. However, 

the application of this coding standard will result in high-quality systems that are reliable, 

robust, and resistant to attack. 

Each rule consists of a title, a description, and noncompliant code examples and compliant solutions. 

The title is a concise, but sometimes imprecise, description of the rule. The description 

specifies the normative requirements of the rule. The noncompliant code examples are examples 

of code that would constitute a violation of the rule. The accompanying compliant solutions 

demonstrate equivalent code that does not violate the rule or any other rules in this coding standard. 

A well-documented and enforceable coding standard is an essential element of coding in the C 

programming language. Coding standards encourage programmers to follow a uniform set of rules 

determined by the requirements of the project and organization rather than by the programmer’s 

familiarity. Once established, these standards can be used as a metric to evaluate source code (using 

manual or automated processes). 

CERT’s coding standards are being widely adopted by industry. Cisco Systems, Inc. announced 

its adoption of the CERT C Secure Coding Standard as a baseline programming standard in its 

product development in October 2011 at Cisco’s annual SecCon conference. Recently, Oracle has 

integrated all of CERT’s secure coding standards into its existing secure coding standards. This 

adoption is the most recent step of a long collaboration: CERT and Oracle previously worked together 

in authoring The CERT Oracle Secure Coding Standard for Java (Addison-Wesley, 2011). 

This standard is based on the C rules available on the CERT Secure Coding wiki as of 30 March 

2016. The wiki contains ongoing updates of the standard between official published releases. If 

you are interested in contributing to the rules, create an account on the wiki and then request contributor 

privileges by sending email to info@sei.cmu.edu. 

The Secure Coding eNewsletter contains news from the CERT Secure Coding Initiative as well as 

summaries of recent updates to the standard rules. If you are interested in receiving updates directly, 

subscribe to the eNewsletter through our website or send a request to info@sei.cmu.edu. 

SEI CERT C Coding Standard: Rules for Developing Safe, Reliable, and Secure Systems 1 


[DISTRIBUTION STATEMENT A] Approved for public release and unlimited distribution.

Introduction - Scope 

1.1 Scope 

The CERT C Secure Coding Standard was developed specifically for versions of the C programming 

language defined by 

• ISO/IEC 9899:2011 ISO/IEC, Programming Languages—C, 3rd ed. [ISO/IEC 9899:2011] 

• ISO/IEC 9899:2011/Cor.1:2012, Technical Corrigendum 1 

Although the guidelines for this standard were developed for C11, they can also be applied to earlier 

versions of the C programming language, including C99. Variations between versions of the 

C Standard that would affect the proper application of these guidelines are noted where applicable. 

Most guidelines have a noncompliant code example that is a C11-conforming program to ensure 

that the problem identified by the guideline is within the scope of the standard. However, the best 

solutions to secure coding problems are often platform specific. In many cases, this standard provides 

appropriate compliant solutions for both POSIX and Windows operating systems. Language 

and library extensions that have been published as ISO/IEC technical reports or technical specifications 

are frequently given precedence, such has those described by ISO/IEC TR 24731-2, Extensions 

to the C Library—Part II: Dynamic Allocation Functions [ISO/IEC TR 24731-2:2010]. 

In many cases, compliant solutions are also provided for specific platforms such as Linux or 

OpenBSD. Occasionally, interesting or illustrative implementation-specific behaviors are described. 

1.1.1 Rationale 

A coding standard for the C programming language can create the highest value for the longest 

period of time by focusing on the C Standard (C11) and the relevant post-C11 technical reports. 

The C Standard documents existing practice where possible. That is, most features must be tested 

in an implementation before being included in the standard. The CERT C Coding Standard has a 

different purpose: to establish a set of best practices, which sometimes requires introducing new 

practices that may not be widely known or used when existing practices are inadequate. To put it 

a different way, the CERT C Coding Standard attempts to drive change rather than just document 

it. 

For example, the optional but normative Annex K, “Bounds-Checking Interfaces,” introduced in 

C11, is gaining support but at present is implemented by only a few vendors. It introduces functions 

such as memcpy_s(), which serve the purpose of security by adding the destination buffer 

size to the API. A forward-looking document could not reasonably ignore these functions simply 

because they are not yet widely implemented. The base C Standard is more widely implemented 

than Annex K, but even if it were not, it is the direction in which the industry is moving. Developers 

of new C code, especially, need guidance that is usable, on and makes the best use of, the 

compilers and tools that are now being developed and are being supported into the future. 




Introduction - Audience 

Some vendors have extensions to C, and some also have implemented only part of the C Standard 

before stopping development. Consequently, it is not possible to back up and discuss only C95, 

C90, or C99. The vendor support equation is too complicated to draw a line and say that a certain 

compiler supports exactly a certain standard. Whatever demarcation point is selected, different 

vendors are on opposite sides of it for different parts of the language. Supporting all possibilities 

would require testing the cross-product of each compiler with each language feature. Consequently, 

we have selected a demarcation point that is the most recent in time so that the rules and 

recommendations defined by the standard will be applicable for as long as possible. As a result of 

the variations in support, source-code portability is enhanced when the programmer uses only the 

features specified by C99. This is one of many trade-offs between security and portability inherent 

to C language programming. 

The value of forward-looking information increases with time before it starts to decrease. The 

value of backward-looking information starts to decrease immediately. 

For all of these reasons, the priority of this standard is to support new code development using 

C11 and the post-C11 technical reports that have not been incorporated into the C Standard. A 

close-second priority is supporting remediation of old code using C99 and the technical reports. 

This coding standard does make contributions to support older compilers when these contributions 

can be significant and doing so does not compromise other priorities. The intent is not to 

capture all deviations from the standard but to capture only a few important ones. 

1.1.2 Issues Not Addressed 

A number of issues are not addressed by this secure coding standard. 

1.1.2.1 Coding Style 

Coding style issues are subjective, and it has proven impossible to develop a consensus on appropriate 

style guidelines. Consequently, the CERT C Secure Coding Standard does not require the 

enforcement of any particular coding style but only suggests that development organizations define 

or adopt style guidelines and apply these guidelines consistently. The easiest way to apply a 

coding style consistently is to use a code-formatting tool. Many interactive development environments 

(IDEs) provide such capabilities. 

1.1.2.2 Controversial Rules 

In general, the CERT coding standards try to avoid the inclusion of controversial rules that lack a 

broad consensus. 

1.2 Audience 

The CERT C Secure Coding Standard is primarily intended for developers of C language programs 

but may also be used by software acquirers to define the requirements for software. The 




Introduction - History 

standard is of particular interest to developers who are building high-quality systems that are reliable, 

robust, and resistant to attack. 

While not intended for C++ programmers, the standard may also be of some value to these developers 

because the vast majority of issues identified for C language programs are also issues in 

C++ programs, although in many cases the solutions are different. 

1.3 History 

The idea of a CERT secure coding standard arose at the Spring 2006 meeting of the C Standards 

Committee (more formally, ISO/IEC JTC1/SC22/WG14) in Berlin, Germany [Seacord 2013a]. 

The C Standard is an authoritative document, but its audience is primarily compiler implementers, 

and, as noted by many, its language is obscure and often impenetrable. A secure coding standard 

would be targeted primarily toward C language programmers and would provide actionable guidance 

on how to code securely in the language. 

The CERT C Secure Coding Standard was developed on the CERT Secure Coding wiki following 

a community-based development process. Experts from the community, including members of the 

WG14 C Standards Committee, were invited to contribute and were provided with edit privileges 

on the wiki. Members of the community can register for a free account on the wiki to comment on 

the coding standards and individual rules. Reviewers who provide high-quality comments are frequently 

extended edit privileges so they can directly contribute to the development and evolution 

of the coding standard. Today, the CERT Secure Coding wiki has 2,502 registered users. 

This wiki-based community development process has many advantages. Most important, it engages 

a broad group of experts to form a consensus opinion on the content of the rules. The main 

disadvantage of developing a secure coding standard on a wiki is that the content is constantly 

evolving. This instability may be acceptable if you want the latest information and are willing to 

entertain the possibility that a recent change has not yet been fully vetted. However, many software 

development organizations require a static set of rules and recommendations that they can 

adopt as requirements for their software development process. 

Toward this end, a stable snapshot of the CERT C Secure Coding Standard was produced after 

two and a half years of community development and published as The CERT C Secure Coding 

Standard. With the production of the manuscript for the book in June 2008, version 1.0 (the book) 

and the wiki versions of the secure coding standard began to diverge. A second snapshot was 

taken in December 2013 and was published in April 2014 as The CERT C Coding Standard, second 

edition. The wiki had become so comprehensive by this time that only the rules were included 

in the second edition of the book. A third snapshot was taken in March 2016 and published in 

June 2016 as SEI CERT C Coding Standard, 2016 edition, as a downloadable PDF document. 

The CERT C secure coding guidelines were first reviewed by WG14 at the London meeting in 

April 2007 and again at the Kona, Hawaii, meeting in August 2007. 




Introduction - ISO/IEC TS 17961 C Secure Coding Rules 

The topic of whether INCITS PL22.11 should submit the CERT C Secure Coding Standard to 

WG14 as a candidate for publication as a type 2 or type 3 technical report was discussed at the 

J11/U.S. TAG Meeting, April 15, 2008, as reported in the minutes. J11 is now Task Group 

PL22.11, Programming Language C, and this technical committee is the U.S. Technical Advisory 

Group to ISO/IEC JTC 1 SC22/WG14. 

A straw poll was taken on the question, “Who has time to work on this project?” for which the 

vote was 4 (has time) to 12 (has no time). Some of the feedback we received afterwards was that 

although the CERT C Secure Coding Standard was a strong set of guidelines that had been developed 

with input from many of the technical experts at WG14 and had been reviewed by WG14 on 

several occasions, WG14 was not normally in the business of “blessing” guidance to developers. 

However, WG14 was certainly in the business of defining normative requirements for tools such 

as compilers. 

Armed with this knowledge, we proposed that WG14 establish a study group to consider the problem 

of producing analyzable secure coding guidelines for the C language. The study group first 

met on October 27, 2009. CERT contributed an automatically enforceable subset of the C secure 

coding rules to ISO/IEC for use in the standardization process. 

Participants in the study group included analyzer vendors such as Coverity, Fortify, GammaTech, 

Gimpel, Klocwork, and LDRA; security experts; language experts; and consumers. A new work 

item to develop and publish ISO/IEC TS 17961, C Secure Coding Rules, was approved for WG14 

in March 2012, and the study group concluded. Roberto Bagnara, the Italian National Body representative 

to WG 14, later joined the WG14 editorial committee. ISO/IEC TS 17961:2013(E), Information 

Technology—Programming Languages, Their Environments and System Software Interfaces—C 

Secure Coding Rules [ISO/IEC TS 17961:2013] was officially published in 

November 2013 and is available for purchase at the ISO store. 

The CERT Secure Coding wiki contains ongoing updates of the standard between official published 

releases. If you are interested in contributing to the rules, create an account on the wiki and 

then request contributor privileges by sending a request to info@sei.cmu.edu. 

The Secure Coding eNewsletter contains news from the CERT Secure Coding Initiative as well as 

summaries of recent updates to the standard rules. If you are interested in getting updates, subscribe 

to the eNewsletter through our website or by sending a request to info@sei.cmu.edu. 

1.4 ISO/IEC TS 17961 C Secure Coding Rules 

The purpose of ISO/IEC TS 17961 [ISO/IEC TS 17961:2013] is to establish a baseline set of requirements 

for analyzers, including static analysis tools and C language compilers, to be applied 

by vendors that wish to diagnose insecure code beyond the requirements of the language standard. 

All rules are meant to be enforceable by static analysis. The criterion for selecting these rules is 

that analyzers that implement these rules must be able to effectively discover secure coding errors 

without generating excessive false positives. 




Introduction - ISO/IEC TS 17961 C Secure Coding Rules 

To date, the application of static analysis to security has been performed in an ad-hoc manner by 

different vendors, resulting in non-uniform coverage of significant security issues. ISO/IEC TS 

17961 enumerates secure coding rules and requires analysis engines to diagnose violations of 

these rules as a matter of conformance to the specification [ISO/IEC TS 17961:2013]. These rules 

may be extended in an implementation-dependent manner, which provides a minimum coverage 

guarantee to customers of any and all conforming static analysis implementations. 

ISO/IEC TS 17961 specifies rules for secure coding in the C programming language and includes 

code examples for each rule. Noncompliant code examples demonstrate language constructs that 

have weaknesses with potentially exploitable security implications; such examples are expected to 

elicit a diagnostic from a conforming analyzer for the affected language construct. Compliant examples 

are expected not to elicit a diagnostic. ISO/IEC TS 17961 does not specify the mechanism 

by which these rules are enforced or any particular coding style to be enforced [ISO/IEC TS 

17961:2013]. 

The following table shows how ISO/IEC TS 17961 relates to other standards and guidelines. Of 

the publications listed, ISO/IEC TS 17961 is the only one for which the immediate audience is analyzers 

and not developers. 

ISO/IEC TS 17961 Compared with Other Standards 

Coding 

Standard 

C Standard 

Security 


Safety 


International 


CWE None/all Yes No No N/A 

MISRA C2 C89 No Yes No No 

MISRA C3 C99 No Yes No No 

CERT C99 C99 Yes No No Yes 

CERT C11 C11 Yes Yes No Yes 

ISO/IEC TS 

17961 

C11 Yes No Yes Yes 

Whole 

Language 

A conforming analyzer must be capable of producing a diagnostic for each distinct rule in the 

technical specification upon detecting a violation of that rule in isolation. If the same program text 

violates multiple rules simultaneously, a conforming analyzer may aggregate diagnostics but must 

produce at least one diagnostic. The diagnostic message might be of the form 

Accessing freed memory in function abc, file xyz.c, line nnn. 

ISO/IEC TS 17961 does not require an analyzer to produce a diagnostic message for any violation 

of any syntax rule or constraint specified by the C Standard [ISO/IEC TS 17961:2013]. Conformance 

is defined only with respect to source code that is visible to the analyzer. Binary-only libraries, 

and calls to them, are outside the scope of these rules. 

An interesting aspect of the technical specification is the portability assumptions, known within 

the group as the “San Francisco rule” because the assumptions evolved at a meeting hosted by 

Coverity at its headquarters. The San Francisco rule states that a conforming analyzer must be 

able to diagnose violations of guidelines for at least one C implementation but does not need to 




Introduction - Tool Selection and Validation 

diagnose a rule violation if the result is documented for the target implementation and does not 

cause a security flaw. Variations in quality of implementation permit an analyzer to produce diagnostics 

concerning portability issues. For example, the following program fragment can produce a 

diagnostic, such as the mismatch between %d and long int: 

long i; printf ("i = %d", i); 

This mismatch might not be a problem for all target implementations, but it is a portability problem 

because not all implementations have the same representation for int and long. 

In addition to other goals already stated, the CERT C Coding Standard has been updated for consistency 

with ISO/IEC TS 17961. Although the documents serve different audiences, consistency 

between the documents should improve the ability of developers to use ISO/IEC TS 17961–conforming 

analyzers to find violations of rules from this coding standard. The Secure Coding Validation 

Suite is a set of tests developed by CERT to validate the rules defined in ISO/IEC TS 

17961. These tests are based on the examples in this technical specification and are distributed 

with a BSD-style license. 

1.5 Tool Selection and Validation 

Although rule checking can be performed manually, with increasing program size and complexity, 

it rapidly becomes infeasible. For this reason, the use of static analysis tools is recommended. 

When choosing a compiler (which should be understood to include the linker), a C-compliant 

compiler should be used whenever possible. A conforming implementation will produce at least 

one diagnostic message if a preprocessing translation unit or translation unit contains a violation 

of any syntax rule or constraint, even if the behavior is also explicitly specified as undefined or 

implementation-defined. It is also likely that any analyzers you may use assume a C-compliant 

compiler. 

When choosing a source code analysis tool, it is clearly desirable that the tool be able to enforce 

as many of the guidelines on the wiki as possible. Not all recommendations are enforceable; some 

are strictly meant to be informative. 

Although CERT recommends the use of an ISO/IEC TS 17961–conforming analyzer, the Software 

Engineering Institute, as a federally funded research and development center (FFRDC), is 

not in a position to endorse any particular vendor or tool. Vendors are encouraged to develop conforming 

analyzers, and users of this coding standard are free to evaluate and select whichever analyzers 

best suit their purposes. 

1.5.1 Completeness and Soundness 

It should be recognized that, in general, determining conformance to coding rules and recommendations 

is computationally undecidable. The precision of static analysis has practical limitations. 

For example, the halting theorem of computer science states that programs exist in which exact 




Introduction - Tool Selection and Validation 

control flow cannot be determined statically. Consequently, any property dependent on control 

flow—such as halting—may be indeterminate for some programs. A consequence of undecidability 

is that it may be impossible for any tool to determine statically whether a given guideline is 

satisfied in specific circumstances. The widespread presence of such code may also lead to unexpected 

results from an analysis tool. 

Regardless of how checking is performed, the analysis may generate 

• False negatives: Failure to report a real flaw in the code is usually regarded as the most serious 

analysis error, as it may leave the user with a false sense of security. Most tools err on the 

side of caution and consequently generate false positives. However, in some cases, it may be 

deemed better to report some high-risk flaws and miss others than to overwhelm the user with 

false positives. 

• False positives: The tool reports a flaw when one does not exist. False positives may occur 

because the code is too complex for the tool to perform a complete analysis. The use of features 

such as function pointers and libraries may make false positives more likely. 

To the greatest extent feasible, an analyzer should be both complete and sound with respect to enforceable 

guidelines. An analyzer is considered sound with respect to a specific guideline if it cannot 

give a false-negative result, meaning it finds all violations of the guideline within the entire 

program. An analyzer is considered complete if it cannot issue false-positive results, or false 

alarms. The possibilities for a given guideline are outlined in the following figure. 

False Positives 

Y 

N 

False Negatives 

N 

Y 

Complete 

with False 

Positives 

Incomplete 

with False 

Positives 

Complete 

and Sound 

Incomplete 

Compilers and source code analysis tools are trusted processes, meaning that a degree of reliance 

is placed on the output of the tools. Accordingly, developers must ensure that this trust is not misplaced. 

Ideally, trust should be achieved by the tool supplier running appropriate validation tests 

such as the Secure Coding Validation Suite. 

1.5.2 False Positives 

Although many guidelines list common exceptions, it is difficult if not impossible to develop a 

complete list of exceptions for each guideline. Consequently, it is important that source code complies 

with the intent of each guideline and that tools, to the greatest extent possible, minimize 




Introduction - Taint Analysis 

false positives that do not violate the intent of the guideline. The degree to which tools minimize 

false-positive diagnostics is a quality-of-implementation issue. 

1.6 Taint Analysis 

1.6.1 Taint and Tainted Sources 

Certain operations and functions have a domain that is a subset of the type domain of their operands 

or parameters. When the actual values are outside of the defined domain, the result might be 

undefined or at least unexpected. If the value of an operand or argument may be outside the domain 

of an operation or function that consumes that value, and the value is derived from any external 

input to the program (such as a command-line argument, data returned from a system call, 

or data in shared memory), that value is tainted, and its origin is known as a tainted source. A 

tainted value is not necessarily known to be out of the domain; rather, it is not known to be in the 

domain. Only values, and not the operands or arguments, can be tainted; in some cases, the same 

operand or argument can hold tainted or untainted values along different paths. In this regard, 

taint is an attribute of a value that is assigned to any value originating from a tainted source. 

1.6.2 Restricted Sinks 

Operands and arguments whose domain is a subset of the domain described by their types are 

called restricted sinks. Any integer operand used in a pointer arithmetic operation is a restricted 

sink for that operand. Certain parameters of certain library functions are restricted sinks because 

these functions perform address arithmetic with these parameters, or control the allocation of a resource, 

or pass these parameters on to another restricted sink. All string input parameters to library 

functions are restricted sinks because it is possible to pass in a character sequence that is not 

null terminated. The exceptions are input parameters to strncpy() and strncpy_s(), which 

explicitly allow the source character sequence not to be null terminated. 

1.6.3 Propagation 

Taint is propagated through operations from operands to results unless the operation itself imposes 

constraints on the value of its result that subsume the constraints imposed by restricted 

sinks. In addition to operations that propagate the same sort of taint, there are operations that 

propagate taint of one sort of an operand to taint of a different sort for their results, the most notable 

example of which is strlen() propagating the taint of its argument with respect to string 

length to the taint of its return value with respect to range. Although the exit condition of a loop is 

not normally considered to be a restricted sink, a loop whose exit condition depends on a tainted 

value propagates taint to any numeric or pointer variables that are increased or decreased by 

amounts proportional to the number of iterations of the loop. 

1.6.4 Sanitization 

To remove the taint from a value, the value must be sanitized to ensure that it is in the defined domain 

of any restricted sink into which it flows. Sanitization is performed by replacement or termination. 

In replacement, out-of-domain values are replaced by in-domain values, and processing 




Introduction - Rules Versus Recommendations 

continues using an in-domain value in place of the original. In termination, the program logic terminates 

the path of execution when an out-of-domain value is detected, often simply by branching 

around whatever code would have used the value. 

In general, sanitization cannot be recognized exactly using static analysis. Analyzers that perform 

taint analysis usually provide some extralinguistic mechanism to identify sanitizing functions that 

sanitize an argument (passed by address) in place, return a sanitized version of an argument, or 

return a status code indicating whether the argument is in the required domain. Because such extralinguistic 

mechanisms are outside the scope of this coding standard, we use a set of rudimentary 

definitions of sanitization that is likely to recognize real sanitization but might cause nonsanitizing 

or ineffectively sanitizing code to be misconstrued as sanitizing. 

The following definition of sanitization presupposes that the analysis is in some way maintaining 

a set of constraints on each value encountered as the simulated execution progresses: a given path 

through the code sanitizes a value with respect to a given restricted sink if it restricts the range of 

that value to a subset of the defined domain of the restricted sink type. For example, sanitization 

of signed integers with respect to an array index operation must restrict the range of that integer 

value to numbers between zero and the size of the array minus one. 

This description is suitable for numeric values, but sanitization of strings with respect to content is 

more difficult to recognize in a general way. 

1.7 Rules Versus Recommendations 

This coding standard contains 99 coding rules. The CERT Coding Standards wiki also has 185 

recommendations at the time of this writing. Rules are meant to provide normative requirements 

for code; recommendations are meant to provide guidance that, when followed, should improve 

the safety, reliability, and security of software systems. However, a violation of a recommendation 

does not necessarily indicate the presence of a defect in the code. Rules and recommendations 

are collectively referred to as guidelines. 

The wiki also contains two platform-specific annexes at the time of this writing; one annex is for 

POSIX and the other one is for Windows. These annexes have been omitted from this standard 

because they are not part of the core standard. 

This standard is based on the C rules available on the CERT Secure Coding wiki as of 30 March 

2016. 

1.7.1 Rules 

Rules must meet the following criteria: 

1. Violation of the guideline is likely to result in a defect that may adversely affect the safety, 

reliability, or security of a system, for example, by introducing a security flaw that may result 

in an exploitable vulnerability. 




Introduction - Conformance Testing 

2. The guideline does not rely on source code annotations or assumptions. 

3. Conformance to the guideline can be determined through automated analysis (either static or 

dynamic), formal methods, or manual inspection techniques. 

1.7.2 Recommendations 

Recommendations are suggestions for improving code quality. Guidelines are defined to be recommendations 

when all of the following conditions are met: 

1. Application of a guideline is likely to improve the safety, reliability, or security of software 

systems. 

2. One or more of the requirements necessary for a guideline to be considered a rule cannot be 

met. 

The set of recommendations that a particular development effort adopts depends on the requirements 

of the final software product. Projects with stricter requirements may decide to dedicate 

more resources to ensuring the safety, reliability, and security of a system and consequently are 

likely to adopt a broader set of recommendations. 

1.8 Conformance Testing 

To ensure that the source code conforms to this coding standard, it is necessary to have measures 

in place that check for rule violations. The most effective means of achieving this goal is to use 

one or more ISO/IEC TS 17961–conforming analyzers. Where a guideline cannot be checked by a 

tool, a manual review is required. 

The Source Code Analysis Laboratory (SCALe) provides a means for evaluating the conformance 

of software systems against this and other coding standards. CERT coding standards provide a 

normative set of rules against which software systems can be evaluated. Conforming software 

systems should demonstrate improvements in the safety, reliability, and security over nonconforming 

systems. 

The SCALe team at CERT analyzes a developer’s source code and provides a detailed report of 

findings to guide the code’s repair. After the developer has addressed these findings and the 

SCALe team determines that the product version conforms to the standard, CERT issues the developer 

a certificate and lists the system in a registry of conforming systems. 

1.8.1 Conformance 

Conformance to the CERT C Coding Standard requires that the code not contain any violations of 

the rules specified in this standard. If an exceptional condition is claimed, the exception must correspond 

to a predefined exceptional condition, and the application of this exception must be documented 

in the source code. Conformance with the recommendations on the wiki is not necessary 

to claim conformance with the CERT C Coding Standard. Conformance to the recommendations 




Introduction - Development Process 

will, in many cases, make it easier to conform to the rules, eliminating many potential sources of 

defects. 

1.8.2 Levels 

Rules and recommendations in this standard are classified into three levels (see How this Coding 

Standard is Organized). Emphasis should be placed on conformance Level 1 (L1) rules. Software 

systems that have been validated as complying with all Level 1 rules are considered to be L1 conforming. 

Software systems can be assessed as L1, L2, or fully conforming, depending on the set 

of rules to which the system has been validated. 

1.8.3 Deviation Procedure 

Strict adherence to all rules is unlikely and, consequently, deviations associated with specific rule 

violations are necessary. Deviations can be used in cases where a true-positive finding is uncontested 

as a rule violation but the code is nonetheless determined to be correct. An uncontested 

true-positive finding may be the result of a design or architecture feature of the software or may 

occur for a valid reason that was unanticipated by the coding standard. In this respect, the deviation 

procedure allows for the possibility that coding rules are overly strict [Seacord 2013]. 

Deviations are not granted for reasons of performance or usability. A software system that successfully 

passes conformance testing must not contain defects or exploitable vulnerabilities. Deviation 

requests are evaluated by the lead assessor, and if the developer can provide sufficient evidence 

that the deviation will not result in a vulnerability, the deviation request is accepted. 

Deviations are used infrequently because it is almost always easier to fix a coding error than it is 

to provide an argument that the coding error does not result in a vulnerability. 

1.9 Development Process 

The development of a coding standard for any programming language is a difficult undertaking 

that requires significant community involvement. The following development process has been 

used to create this standard: 

1. Rules and recommendations for a coding standard are solicited from the communities involved 

in the development and application of each programming language, including the formal 

or de facto standards bodies responsible for the documented standard. 

2. These rules and recommendations are edited by members of the CERT technical staff for 

content and style and placed on the CERT Secure Coding Standards website for comment 

and review. 

3. The user community may then comment on the publicly posted content using threaded discussions 

and other communication tools. Once a consensus develops that the rule or recommendation 

is appropriate and correct, the final rule is incorporated into an officially released 

version of the secure coding standard. 




Introduction - Usage 

Early drafts of the CERT C Secure Coding Standard have been reviewed by the ISO/IEC 

JTC1/SC22/WG14 international standardization working group for the C programming language 

and by other industry groups as appropriate. 

1.10 Usage 

The rules in this standard may be extended with organization-specific rules. However, the rules in 

the standard must be obeyed to claim conformance with the standard. 

Training may be developed to educate software professionals regarding the appropriate application 

of coding standards. After passing an examination, these trained programmers may also be 

certified as coding professionals. For example, the Software Developer Certification (SDC) is a 

credentialing program developed at Carnegie Mellon University. The SDC uses authentic examination 

to 

1. Identify job candidates with specific programming skills 

2. Demonstrate the presence of a well-trained software workforce 

3. Provide guidance to educational and training institutions 

Once a coding standard has been established, tools and processes can be developed or modified to 

determine conformance with the standard. 

1.11 System Qualities 

The goal of this coding standard is to produce safe, reliable, and secure systems. Additional requirements 

might exist for safety-critical systems, such as the absence of dynamic memory allocation. 

Other software quality attributes of interest include portability, usability, availability, maintainability, 

readability, and performance. 

Many of these attributes are interrelated in interesting ways. For example, readability is an attribute 

of maintainability; both are important for limiting the introduction of defects during maintenance 

that can result in security flaws or reliability issues. In addition, readability aids code inspection 

by safety officers. Reliability and availability require proper resource management, 

which also contributes to the safety and security of the system. System attributes such as performance 

and security are often in conflict, requiring trade-offs to be considered. 

1.12 Vulnerability Metric 

The CERT vulnerability metric value is a number between 0 and 180 that assigns an approximate 

severity to the vulnerability. This number considers several factors: 

• Is information about the vulnerability widely available or known? 




Introduction - How This Coding Standard Is Organized 

• Is the vulnerability being exploited in incidents reported to CERT or other incident response 

teams? 

• Is the Internet infrastructure (for example, routers, name servers, critical Internet protocols) at 

risk because of this vulnerability? 

• How many systems on the Internet are at risk from this vulnerability? 

• What is the impact of exploiting the vulnerability? 

• How easy is it to exploit the vulnerability? 

• What are the preconditions required to exploit the vulnerability? 

Because the questions are answered with approximate values based on our own judgments and 

may differ significantly from one site to another, readers should not rely too heavily on the metric 

for prioritizing their response to vulnerabilities. Rather, this metric may be useful for separating 

the serious vulnerabilities from the larger number of less severe vulnerabilities described in the 

database. Because the questions are not all weighted equally, the resulting score is not linear; that 

is, a vulnerability with a metric of 40 is not twice as severe as one with a metric of 20. 

An alternative vulnerability severity metric is the Common Vulnerability Scoring System 

(CVSS). 

1.13 How This Coding Standard Is Organized 

This coding standard is organized into 15 chapters containing rules in specific topic areas followed 

by four appendices. Appendix A contains the bibliography. Appendix B lists the definitions 

of terms used throughout the standard. Appendix C lists the undefined behaviors from the C 

Standard, Annex J, J.2 [ISO/IEC 9899:2011], numbered and classified for easy reference. These 

numbered undefined behaviors are referenced frequently from the rules. Appendix D lists unspecified 

behaviors from the C Standard, Annex J, J.2 [ISO/IEC 9899:2011]. These unspecified behaviors 

are occasionally referenced from the rules as well. 

Most rules have a consistent structure. Each rule in this standard has a unique identifier, which is 

included in the title. The title and the introductory paragraphs define the rule and are typically followed 

by one or more pairs of noncompliant code examples and compliant solutions. Each rule 

also includes a risk assessment, related guidelines, and a bibliography (where applicable). Rules 

may also include a table of related vulnerabilities. The recommendations on the CERT Coding 

Standards wiki are organized in a similar fashion. 

1.13.1 Identifiers 

Each rule and recommendation is given a unique identifier. These identifiers consist of three 

parts: 

• A three-letter mnemonic representing the section of the standard 

• A two-digit numeric value in the range of 00 to 99 

• The letter C indicating that this is a C language guideline 





The three-letter mnemonic can be used to group similar coding practices and to indicate which 

category a coding practice belongs to. 

The numeric value is used to give each coding practice a unique identifier. Numeric values in the 

range of 00 to 29 are reserved for recommendations, and values in the range of 30 to 99 are reserved 

for rules. Rules and recommendations are frequently referenced from the rules in this 

standard by their identifier and title. Rules can be found in this standard’s table of contents, 

whereas recommendations can be found only on the wiki. 

1.13.2 Noncompliant Code Examples and Compliant Solutions 

Noncompliant code examples illustrate code that violates the guideline under discussion. It is important 

to note that these are only examples, and eliminating all occurrences of the example does 

not necessarily mean that the code being analyzed is now compliant with the guideline. 

Noncompliant code examples are typically followed by compliant solutions, which show how the 

noncompliant code example can be recoded in a secure, compliant manner. Except where noted, 

noncompliant code examples should contain violations only of the guideline under discussion. 

Compliant solutions should comply with all of the secure coding rules but may on occasion fail to 

comply with a recommendation. 

1.13.3 Exceptions 

Any rule or recommendation may specify a small set of exceptions detailing the circumstances 

under which the guideline is not necessary to ensure the safety, reliability, or security of software. 

Exceptions are informative only and are not required to be followed. 

1.13.4 Risk Assessment 

Each guideline in the CERT C Coding Standard contains a risk assessment section that attempts to 

provide software developers with an indication of the potential consequences of not addressing a 

particular rule or recommendation in their code (along with some indication of expected remediation 

costs). This information may be used to prioritize the repair of rule violations by a development 

team. The metric is designed primarily for remediation projects. It is generally assumed that 

new code will be developed to be compliant with the entire coding standard and applicable recommendations. 

Each rule and recommendation has an assigned priority. Priorities are assigned using a metric 

based on Failure Mode, Effects, and Criticality Analysis (FMECA) [IEC 60812]. Three values are 

assigned for each rule on a scale of 1 to 3 for severity, likelihood, and remediation cost. 

Severity—How serious are the consequences of the rule being ignored? 

Value Meaning Examples of Vulnerability 

1 Low Denial-of-service attack, abnormal 

termination 

2 Medium Data integrity violation, unintentional 

information disclosure 





3 High Run arbitrary code 

Likelihood—How likely is it that a flaw introduced by ignoring the rule can lead to an exploitable 

vulnerability? 

Value 

Meaning 

1 Unlikely 

2 Probable 

3 Likely 

Remediation Cost—How expensive is it to comply with the rule? 

Value Meaning Detection Correction 

1 High Manual Manual 

2 Medium Automatic Manual 

3 Low Automatic Automatic 

The three values are then multiplied together for each rule. This product provides a measure that 

can be used in prioritizing the application of the rules. The products range from 1 to 27, although 

only the following 10 distinct values are possible: 1, 2, 3, 4, 6, 8, 9, 12, 18, and 27. Rules and recommendations 

with a priority in the range of 1 to 4 are Level 3 rules, 6 to 9 are Level 2, and 12 to 

27 are Level 1. The following are possible interpretations of the priorities and levels. 

Priorities and Levels 

Level Priorities Possible Interpretation 

L1 12, 18, 27 High severity, likely, inexpensive 

to repair 

L2 6, 8, 9 Medium severity, probable, medium 

cost to repair 

L3 1, 2, 3, 4 Low severity, unlikely, expensive 

to repair 

Specific projects may begin remediation by implementing all rules at a particular level before proceeding 

to the lower priority rules, as shown in the following illustration. 





Recommendations are not compulsory and are provided for information purposes only. 

1.13.5 Automated Detection 

On the wiki, both rules and recommendations frequently have sections that describe automated 

detection. These sections provide additional information on analyzers that can automatically diagnose 

violations of coding guidelines. Most automated analyses for the C programming language 

are neither sound nor complete, so the inclusion of a tool in this section typically means that the 

tool can diagnose some violations of this particular rule. Although the Secure Coding Validation 

Suite can be used to test the ability of analyzers to diagnose violations of rules from ISO/IEC TS 

19761, no currently available conformance test suite can assess the ability of analyzers to diagnose 

violations of the rules in this standard. Consequently, the information in automated detection 

sections on the wiki may be 

• Provided by the vendors 

• Determined by CERT by informally evaluating the analyzer 

• Determined by CERT by reviewing the vendor documentation 

Where possible, we try to reference the exact version of the tool for which the results were obtained. 

Because these tools evolve continuously, this information can rapidly become dated and 

obsolete. 




Introduction - Automatically Generated Code 

1.13.6 Related Vulnerabilities 

The risk assessment sections on the wiki also contain a link to search for related vulnerabilities on 

the CERT website. Whenever possible, CERT Vulnerability Notes are tagged with a keyword corresponding 

to the unique ID of the coding guideline. This search provides you with an up-to-date 

list of real-world vulnerabilities that have been determined to be at least partially caused by a violation 

of this specific guideline. These vulnerabilities are labeled as such only when the vulnerability 

analysis team at the CERT/CC is able to evaluate the source code and precisely determine 

the cause of the vulnerability. Because many vulnerability notes refer to vulnerabilities in closedsource 

software systems, it is not always possible to provide this additional analysis. Consequently, 

the related vulnerabilities field tends to be somewhat sparsely populated. 

Related vulnerability sections are included only for specific rules in this standard, when the information 

is both relevant and interesting. 

1.13.7 Related Guidelines 

The related guidelines sections contain links to guidelines in related standards, technical specifications, 

and guideline collections such as Information Technology—Programming Languages, Their 

Environments and System Software Interfaces—C Secure Coding Rules [ISO/IEC TS 

17961:2013]; Information Technology—Programming Languages—Guidance to Avoiding Vulnerabilities 

in Programming Languages through Language Selection and Use [ISO/IEC TR 

24772:2013]; MISRA C 2012: Guidelines for the Use of the C Language in Critical Systems 

[MISRA C:2012]; and CWE IDs in MITRE’s Common Weakness Enumeration (CWE) [MITRE 

2010]. 

You can create a unique URL to get more information on CWEs by appending the relevant ID to 

the end of a fixed string. For example, to find more information about CWE-192, Integer Coercion 

Error,” you can append 192.html to http://cwe.mitre.org/data/definitions/ and enter the resulting 

URL in your browser: http://cwe.mitre.org/data/definitions/192.html. 

The other referenced technical specifications, technical reports, and guidelines are commercially 

available. 

1.13.8 Bibliography 

Most guidelines have a small bibliography section that lists documents and section in those documents 

that provide information relevant to the rule. 

1.14 Automatically Generated Code 

If a code-generating tool is to be used, it is necessary to select an appropriate tool and undertake 

validation. Adherence to the requirements of this document may provide one criterion for assessing 

a tool. 




Introduction - Government Regulations 

Coding guidance varies depending on how code is generated and maintained. Categories of code 

include the following: 

• Tool-generated, tool-maintained code that is specified and maintained in a higher level format 

from which language-specific source code is generated. The source code is generated from 

this higher level description and then provided as input to the language compiler. The generated 

source code is never viewed or modified by the programmer. 

• Tool-generated, hand-maintained code that is specified and maintained in a higher level format 

from which language-specific source code is generated. It is expected or anticipated, 

however, that at some point in the development cycle, the tool will cease to be used and the 

generated source code will be visually inspected and/or manually modified and maintained. 

• Hand-coded code is manually written by a programmer using a text editor or interactive development 

environment; the programmer maintains source code directly in the source-code 

format provided to the compiler. 

Source code that is written and maintained by hand must have the following properties: 

• Readability 

• Program comprehension 

These requirements are not applicable for source code that is never directly handled by a programmer, 

although requirements for correct behavior still apply. Reading and comprehension requirements 

apply to code that is tool generated and hand maintained but do not apply to code that is 

tool generated and tool maintained. Tool-generated, tool-maintained code can impose consistent 

constraints that ensure the safety of some constructs that are risky in hand-generated code. 

1.15 Government Regulations 

Developing software to secure coding rules is a good idea and is increasingly a requirement. The 

National Defense Authorization Act for Fiscal Year 2013, Section 933, “Improvements in Assurance 

of Computer Software Procured by the Department of Defense,” requires evidence that government 

software development and maintenance organizations and contractors are conforming in 

computer software coding to approved secure coding standards of the Department of Defense 

(DoD) during software development, upgrade, and maintenance activities, including through the 

use of inspection and appraisals. 

DoD acquisition programs are specifying The Application Security and Development Security 

Technical Implementation Guide (STIG), Version 3, Release 10 [DISA 2015] in requests for proposal 

(RFPs). Section 2.1.5, “Coding Standards,” requires that “the Program Manager will ensure 

the development team follows a set of coding standards.” 

The proper application of this standard would enable a system to comply with the following requirements 

from the Application Security and Development Security Technical Implementation 

Guide, Version 3, Release 10 [DISA 2015]: 




Introduction - Acknowledgments 

• (APP2060.1: CAT II) The Program Manager will ensure the development team follows a set 

of coding standards. 

• (APP2060.2: CAT II) The Program Manager will ensure the development team creates a list 

of unsafe functions to avoid and document this list in the coding standards. 

• (APP3550: CAT I) The Designer will ensure the application is not vulnerable to integer 

arithmetic issues. 

• (APP3560: CAT I) The Designer will ensure the application does not contain format string 

vulnerabilities. 

• (APP3570: CAT I) The Designer will ensure the application does not allow command injection. 

• (APP3590.1: CAT I) The Designer will ensure the application does not have buffer overflows. 

• (APP3590.2: CAT I) The Designer will ensure the application does not use functions known 

to be vulnerable to buffer overflows. 

• (APP3590.3: CAT II) The Designer will ensure the application does not use signed values 

for memory allocation where permitted by the programming language. 

• (APP3600: CAT II) The Designer will ensure the application has no canonical representation 

vulnerabilities. 

• (APP3630.1: CAT II) The Designer will ensure the application is not vulnerable to race conditions. 

• (APP3630.2: CAT III) The Designer will ensure the application does not use global variables 

when local variables could be used. 

Training programmers and software testers on the standard will satisfy the following requirements: 

• (APP2120.3: CAT II) The Program Manager will ensure developers are provided with training 

on secure design and coding practices on at least an annual basis. 

• (APP2120.4: CAT II) The Program Manager will ensure testers are provided training on at 

least an annual basis. 

• (APP2060.3: CAT II) The Designer will follow the established coding standards established 

for the project. 

• (APP2060.4: CAT II) The Designer will not use unsafe functions documented in the project 

coding standards. 

• (APP5010: CAT III) The Test Manager will ensure at least one tester is designated to test for 

security flaws in addition to functional testing. 

1.16 Acknowledgments 

This standard was made possible through a broad community effort. We thank all those who contributed 

and provided reviews on the CERT Secure Coding wiki that helped to make the standards 





a success. If you are interested in contributing to the rules, create an account on the wiki and then 

request contributor privileges by sending email to info@sei.cmu.edu. 

1.16.1 Contributors to this Edition of the Standard 

Eric Azebu, Aaron Ballman, Jill Britton, Vaclav Bubnik, G. Ann Campbell, Geoff Clare, Lori 

Flynn, Amy Gale, Arthur Hicken, David Keaton, Will Klieber, Masaki Kubo, Carol Lallier, Fred 

Long, Daniel Marjamäki, Robert Seacord, Martin Sebor, Sandy Shrum, Will Snavely, David Svoboda, 

Yozo Toda, Barbara White, and Liz Whiting 

1.16.2 Contributors and Reviewers of Previous Editions of the Standard 

Arbob Ahmad, Juan Alvarado, Dave Aronson, Abhishek Arya, Berin Babcock-McConnell, Roberto 

Bagnara, Aaron Ballman, BJ Bayha, John Benito, Joe Black, Jodi Blake, Jill Britton, Levi 

Broderick, Hal Burch, J. L. Charton, Steven Christey, Ciera Christopher, Geoff Clare, Frank Costello, 

Joe Damato, Stephen C. Dewhurst, Susan Ditmore, Chad Dougherty, Mark Dowd, Apoorv 

Dutta, Emily Evans, Xiaoyi Fei, William Fithen, Hallvard Furuseth, Jeffrey Gennari, Andrew 

Gidwani, Ankur Goyal, Douglas A. Gwyn, Shaun Hedrick, Michael Howard, Sujay Jain, Christina 

Johns, Pranjal Jumde, David Keaton, Andrew Keeton, David Kohlbrenner, Takuya Kondo, 

Masaki Kubo, Pranav Kukreja, Richard Lane, Stephanie Wan-Ruey Lee, Jonathan Leffler, 

Pengfei Li, Fred Long, Justin Loo, Gregory K. Look, Nat Lyle, Larry Maccherone, Aditya Mahendrakar, 

Lee Mancuso, John McDonald, James McNellis, Randy Meyers, Dhruv Mohindra, 

Bhaswanth Nalabothula, Todd Nowacki, Adrian Trejo Nuñez, Bhadrinath Pani, Vishal Patel, David 

M. Pickett, Justin Pincar, Dan Plakosh, Thomas Plum, Abhijit Rao, Raunak Rungta, Dan 

Saks, Alexandre Santos, Brendan Saulsbury, Robert C. Seacord, Martin Sebor, Jason Michael 

Sharp, Astha Singhal, Will Snavely, Nick Stoughton, Alexander E. Strommen, Glenn Stroz, David 

Svoboda, Dean Sutherland, Kazunori Takeuchi, Chris Tapp, Chris Taschner, Mira Sri Divya 

Thambireddy, Melanie Thompson, Elpiniki Tsakalaki, Ben Tucker, Fred J. Tydeman, Abhishek 

Veldurthy, Wietse Venema, Alex Volkovitsky, Michael Shaye-Wen Wang, Grant Watters, Tim 

Wilson, Eric Wong, Lutz Wrage, Shishir Kumar Yadav, Gary Yuan, Ricky Zhou, and Alen Zukich 

Stefan Achatz, Arbob Ahmad, Laurent Alebarde, Kevin Bagust, Greg Beeley, Arjun Bijanki, John 

Bode, Konrad Borowski, Stewart Brodie, Jordan Brown, Andrew Browne, G Bulmer, Kyle 

Comer, Sean Connelly, Ale Contenti, Tom Danielsen, Eric Decker, Mark Dowd, T. Edwin, Brian 

Ewins, Justin Ferguson, William L. Fithen, Stephen Friedl, Hallvard Furuseth, Shay Green, Samium 

Gromoff, Kowsik Guruswamy, Jens Gustedt, Peter Gutmann, Douglas A. Gwyn, Richard 

Heathfield, Darryl Hill, Paul Hsieh, Ivan Jager, Steven G. Johnson, Anders Kaseorg, Matt Kraai, 

Piotr Krukowiecki, Jerry Leichter, Nicholas Marriott, Frank Martinez, Scott Meyers, Eric Miller, 

Charles-Francois Natali, Ron Natalie, Adam O’Brien, Heikki Orsila, Balog Pal, Jonathan Paulson, 

P.J. Plauger, Leslie Satenstein, Kirk Sayre, Neil Schellenberger, Michel Schinz, Eric Sosman, 

Chris Tapp, Andrey Tarasevich, Yozo Toda, Josh Triplett, Pavel Vasilyev, Ivan Vecerina, Zeljko 

Vrba, David Wagner, Henry S. Warren, Colin Watson, Zhenyu Wu, Drew Yao, Christopher Yeleighton, 

and Robin Zhu 





1.16.3 The SEI CERT Secure Coding Team 

Aaron Ballman, Lori Flynn, David Keaton, William Klieber, Robert Schiela, William Snavely, 

and David Svoboda 




Preprocessor (PRE) - PRE30-C. Do not create a universal character name through concatenation 

2 Preprocessor (PRE) 

2.1 PRE30-C. Do not create a universal character name through 

concatenation 

The C Standard supports universal character names that may be used in identifiers, character constants, 

and string literals to designate characters that are not in the basic character set. The universal 

character name \Unnnnnnnn designates the character whose 8-digit short identifier (as specified 

by ISO/IEC 10646) is nnnnnnnn. Similarly, the universal character name \unnnn designates 

the character whose 4-digit short identifier is nnnn (and whose 8-digit short identifier is 

0000nnnn). 

The C Standard, 5.1.1.2, paragraph 4 [ISO/IEC 9899:2011], says 

If a character sequence that matches the syntax of a universal character name is produced 

by token concatenation (6.10.3.3), the behavior is undefined. 

See also undefined behavior 3. 

In general, avoid universal character names in identifiers unless absolutely necessary. 

2.1.1 Noncompliant Code Example 

This code example is noncompliant because it produces a universal character name by token concatenation: 

#define assign(uc1, uc2, val) uc1##uc2 = val 

void func(void) { 

int \u0401; 

/* ... */ 

assign(\u04, 01, 4); 

/* ... */ 

} 

2.1.1.1 Implementation Details 

This code compiles and runs with Microsoft Visual Studio 2013, assigning 4 to the variable as expected. 

GCC 4.8.1 on Linux refuses to compile this code; it emits a diagnostic reading, “stray '\' in program,” 

referring to the universal character fragment in the invocation of the assign macro. 




Preprocessor (PRE) - PRE30-C. Do not create a universal character name through concatenation 

2.1.2 Compliant Solution 

This compliant solution uses a universal character name but does not create it by using token concatenation: 

#define assign(ucn, val) ucn = val 


int \u0401; 

/* ... */ 

assign(\u0401, 4); 

/* ... */ 

} 


Creating a universal character name through token concatenation results in undefined behavior. 

Rule Severity Likelihood Remediation Cost Priority Level 

PRE30-C Low Unlikely Medium P2 L3 


[ISO/IEC 10646-2003] 

[ISO/IEC 9899:2011] 

Subclause 5.1.1.2, “Translation Phases” 




Preprocessor (PRE) - PRE31-C. Avoid side effects in arguments to unsafe macros 

2.2 PRE31-C. Avoid side effects in arguments to unsafe macros 

An unsafe function-like macro is one whose expansion results in evaluating one of its parameters 

more than once or not at all. Never invoke an unsafe macro with arguments containing an assignment, 

increment, decrement, volatile access, input/output, or other expressions with side effects 

(including function calls, which may cause side effects). 

The documentation for unsafe macros should warn against invoking them with arguments with 

side effects, but the responsibility is on the programmer using the macro. Because of the risks associated 

with their use, it is recommended that the creation of unsafe function-like macros be 

avoided. (See PRE00-C. Prefer inline or static functions to function-like macros.) 

This rule is similar to EXP44-C. Do not rely on side effects in operands to sizeof, _Alignof, or 

_Generic. 


One problem with unsafe macros is side effects on macro arguments, as shown by this noncompliant 

code example: 

#define ABS(x) (((x) < 0) ? -(x) : (x)) 

void func(int n) { 

/* Validate that n is within the desired range */ 

int m = ABS(++n); 

} 

/* ... */ 

The invocation of the ABS() macro in this example expands to 

m = (((++n) < 0) ? -(++n) : (++n)); 

The resulting code is well defined but causes n to be incremented twice rather than once. 


In this compliant solution, the increment operation ++n is performed before the call to the unsafe 

macro. 

#define ABS(x) (((x) < 0) ? -(x) : (x)) /* UNSAFE */ 



++n; 

int m = ABS(n); 





} 

/* ... */ 

Note the comment warning programmers that the macro is unsafe. The macro can also be renamed 

ABS_UNSAFE() to make it clear that the macro is unsafe. This compliant solution, like all 

the compliant solutions for this rule, has undefined behavior if the argument to ABS() is equal to 

the minimum (most negative) value for the signed integer type. (See INT32-C. Ensure that operations 

on signed integers do not result in overflow for more information.) 


This compliant solution follows the guidance of PRE00-C. Prefer inline or static functions to 

function-like macros by defining an inline function iabs() to replace the ABS() macro. Unlike 

the ABS() macro, which operates on operands of any type, the iabs() function will truncate arguments 

of types wider than int whose value is not in range of the latter type. 

#include 

#include 

static inline int iabs(int x) { 

return (((x) < 0) ? -(x) : (x)); 

} 



int m = iabs(++n); 

} 

/* ... */ 


A more flexible compliant solution is to declare the ABS() macro using a _Generic selection. 

To support all arithmetic data types, this solution also makes use of inline functions to compute 

integer absolute values. (See PRE00-C. Prefer inline or static functions to function-like macros 

and PRE12-C. Do not define unsafe macros.) 

According to the C Standard, 6.5.1.1, paragraph 3 [ISO/IEC 9899:2011]: 

The controlling expression of a generic selection is not evaluated. If a generic selection 

has a generic association with a type name that is compatible with the type of the controlling 

expression, then the result expression of the generic selection is the expression 

in that generic association. Otherwise, the result expression of the generic selection is 

the expression in the default generic association. None of the expressions from any 

other generic association of the generic selection is evaluated. 





Because the expression is not evaluated as part of the generic selection, the use of a macro in this 

solution is guaranteed to evaluate the macro parameter v only once. 

#include 

#include 

static inline long llabs(long long v) { 

return v < 0 ? -v : v; 

} 

static inline long labs(long v) { 

return v < 0 ? -v : v; 

} 

static inline int iabs(int v) { 

return v < 0 ? -v : v; 

} 

static inline int sabs(short v) { 

return v < 0 ? -v : v; 

} 

static inline int scabs(signed char v) { 

return v < 0 ? -v : v; 

} 

#define ABS(v) _Generic(v, signed char : scabs, \ 

short : sabs, \ 

int : iabs, \ 

long : labs, \ 

long : llabs, \ 

float : fabsf, \ 

double : fabs, \ 

long double : fabsl, \ 

double complex : cabs, \ 

float complex : cabsf, \ 

long double complex : cabsl)(v) 



int m = ABS(++n); 

/* ... */ 

} 

Generic selections were introduced in C11 and are not available in C99 and earlier editions of the 

C Standard. 

2.2.5 Compliant Solution (GCC) 

GCC’s __typeof extension makes it possible to declare and assign the value of the macro operand 

to a temporary of the same type and perform the computation on the temporary, consequently 

guaranteeing that the operand will be evaluated exactly once. Another GCC extension, known as 





statement expression, makes it possible for the block statement to appear where an expression is 

expected: 

#define ABS(x) __extension__ ({ __typeof (x) tmp = x; \ 

tmp < 0 ? -tmp : tmp; }) 

Relying on such extensions makes code nonportable and violates MSC14-C. Do not introduce unnecessary 

platform dependencies. 

2.2.6 Noncompliant Code Example (assert()) 

The assert() macro is a convenient mechanism for incorporating diagnostic tests in code. (See 

MSC11-C. Incorporate diagnostic tests using assertions.) Expressions used as arguments to the 

standard assert() macro should not have side effects. The behavior of the assert() macro depends 

on the definition of the object-like macro NDEBUG. If the macro NDEBUG is undefined, the 

assert() macro is defined to evaluate its expression argument and, if the result of the expression 

compares equal to 0, call the abort() function. If NDEBUG is defined, assert is defined to 

expand to ((void)0). Consequently, the expression in the assertion is not evaluated, and no side 

effects it may have had otherwise take place in non-debugging executions of the code. 

This noncompliant code example includes an assert() macro containing an expression (index++) 

that has a side effect: 

#include 

#include 

void process(size_t index) { 

assert(index++ > 0); /* Side effect */ 

/* ... */ 

} 

2.2.7 Compliant Solution (assert()) 

This compliant solution avoids the possibility of side effects in assertions by moving the expression 

containing the side effect outside of the assert() macro. 

#include 

#include 

void process(size_t index) { 

assert(index > 0); /* No side effect */ 

++index; 





} 

/* ... */ 


PRE31-C-EX1: An exception can be made for invoking an unsafe macro with a function call argument 

provided that the function has no side effects. However, it is easy to forget about obscure 

side effects that a function might have, especially library functions for which source code is not 

available; even changing errno is a side effect. Unless the function is user-written and does nothing 

but perform a computation and return its result without calling any other functions, it is likely 

that many developers will forget about some side effect. Consequently, this exception must be 

used with great care. 


Invoking an unsafe macro with an argument that has side effects may cause those side effects to 

occur more than once. This practice can lead to unexpected program behavior. 


PRE31-C Low Unlikely Low P3 L3 


SEI CERT C Coding Standard 

PRE00-C. Prefer inline or static functions to 

function-like macros 

PRE12-C. Do not define unsafe macros 

MSC14-C. Do not introduce unnecessary platform 

dependencies 

DCL37-C. Do not declare or define a reserved 

identifier 

SEI CERT C++ Coding Standard 

PRE31-CPP. Avoid side-effects in arguments 

to unsafe macros 

CERT Oracle Secure Coding Standard for Java EXP06-J. Expressions used in assertions must 

not produce side effects 

ISO/IEC TR 24772:2013 

Pre-processor Directives [NMP] 

MISRA C:2012 

Rule 20.5 (advisory) 


[Dewhurst 2002] 

Gotcha #28, “Side Effects in Assertions” 

[ISO/IEC 9899:2011] 

Subclause 6.5.1.1, “Generic Selection” 

[Plum 1985] Rule 1-11 




Preprocessor (PRE) - PRE32-C. Do not use preprocessor directives in invocations of function-like macros 

2.3 PRE32-C. Do not use preprocessor directives in invocations of 


The arguments to a macro must not include preprocessor directives, such as #define, #ifdef, 

and #include. Doing so results in undefined behavior, according to the C Standard, 6.10.3, paragraph 

11 [ISO/IEC 9899:2011]: 

The sequence of preprocessing tokens bounded by the outside-most matching parentheses 

forms the list of arguments for the function-like macro. The individual arguments 

within the list are separated by comma preprocessing tokens, but comma preprocessing 

tokens between matching inner parentheses do not separate arguments. If there are 

sequences of preprocessing tokens within the list of arguments that would otherwise 

act as preprocessing directives, the behavior is undefined. 


This rule also applies to the use of preprocessor directives in arguments to a function where it is 

unknown whether or not the function is implemented using a macro. For example, standard library 

functions, such as memcpy(), printf(), and assert(), may be implemented as macros. 


In this noncompliant code example [GCC Bugs], the programmer uses preprocessor directives to 

specify platform-specific arguments to memcpy(). However, if memcpy() is implemented using a 

macro, the code results in undefined behavior. 

#include 

void func(const char *src) { 

/* Validate the source string; calculate size */ 

char *dest; 

/* malloc() destination string */ 

memcpy(dest, src, 

#ifdef PLATFORM1 

12 

#else 

24 

#endif 

); 

/* ... */ 

); 




Preprocessor (PRE) - PRE32-C. Do not use preprocessor directives in invocations of function-like macros 


In this compliant solution [GCC Bugs], the appropriate call to memcpy() is determined outside 

the function call: 

#include 

void func(const char *src) { 

/* Validate the source string; calculate size */ 

char *dest; 

/* malloc() destination string */ 

#ifdef PLATFORM1 

memcpy(dest, src, 12); 

#else 

memcpy(dest, src, 24); 

#endif 

/* ... */ 

} 


Including preprocessor directives in macro arguments is undefined behavior. 


PRE32-C Low Unlikely Medium P2 L3 


[GCC Bugs] 

[ISO/IEC 9899:2011] 

“Non-bugs” 

6.10.3, “Macro Replacement” 




Declarations and Initialization (DCL) - DCL30-C. Declare objects with appropriate storage durations 

3 Declarations and Initialization (DCL) 

3.1 DCL30-C. Declare objects with appropriate storage durations 

Every object has a storage duration that determines its lifetime: static, thread, automatic, or allocated. 

According to the C Standard, 6.2.4, paragraph 2 [ISO/IEC 9899:2011], 

The lifetime of an object is the portion of program execution during which storage is 

guaranteed to be reserved for it. An object exists, has a constant address, and retains its 

last-stored value throughout its lifetime. If an object is referred to outside of its lifetime, 

the behavior is undefined. The value of a pointer becomes indeterminate when the object 

it points to reaches the end of its lifetime. 

Do not attempt to access an object outside of its lifetime. Attempting to do so is undefined behavior 

and can lead to an exploitable vulnerability. (See also undefined behavior 9 in the C Standard, 

Annex J.) 

3.1.1 Noncompliant Code Example (Differing Storage Durations) 

In this noncompliant code example, the address of the variable c_str with automatic storage duration 

is assigned to the variable p, which has static storage duration. The assignment itself is 

valid, but it is invalid for c_str to go out of scope while p holds its address, as happens at the 

end of dont_do_this(). 

#include 

const char *p; 

void dont_do_this(void) { 

const char c_str[] = "This will change"; 

p = c_str; /* Dangerous */ 

} 

void innocuous(void) { 

printf("%s\n", p); 

} 

int main(void) { 

dont_do_this(); 

innocuous(); 

return 0; 

} 





3.1.2 Compliant Solution (Same Storage Durations) 

In this compliant solution, p is declared with the same storage duration as c_str, preventing p 

from taking on an indeterminate value outside of this_is_OK(): 

void this_is_OK(void) { 

const char c_str[] = "Everything OK"; 

const char *p = c_str; 

/* ... */ 

} 

/* p is inaccessible outside the scope of string c_str */ 

Alternatively, both p and c_str could be declared with static storage duration. 

3.1.3 Compliant Solution (Differing Storage Durations) 

If it is necessary for p to be defined with static storage duration but c_str with a more limited 

duration, then p can be set to NULL before c_str is destroyed. This practice prevents p from taking 

on an indeterminate value, although any references to p must check for NULL. 

const char *p; 

void is_this_OK(void) { 

const char c_str[] = "Everything OK?"; 

p = c_str; 

/* ... */ 

p = NULL; 

} 

3.1.4 Noncompliant Code Example (Return Values) 

In this noncompliant code sample, the function init_array() returns a pointer to a character 

array with automatic storage duration, which is accessible to the caller: 

char *init_array(void) { 

char array[10]; 

/* Initialize array */ 

return array; 

} 

Some compilers generate a diagnostic message when a pointer to an object with automatic storage 

duration is returned from a function, as in this example. Programmers should compile code at high 

warning levels and resolve any diagnostic messages. (See MSC00-C. Compile cleanly at high 

warning levels.) 





3.1.5 Compliant Solution (Return Values) 

The solution, in this case, depends on the intent of the programmer. If the intent is to modify the 

value of array and have that modification persist outside the scope of init_array(), the desired 

behavior can be achieved by declaring array elsewhere and passing it as an argument to 

init_array(): 

#include 

void init_array(char *array, size_t len) { 


return; 

} 


char array[10]; 

init_array(array, sizeof(array) / sizeof(array[0])); 

/* ... */ 

return 0; 

} 

3.1.6 Noncompliant Code Example (Output Parameter) 

In this noncompliant code example, the function squirrel_away() stores a pointer to local variable 

local into a location pointed to by function parameter ptr_param. Upon the return of 

squirrel_away(), the pointer ptr_param points to a variable that has an expired lifetime. 

void squirrel_away(char **ptr_param) { 

char local[10]; 


*ptr_param = local; 

} 

void rodent(void) { 

char *ptr; 

squirrel_away(&ptr); 

/* ptr is live but invalid here */ 

} 

3.1.7 Compliant Solution (Output Parameter) 

In this compliant solution, the variable local has static storage duration; consequently, ptr can 

be used to reference the local array within the rodent() function: 

char local[10]; 

void squirrel_away(char **ptr_param) { 


*ptr_param = local; 

} 





void rodent(void) { 

char *ptr; 

squirrel_away(&ptr); 

/* ptr is valid in this scope */ 

} 


Referencing an object outside of its lifetime can result in an attacker being able to execute arbitrary 

code. 


DCL30-C High Probable High P6 L2 


CERT C Secure Coding Standard 


ISO/IEC TR 24772:2013 

ISO/IEC TS 17961 

MISRA C:2012 

MSC00-C. Compile cleanly at high warning 

levels 

EXP54-CPP. Do not access an object outside of 

its lifetime 

Dangling References to Stack Frames [DCM] 

Escaping of the address of an automatic object 

[addrescape] 

Rule 18.6 (required) 


[Coverity 2007] 

[ISO/IEC 9899:2011] 

6.2.4, “Storage Durations of Objects” 




Declarations and Initialization (DCL) - DCL31-C. Declare identifiers before using them 

3.2 DCL31-C. Declare identifiers before using them 

The C11 Standard requires type specifiers and forbids implicit function declarations. The C90 

Standard allows implicit typing of variables and functions. Consequently, some existing legacy 

code uses implicit typing. Some C compilers still support legacy code by allowing implicit typing, 

but it should not be used for new code. Such an implementation may choose to assume an implicit 

declaration and continue translation to support existing programs that used this feature. 

3.2.1 Noncompliant Code Example (Implicit int) 

C no longer allows the absence of type specifiers in a declaration. The C Standard, 6.7.2 [ISO/IEC 

9899:2011], states 

At least one type specifier shall be given in the declaration specifiers in each declaration, 

and in the specifier-qualifier list in each struct declaration and type name. 

This noncompliant code example omits the type specifier: 

extern foo; 

Some C implementations do not issue a diagnostic for the violation of this constraint. These nonconforming 

C translators continue to treat such declarations as implying the type int. 

3.2.2 Compliant Solution (Implicit int) 

This compliant solution explicitly includes a type specifier: 

extern int foo; 

3.2.3 Noncompliant Code Example (Implicit Function Declaration) 

Implicit declaration of functions is not allowed; every function must be explicitly declared before 

it can be called. In C90, if a function is called without an explicit prototype, the compiler provides 

an implicit declaration. 

The C90 Standard [ISO/IEC 9899:1990] includes this requirement: 

If the expression that precedes the parenthesized argument list in a function call consists 

solely of an identifier, and if no declaration is visible for this identifier, the identifier 

is implicitly declared exactly as if, in the innermost block containing the function call, the 

declaration extern int identifier(); appeared. 

If a function declaration is not visible at the point at which a call to the function is made, C90- 

compliant platforms assume an implicit declaration of extern int identifier();. 





This declaration implies that the function may take any number and type of arguments and return 

an int. However, to conform to the current C Standard, programmers must explicitly prototype 

every function before invoking it. An implementation that conforms to the C Standard may or 

may not perform implicit function declarations, but C does require a conforming implementation 

to issue a diagnostic if it encounters an undeclared function being used. 

In this noncompliant code example, if malloc() is not declared, either explicitly or by including 

stdlib.h, a compiler that conforms only to C90 may implicitly declare malloc() as int malloc(). 

If the platform’s size of int is 32 bits, but the size of pointers is 64 bits, the resulting 

pointer would likely be truncated as a result of the implicit declaration of malloc(), returning a 

32-bit integer. 

#include 

/* #include is missing */ 


for (size_t i = 0; i < 100; ++i) { 

/* int malloc() assumed */ 

char *ptr = (char *)malloc(0x10000000); 

*ptr = 'a'; 

} 

return 0; 

} 


When compiled with Microsoft Visual Studio 2013 for a 64-bit platform, this noncompliant code 

example will eventually cause an access violation when dereferencing ptr in the loop. 

3.2.4 Compliant Solution (Implicit Function Declaration) 

This compliant solution declares malloc() by including the appropriate header file: 

#include 


for (size_t i = 0; i < 100; ++i) { 

char *ptr = (char *)malloc(0x10000000); 

*ptr = 'a'; 

} 

return 0; 

} 

For more information on function declarations, see DCL07-C. Include the appropriate type information 

in function declarators. 





3.2.5 Noncompliant Code Example (Implicit Return Type) 

Do not declare a function with an implicit return type. For example, if a function returns a meaningful 

integer value, declare it as returning int. If it returns no meaningful value, declare it as returning 

void. 

#include 

#include 

foo(void) { 

return UINT_MAX; 

} 


long long int c = foo(); 

printf("%lld\n", c); 

return 0; 

} 

Because the compiler assumes that foo() returns a value of type int for this noncompliant code 

example, UINT_MAX is incorrectly converted to −1. 

3.2.6 Compliant Solution (Implicit Return Type) 

This compliant solution explicitly defines the return type of foo() as unsigned int. As a result, 

the function correctly returns UINT_MAX. 

#include 

#include 

unsigned int foo(void) { 

return UINT_MAX; 

} 


long long int c = foo(); 

printf("%lld\n", c); 

return 0; 

} 






Because implicit declarations lead to less stringent type checking, they can introduce unexpected 

and erroneous behavior. Occurrences of an omitted type specifier in existing code are rare, and the 

consequences are generally minor, perhaps resulting in abnormal program termination. 


DCL31-C Low Unlikely Low P3 L3 



ISO/IEC TR 24772:2013 

MISRA C:2012 

DCL07-C. Include the appropriate type information 

in function declarators 

Subprogram Signature Mismatch [OTR] 



[ISO/IEC 9899:1990] 

[ISO/IEC 9899:2011] 

[Jones 2008] 

Subclause 6.7.2, “Type Specifiers” 




Declarations and Initialization (DCL) - DCL36-C. Do not declare an identifier with conflicting linkage classifications 

3.3 DCL36-C. Do not declare an identifier with conflicting linkage 

classifications 

Linkage can make an identifier declared in different scopes or declared multiple times within the 

same scope refer to the same object or function. Identifiers are classified as externally linked, internally 

linked, or not linked. These three kinds of linkage have the following characteristics 

[Kirch-Prinz 2002]: 

• External linkage: An identifier with external linkage represents the same object or function 

throughout the entire program, that is, in all compilation units and libraries belonging to the 

program. The identifier is available to the linker. When a second declaration of the same identifier 

with external linkage occurs, the linker associates the identifier with the same object or 

function. 

• Internal linkage: An identifier with internal linkage represents the same object or function 

within a given translation unit. The linker has no information about identifiers with internal 

linkage. Consequently, these identifiers are internal to the translation unit. 

• No linkage: If an identifier has no linkage, then any further declaration using the identifier 

declares something new, such as a new variable or a new type. 

According to the C Standard, 6.2.2 [ISO/IEC 9899:2011], linkage is determined as follows: 

If the declaration of a file scope identifier for an object or a function contains the storage 

class specifier static, the identifier has internal linkage. 

For an identifier declared with the storage-class specifier extern in a scope in which a 

prior declaration of that identifier is visible, if the prior declaration specifies internal or external 

linkage, the linkage of the identifier at the later declaration is the same as the linkage 

specified at the prior declaration. If no prior declaration is visible, or if the prior declaration 

specifies no linkage, then the identifier has external linkage. 

If the declaration of an identifier for a function has no storage-class specifier, its linkage 

is determined exactly as if it were declared with the storage-class specifier extern. If 

the declaration of an identifier for an object has file scope and no storage-class specifier, 

its linkage is external. 

The following identifiers have no linkage: an identifier declared to be anything other than 

an object or a function; an identifier declared to be a function parameter; a block scope 

identifier for an object declared without the storage-class specifier extern. 

Use of an identifier (within one translation unit) classified as both internally and externally linked 

is undefined behavior. (See also undefined behavior 8.) A translation unit includes the source file 

together with its headers and all source files included via the preprocessing directive #include. 





The following table identifies the linkage assigned to an object that is declared twice in a single 

translation unit. The column designates the first declaration, and the row designates the redeclaration. 


In this noncompliant code example, i2 and i5 are defined as having both internal and external 

linkage. Future use of either identifier results in undefined behavior. 

int i1 = 10; /* Definition, external linkage */ 

static int i2 = 20; /* Definition, internal linkage */ 

extern int i3 = 30; /* Definition, external linkage */ 

int i4; /* Tentative definition, external linkage */ 

static int i5; /* Tentative definition, internal linkage */ 

int i1; /* Valid tentative definition */ 

int i2; /* Undefined, linkage disagreement with previous */ 



int i5; /* Undefined, linkage disagreement with previous */ 


/* ... */ 

return 0; 

} 


Microsoft Visual Studio 2013 issues no warnings about this code, even at the highest diagnostic 

levels. 

The GCC compiler generates a fatal diagnostic for the conflicting definitions of i2 and i5. 






This compliant solution does not include conflicting definitions: 

int i1 = 10; /* Definition, external linkage */ 

static int i2 = 20; /* Definition, internal linkage */ 

extern int i3 = 30; /* Definition, external linkage */ 

int i4; /* Tentative definition, external linkage */ 

static int i5; /* Tentative definition, internal linkage */ 


/* ... */ 

return 0; 

} 


Use of an identifier classified as both internally and externally linked is undefined behavior. 


DCL36-C Medium Probable Medium P8 L2 


MISRA C:2012 




Rule 17.3 (mandatory) 


[Banahan 2003] 

[ISO/IEC 9899:2011] 

[Kirch-Prinz 2002] 

Section 8.2, “Declarations, Definitions and Accessibility” 

6.2.2, “Linkages of Identifiers” 




Declarations and Initialization (DCL) - DCL37-C. Do not declare or define a reserved identifier 

3.4 DCL37-C. Do not declare or define a reserved identifier 

According to the C Standard, 7.1.3 [ISO/IEC 9899:2011], 

All identifiers that begin with an underscore and either an uppercase letter or another underscore 

are always reserved for any use. 

All identifiers that begin with an underscore are always reserved for use as identifiers 

with file scope in both the ordinary and tag name spaces. 

Each macro name in any of the following subclauses (including the future library directions) 

is reserved for use as specified if any of its associated headers is included, unless 

explicitly stated otherwise. 

All identifiers with external linkage (including future library directions) and errno are always 

reserved for use as identifiers with external linkage. 

Each identifier with file scope listed in any of the following subclauses (including the future 

library directions) is reserved for use as a macro name and as an identifier with file 

scope in the same name space if any of its associated headers is included. 

Additionally, subclause 7.31 defines many other reserved identifiers for future library directions. 

No other identifiers are reserved. (The POSIX standard extends the set of identifiers reserved by 

the C Standard to include an open-ended set of its own. See Portable Operating System Interface 

[POSIX®], Base Specifications, Issue 7, Section 2.2, “The Compilation Environment” [IEEE Std 

1003.1-2013].) The behavior of a program that declares or defines an identifier in a context in 

which it is reserved or that defines a reserved identifier as a macro name is undefined. (See undefined 

behavior 106.) 

3.4.1 Noncompliant Code Example (Header Guard) 

A common, but noncompliant, practice is to choose a reserved name for a macro used in a preprocessor 

conditional guarding against multiple inclusions of a header file. (See also PRE06-C. Enclose 

header files in an inclusion guard.) The name may clash with reserved names defined by the 

implementation of the C standard library in its headers or with reserved names implicitly predefined 

by the compiler even when no C standard library header is included. 

#ifndef _MY_HEADER_H_ 

#define _MY_HEADER_H_ 

/* Contents of */ 

#endif /* _MY_HEADER_H_ */ 





3.4.2 Compliant Solution (Header Guard) 

This compliant solution avoids using leading underscores in the name of the header guard: 

#ifndef MY_HEADER_H 

#define MY_HEADER_H 

/* Contents of */ 

#endif /* MY_HEADER_H */ 

3.4.3 Noncompliant Code Example (File Scope Objects) 

In this noncompliant code example, the names of the file scope objects _max_limit and _limit 

both begin with an underscore. Because _max_limit is static, this declaration might seem to be 

impervious to clashes with names defined by the implementation. However, because the header 

is included to define size_t, a potential for a name clash exists. (Note, however, 

that a conforming compiler may implicitly declare reserved names regardless of whether any C 

standard library header is explicitly included.) 

In addition, because _limit has external linkage, it may clash with a symbol of the same name 

defined in the language runtime library even if such a symbol is not declared in any header. Consequently, 

it is not safe to start the name of any file scope identifier with an underscore even if its 

linkage limits its visibility to a single translation unit. 

#include 

static const size_t _max_limit = 1024; 

size_t _limit = 100; 

unsigned int getValue(unsigned int count) { 

return count < _limit ? count : _limit; 

} 

3.4.4 Compliant Solution (File Scope Objects) 

In this compliant solution, names of file scope objects do not begin with an underscore: 

#include 

static const size_t max_limit = 1024; 

size_t limit = 100; 

unsigned int getValue(unsigned int count) { 





} 

return count < limit ? count : limit; 

3.4.5 Noncompliant Code Example (Reserved Macros) 

In this noncompliant code example, because the C standard library header is 

specified to include , the name SIZE_MAX conflicts with a standard macro of the 

same name, which is used to denote the upper limit of size_t. In addition, although the name 

INTFAST16_LIMIT_MAX is not defined by the C standard library, it is a reserved identifier because 

it begins with the INT prefix and ends with the _MAX suffix. (See the C Standard, 7.31.10.) 

#include 

#include 

static const int_fast16_t INTFAST16_LIMIT_MAX = 12000; 

void print_fast16(int_fast16_t val) { 

enum { SIZE_MAX = 80 }; 

char buf[SIZE_MAX]; 

if (INTFAST16_LIMIT_MAX < val) { 

sprintf(buf, "The value is too large"); 

} else { 

snprintf(buf, SIZE_MAX, "The value is %" PRIdFAST16, val); 

} 

} 

3.4.6 Compliant Solution (Reserved Macros) 

This compliant solution avoids redefining reserved names or using reserved prefixes and suffixes: 

#include 

#include 

static const int_fast16_t MY_INTFAST16_UPPER_LIMIT = 12000; 

void print_fast16(int_fast16_t val) { 

enum { BUFSIZE = 80 }; 

char buf[BUFSIZE]; 

if (MY_INTFAST16_UPPER_LIMIT < val) { 

sprintf(buf, "The value is too large"); 

} else { 

snprintf(buf, BUFSIZE, "The value is %" PRIdFAST16, val); 

} 

} 





3.4.7 Noncompliant Code Example (Identifiers with External Linkage) 

In addition to symbols defined as functions in each C standard library header, identifiers with external 

linkage include errno and math_errhandling, among others, regardless of whether any 

of them are masked by a macro of the same name. 

This noncompliant example provides definitions for the C standard library functions malloc() 

and free(). Although this practice is permitted by many traditional implementations of UNIX 

(for example, the Dmalloc library), it is undefined behavior according to the C Standard. Even on 

systems that allow replacing malloc(), doing so without also replacing aligned_alloc(), 

calloc(), and realloc() is likely to cause problems. 

#include 

void *malloc(size_t nbytes) { 

void *ptr; 

/* Allocate storage from own pool and set ptr */ 

return ptr; 

} 

void free(void *ptr) { 

/* Return storage to own pool */ 

} 

3.4.8 Compliant Solution (Identifiers with External Linkage) 

The compliant, portable solution avoids redefining any C standard library identifiers with external 

linkage. In addition, it provides definitions for all memory allocation functions: 

#include 

void *my_malloc(size_t nbytes) { 

void *ptr; 

/* Allocate storage from own pool and set ptr */ 

return ptr; 

} 

void *my_aligned_alloc(size_t alignment, size_t size) { 

void *ptr; 

/* Allocate storage from own pool, align properly, set ptr */ 

return ptr; 

} 

void *my_calloc(size_t nelems, size_t elsize) { 

void *ptr; 

/* Allocate storage from own pool, zero memory, and set ptr */ 

return ptr; 

} 





void *my_realloc(void *ptr, size_t nbytes) { 

/* Reallocate storage from own pool and set ptr */ 

return ptr; 

} 

void my_free(void *ptr) { 

/* Return storage to own pool */ 

} 

3.4.9 Noncompliant Code Example (errno) 

According to the C Standard, 7.5, paragraph 2 [ISO/IEC 9899:2011], the behavior of a program is 

undefined when 

A macro definition of errno is suppressed in order to access an actual object, or the 

program defines an identifier with the name errno. 

See undefined behavior 114. 

The errno identifier expands to a modifiable lvalue that has type int but is not necessarily the 

identifier of an object. It might expand to a modifiable lvalue resulting from a function call, such 

as *errno(). It is unspecified whether errno is a macro or an identifier declared with external 

linkage. If a macro definition is suppressed to access an actual object, or if a program defines an 

identifier with the name errno, the behavior is undefined. 

Legacy code is apt to include an incorrect declaration, such as the following: 

extern int errno; 

3.4.10 Compliant Solution (errno) 

The correct way to declare errno is to include the header : 

#include 

Implementations conforming to C are required to declare errno in , although some 

historic implementations failed to do so. 


DCL37-C-EX1: Provided that a library function can be declared without reference to any type 

defined in a header, it is permissible to declare that function without including its header provided 

that declaration is compatible with the standard declaration. 

/* Not including stdlib.h */ 

void free(void *); 





void func(void *ptr) { 

free(ptr); 

} 

Such code is compliant because the declaration matches what stdlib.h would provide and does 

not redefine the reserved identifier. However, it would not be acceptable to provide a definition 

for the free() function in this example. 

DCL37-C-EX2: For compatibility with other compiler vendors or language standard modes, it is 

acceptable to create a macro identifier that is the same as a reserved identifier so long as the behavior 

is idempotent, as in this example: 

/* Sometimes generated by configuration tools such as autoconf */ 

#define const 

/* Allowed compilers with semantically equivalent extension behavior 

*/ 

#define inline __inline 

DCL37-C-EX3: As a compiler vendor or standard library developer, it is acceptable to use identifiers 

reserved for your implementation. Reserved identifiers may be defined by the compiler, in 

standard library headers or headers included by a standard library header, as in this example declaration 

from the glibc standard C library implementation: 

/* 

The following declarations of reserved identifiers exist in the 

glibc implementation of 

. The original source code may be found at: 

https://sourceware.org/git/?p=glibc.git;a=blob_plain;f=include/stdio.h;hb=HEAD 

*/ 

# define __need_size_t 

# include 

/* Generate a unique file name (and possibly open it). */ 

extern int __path_search (char *__tmpl, size_t __tmpl_len, 

const char *__dir, const char *__pfx, 

int __try_tempdir); 


Using reserved identifiers can lead to incorrect program operation. 











MISRA C:2012 

PRE00-C. Prefer inline or static functions to 


PRE06-C. Enclose header files in an inclusion 

guard 

PRE31-C. Avoid side effects in arguments to 

unsafe macros 

DCL51-CPP. Do not declare or define a reserved 

identifier 

Using identifiers that are reserved for the implementation 

[resident] 




[IEEE Std 1003.1-2013] 

[ISO/IEC 9899:2011] 

Section 2.2, “The Compilation Environment” 

7.1.3, “Reserved Identifiers” 

7.31.10, “Integer Types “ 




Declarations and Initialization (DCL) - DCL38-C. Use the correct syntax when declaring a flexible array member 

3.5 DCL38-C. Use the correct syntax when declaring a flexible array 

member 

Flexible array members are a special type of array in which the last element of a structure with 

more than one named member has an incomplete array type; that is, the size of the array is not 

specified explicitly within the structure. This “struct hack” was widely used in practice and supported 

by a variety of compilers. Consequently, a variety of different syntaxes have been used for 

declaring flexible array members. For conforming C implementations, use the syntax guaranteed 

to be valid by the C Standard. 

Flexible array members are defined in the C Standard, 6.7.2.1, paragraph 18 [ISO/IEC 

9899:2011], as follows: 

As a special case, the last element of a structure with more than one named member 

may have an incomplete array type; this is called a flexible array member. In most situations, 

the flexible array member is ignored. In particular, the size of the structure is as if 

the flexible array member were omitted except that it may have more trailing padding 

than the omission would imply. However, when a . (or ->) operator has a left operand 

that is (a pointer to) a structure with a flexible array member and the right operand 

names that member, it behaves as if that member were replaced with the longest array 

(with the same element type) that would not make the structure larger than the object 

being accessed; the offset of the array shall remain that of the flexible array member, 

even if this would differ from that of the replacement array. If this array would have no 

elements, it behaves as if it had one element but the behavior is undefined if any attempt 

is made to access that element or to generate a pointer one past it. 

Structures with a flexible array member can be used to produce code with defined behavior. However, 

some restrictions apply: 

1. The incomplete array type must be the last element within the structure. 

2. There cannot be an array of structures that contain a flexible array member. 

3. Structures that contain a flexible array member cannot be used as a member of another structure 

except as the last element of that structure. 

4. The structure must contain at least one named member in addition to the flexible array member. 

MEM33-C. Allocate and copy structures containing a flexible array member dynamically describes 

how to allocate and copy structures containing flexible array members. 


Before the introduction of flexible array members in the C Standard, structures with a one-element 

array as the final member were used to achieve similar functionality. This noncompliant 

code example illustrates how struct flexArrayStruct is declared in this case. 





This noncompliant code example attempts to allocate a flexible array-like member with a one-element 

array as the final member. When the structure is instantiated, the size computed for malloc() 

is modified to account for the actual size of the dynamic array. 

#include 

struct flexArrayStruct { 

int num; 

int data[1]; 

}; 

void func(size_t array_size) { 

/* Space is allocated for the struct */ 

struct flexArrayStruct *structP 

= (struct flexArrayStruct *) 

malloc(sizeof(struct flexArrayStruct) 

+ sizeof(int) * (array_size - 1)); 

if (structP == NULL) { 

/* Handle malloc failure */ 

} 

structP->num = array_size; 

} 

/* 

* Access data[] as if it had been allocated 

* as data[array_size]. 

*/ 

for (size_t i = 0; i < array_size; ++i) { 

structP->data[i] = 1; 

} 

This example has undefined behavior when accessing any element other than the first element of 

the data array. (See the C Standard, 6.5.6.) Consequently, the compiler can generate code that 

does not return the expected value when accessing the second element of data. 

This approach may be the only alternative for compilers that do not yet implement the standard C 

syntax. 


This compliant solution uses a flexible array member to achieve a dynamically sized structure: 

#include 

struct flexArrayStruct{ 

int num; 

int data[]; 





}; 

void func(size_t array_size) { 


struct flexArrayStruct *structP 

= (struct flexArrayStruct *) 

malloc(sizeof(struct flexArrayStruct) 

+ sizeof(int) * array_size); 

if (structP == NULL) { 

/* Handle malloc failure */ 

} 

structP->num = array_size; 

} 

/* 

* Access data[] as if it had been allocated 

* as data[array_size]. 

*/ 


structP->data[i] = 1; 

} 

This compliant solution allows the structure to be treated as if its member data[] was declared to 

be data[array_size] in a manner that conforms to the C Standard. 


Failing to use the correct syntax when declaring a flexible array member can result in undefined 

behavior, although the incorrect syntax will work on most implementations. 





MEM33-C. Allocate and copy structures containing 

flexible array members dynamically 


[ISO/IEC 9899:2011] 

[McCluskey 2001] 

6.5.6, “Additive Operators” 

6.7.2.1, “Structure and Union Specifiers” 

“Flexible Array Members and Designators in 

C9X” 




Declarations and Initialization (DCL) - DCL39-C. Avoid information leakage when passing a structure across a trust boundary 

3.6 DCL39-C. Avoid information leakage when passing a structure 

across a trust boundary 

The C Standard, 6.7.2.1, discusses the layout of structure fields. It specifies that non-bit-field 

members are aligned in an implementation-defined manner and that there may be padding within 

or at the end of a structure. Furthermore, initializing the members of the structure does not guarantee 

initialization of the padding bytes. The C Standard, 6.2.6.1, paragraph 6 [ISO/IEC 

9899:2011], states 

When a value is stored in an object of structure or union type, including in a member object, 

the bytes of the object representation that correspond to any padding bytes take unspecified 

values. 

Additionally, the storage units in which a bit-field resides may also have padding bits. For an object 

with automatic storage duration, these padding bits do not take on specific values and can 

contribute to leaking sensitive information. 

When passing a pointer to a structure across a trust boundary to a different trusted domain, the 

programmer must ensure that the padding bytes and bit-field storage unit padding bits of such a 

structure do not contain sensitive information. 


This noncompliant code example runs in kernel space and copies data from struct test to user 

space. However, padding bytes may be used within the structure, for example, to ensure the 

proper alignment of the structure members. These padding bytes may contain sensitive information, 

which may then be leaked when the data is copied to user space. 

#include 

struct test { 

int a; 

char b; 

int c; 

}; 

/* Safely copy bytes to user space */ 

extern int copy_to_user(void *dest, void *src, size_t size); 

void do_stuff(void *usr_buf) { 

struct test arg = {.a = 1, .b = 2, .c = 3}; 

copy_to_user(usr_buf, &arg, sizeof(arg)); 

} 





3.6.2 Noncompliant Code Example (memset()) 

The padding bytes can be explicitly initialized by calling memset(): 

#include 

struct test { 

int a; 

char b; 

int c; 

}; 




struct test arg; 

/* Set all bytes (including padding bytes) to zero */ 

memset(&arg, 0, sizeof(arg)); 

arg.a = 1; 

arg.b = 2; 

arg.c = 3; 

} 


However, compilers are free to implement arg.b = 2 by setting the low byte of a 32-bit register 

to 2, leaving the high bytes unchanged and storing all 32 bits of the register into memory. This 

implementation could leak the high-order bytes resident in the register to a user. 


This compliant solution serializes the structure data before copying it to an untrusted context: 

#include 

#include 

struct test { 

int a; 

char b; 

int c; 

}; 









/* May be larger than strictly needed */ 

unsigned char buf[sizeof(arg)]; 

size_t offset = 0; 

memcpy(buf + offset, &arg.a, sizeof(arg.a)); 

offset += sizeof(arg.a); 

memcpy(buf + offset, &arg.b, sizeof(arg.b)); 

offset += sizeof(arg.b); 

memcpy(buf + offset, &arg.c, sizeof(arg.c)); 

offset += sizeof(arg.c); 

} 

copy_to_user(usr_buf, buf, offset /* size of info copied */); 

This code ensures that no uninitialized padding bytes are copied to unprivileged users. The structure 

copied to user space is now a packed structure and the copy_to_user() function would 

need to unpack it to recreate the original padded structure. 

3.6.4 Compliant Solution (Padding Bytes) 

Padding bytes can be explicitly declared as fields within the structure. This solution is not portable, 

however, because it depends on the implementation and target memory architecture. The following 

solution is specific to the x86-32 architecture: 

#include 

#include 

struct test { 

int a; 

char b; 

char padding_1, padding_2, padding_3; 

int c; 

}; 




/* Ensure c is the next byte after the last padding byte */ 

static_assert(offsetof(struct test, c) == 

offsetof(struct test, padding_3) + 1, 

"Structure contains intermediate padding"); 

/* Ensure there is no trailing padding */ 

static_assert(sizeof(struct test) == 

offsetof(struct test, c) + sizeof(int), 

"Structure contains trailing padding"); 


arg.padding_1 = 0; 





} 




The C Standard static_assert() macro accepts a constant expression and an error message. 

The expression is evaluated at compile time and, if false, the compilation is terminated and the error 

message is output. (See DCL03-C. Use a static assertion to test the value of a constant expression 

for more details.) The explicit insertion of the padding bytes into the struct should ensure 

that no additional padding bytes are added by the compiler and consequently both static assertions 

should be true. However, it is necessary to validate these assumptions to ensure that the solution is 

correct for a particular implementation. 

3.6.5 Compliant Solution (Structure Packing—GCC) 

GCC allows specifying declaration attributes using the keyword __attribute__((__packed__)). 

When this attribute is present, the compiler will not add padding bytes 

for memory alignment unless otherwise required by the _Alignas alignment specifier, and it will 

attempt to place fields at adjacent memory offsets when possible. 

#include 

struct test { 

int a; 

char b; 

int c; 

} __attribute__((__packed__)); 






} 

3.6.6 Compliant Solution (Structure Packing—Microsoft Visual Studio) 

Microsoft Visual Studio supports #pragma pack() to suppress padding bytes [MSDN]. The 

compiler adds padding bytes for memory alignment, depending on the current packing mode, but 

still honors the alignment specified by __declspec(align()). In this compliant solution, the 

packing mode is set to 1 in an attempt to ensure all fields are given adjacent offsets: 

#include 

#pragma pack(push, 1) /* 1 byte */ 





struct test { 

int a; 

char b; 

int c; 

}; 

#pragma pack(pop) 




struct test arg = {1, 2, 3}; 


} 

The pack pragma takes effect at the first struct declaration after the pragma is seen. 


This noncompliant code example also runs in kernel space and copies data from struct test to 

user space. However, padding bits will be used within the structure due to the bit-field member 

lengths not adding up to the number of bits in an unsigned object. Further, there is an unnamed 

bit-field that causes no further bit-fields to be packed into the same storage unit. These padding 

bits may contain sensitive information, which may then be leaked when the data is copied to user 

space. For instance, the uninitialized bits may contain a sensitive kernel space pointer value that 

can be trivially reconstructed by an attacker in user space. 

#include 

struct test { 

unsigned a : 1; 

unsigned : 0; 

unsigned b : 4; 

}; 




struct test arg = { .a = 1, .b = 10 }; 


} 

However, compilers are free to implement the initialization of arg.a and arg.b by setting the 

low byte of a 32-bit register to the value specified, leaving the high bytes unchanged and storing 

all 32 bits of the register into memory. This implementation could leak the high-order bytes resident 

in the register to a user. 






Padding bits can be explicitly declared, allowing the programmer to specify the value of those 

bits. When explicitly declaring all of the padding bits, any unnamed bit-fields of length 0 must be 

removed from the structure because the explicit padding bits ensure that no further bit-fields will 

be packed into the same storage unit. 

#include 

#include 

#include 

struct test { 

unsigned a : 1; 

unsigned padding1 : sizeof(unsigned) * CHAR_BIT - 1; 

unsigned b : 4; 

unsigned padding2 : sizeof(unsigned) * CHAR_BIT - 4; 

}; 

/* Ensure that we have added the correct number of padding bits. */ 

static_assert(sizeof(struct test) == sizeof(unsigned) * 2, 

"Incorrect number of padding bits for type: unsigned"); 




struct test arg = { .a = 1, .padding1 = 0, .b = 10, .padding2 = 0 

}; 


} 

This solution is not portable, however, because it depends on the implementation and target 

memory architecture. The explicit insertion of padding bits into the struct should ensure that no 

additional padding bits are added by the compiler. However, it is still necessary to validate these 

assumptions to ensure that the solution is correct for a particular implementation. For instance, the 

DEC Alpha is an example of a 64-bit architecture with 32-bit integers that allocates 64 bits to a 

storage unit. 

In addition, this solution assumes that there are no integer padding bits in an unsigned int. 

The portable version of the width calculation from INT35-C. Use correct integer precisions cannot 

be used because the bit-field width must be an integer constant expression. 

From this situation, it can be seen that special care must be taken because no solution to the bitfield 

padding issue will be 100% portable. 






Padding units might contain sensitive data because the C Standard allows any padding to take unspecified 

values. A pointer to such a structure could be passed to other functions, causing information 

leakage. 


DCL39-C Low Unlikely High P1 L3 

3.6.9.1 Related Vulnerabilities 

Numerous vulnerabilities in the Linux Kernel have resulted from violations of this rule. CVE- 

2010-4083 describes a vulnerability in which the semctl() system call allows unprivileged users 

to read uninitialized kernel stack memory because various fields of a semid_ds struct declared 

on the stack are not altered or zeroed before being copied back to the user. 

CVE-2010-3881 describes a vulnerability in which structure padding and reserved fields in certain 

data structures in QEMU-KVM were not initialized properly before being copied to user space. 

A privileged host user with access to /dev/kvm could use this flaw to leak kernel stack memory 

to user space. CVE-2010-3477 describes a kernel information leak in act_police where incorrectly 

initialized structures in the traffic-control dump code may allow the disclosure of kernel 

memory to user space applications. 



DCL03-C. Use a static assertion to test the 

value of a constant expression 


[ISO/IEC 9899:2011] 

[Graff 2003] 

[Sun 1993] 

6.2.6.1, “General” 





Declarations and Initialization (DCL) - DCL40-C. Do not create incompatible declarations of the same function or object 

3.7 DCL40-C. Do not create incompatible declarations of the same 

function or object 

Two or more incompatible declarations of the same function or object must not appear in the 

same program because they result in undefined behavior. The C Standard, 6.2.7, mentions that 

two types may be distinct yet compatible and addresses precisely when two distinct types are 

compatible. 

The C Standard identifies four situations in which undefined behavior (UB) may arise as a result 

of incompatible declarations of the same function or object: 

UB Description Code 

15 Two declarations of the same 

object or function specify types 

that are not compatible (6.2.7). 

31 Two identifiers differ only in nonsignificant 

characters (6.4.2.1). 

37 An object has its stored value 

accessed other than by an lvalue 

of an allowable type (6.5). 

41 A function is defined with a type 

that is not compatible with the 

type (of the expression) pointed 

to by the expression that denotes 

the called function 

(6.5.2.2). 

All noncompliant code in this 

guideline 

Excessively Long Identifiers 

Incompatible Object Declarations 

Incompatible Array Declarations 

Incompatible Function Declarations 

Excessively Long Identifiers 

Although the effect of two incompatible declarations simply appearing in the same program may 

be benign on most implementations, the effects of invoking a function through an expression 

whose type is incompatible with the function definition are typically catastrophic. Similarly, the 

effects of accessing an object using an lvalue of a type that is incompatible with the object definition 

may range from unintended information exposure to memory overwrite to a hardware trap. 

3.7.1 Noncompliant Code Example (Incompatible Object Declarations) 

In this noncompliant code example, the variable i is declared to have type int in file a.c but defined 

to be of type short in file b.c. The declarations are incompatible, resulting in undefined 

behavior 15. Furthermore, accessing the object using an lvalue of an incompatible type, as shown 

in function f(), is undefined behavior 37 with possible observable results ranging from unintended 

information exposure to memory overwrite to a hardware trap. 

/* In a.c */ 

extern int i; /* UB 15 */ 

int f(void) { 

return ++i; /* UB 37 */ 

} 





/* In b.c */ 

short i; /* UB 15 */ 

3.7.2 Compliant Solution (Incompatible Object Declarations) 

This compliant solution has compatible declarations of the variable i: 

/* In a.c */ 

extern int i; 

int f(void) { 

return ++i; 

} 

/* In b.c */ 

int i; 

3.7.3 Noncompliant Code Example (Incompatible Array Declarations) 

In this noncompliant code example, the variable a is declared to have a pointer type in file a.c 

but defined to have an array type in file b.c. The two declarations are incompatible, resulting in 

undefined behavior 15. As before, accessing the object in function f() is undefined behavior 37 

with the typical effect of triggering a hardware trap. 

/* In a.c */ 

extern int *a; /* UB 15 */ 

int f(unsigned int i, int x) { 

int tmp = a[i]; /* UB 37: read access */ 

a[i] = x; /* UB 37: write access */ 

return tmp; 

} 

/* In b.c */ 

int a[] = { 1, 2, 3, 4 }; /* UB 15 */ 

3.7.4 Compliant Solution (Incompatible Array Declarations) 

This compliant solution declares a as an array in a.c and b.c: 

/* In a.c */ 

extern int a[]; 

int f(unsigned int i, int x) { 

int tmp = a[i]; 

a[i] = x; 

return tmp; 

} 





/* In b.c */ 

int a[] = { 1, 2, 3, 4 }; 

3.7.5 Noncompliant Code Example (Incompatible Function Declarations) 

In this noncompliant code example, the function f() is declared in file a.c with one prototype 

but defined in file b.c with another. The two prototypes are incompatible, resulting in undefined 

behavior 15. Furthermore, invoking the function is undefined behavior 41 and typically has catastrophic 

consequences. 

/* In a.c */ 

extern int f(int a); /* UB 15 */ 

int g(int a) { 

return f(a); /* UB 41 */ 

} 

/* In b.c */ 

long f(long a) { /* UB 15 */ 

return a * 2; 

} 

3.7.6 Compliant Solution (Incompatible Function Declarations) 

This compliant solution has compatible prototypes for the function f(): 

/* In a.c */ 

extern int f(int a); 

int g(int a) { 

return f(a); 

} 

/* In b.c */ 

int f(int a) { 

return a * 2; 

} 

3.7.7 Noncompliant Code Example (Incompatible Variadic Function 

Declarations) 

In this noncompliant code example, the function buginf() is defined to take a variable number 

of arguments and expects them all to be signed integers with a sentinel value of -1: 

/* In a.c */ 

void buginf(const char *fmt, ...) { 

/* ... */ 





} 

/* In b.c */ 

void buginf(); 

Although this code appears to be well defined because of the prototype-less declaration of 

buginf(), it exhibits undefined behavior in accordance with the C Standard, 6.7.6.3, paragraph 

15 [ISO/IEC 9899:2011], 

For two function types to be compatible, both shall specify compatible return types. 

Moreover, the parameter type lists, if both are present, shall agree in the number of parameters 

and in use of the ellipsis terminator; corresponding parameters shall have compatible 

types. If one type has a parameter type list and the other type is specified by a 

function declarator that is not part of a function definition and that contains an empty 

identifier list, the parameter list shall not have an ellipsis terminator and the type of each 

parameter shall be compatible with the type that results from the application of the default 

argument promotions. 

3.7.8 Compliant Solution (Incompatible Variadic Function Declarations) 

In this compliant solution, the prototype for the function buginf() is included in the scope in the 

source file where it will be used: 

/* In a.c */ 

void buginf(const char *fmt, ...) { 

/* ... */ 

} 

/* In b.c */ 

void buginf(const char *fmt, ...); 

3.7.9 Noncompliant Code Example (Excessively Long Identifiers) 

In this noncompliant code example, the length of the identifier declaring the function pointer 

bash_groupname_completion_function() in the file bashline.h exceeds by 3 the minimum 

implementation limit of 31 significant initial characters in an external identifier. This introduces 

the possibility of colliding with the bash_groupname_completion_funct integer variable 

defined in file b.c, which is exactly 31 characters long. On an implementation that exactly 

meets this limit, this is undefined behavior 31. It results in two incompatible declarations of the 

same function. (See undefined behavior 15.) In addition, invoking the function leads to undefined 

behavior 41 with typically catastrophic effects. 

/* In bashline.h */ 

/* UB 15, UB 31 */ 

extern char * bash_groupname_completion_function(const char *, 

int); 





/* In a.c */ 

#include "bashline.h" 

void f(const char *s, int i) { 

bash_groupname_completion_function(s, i); /* UB 41 */ 

} 

/* In b.c */ 

int bash_groupname_completion_funct; /* UB 15, UB 31 */ 

NOTE: The identifier bash_groupname_completion_function referenced here was taken 

from GNU Bash, version 3.2. 

3.7.10 Compliant Solution (Excessively Long Identifiers) 

In this compliant solution, the length of the identifier declaring the function pointer 

bash_groupname_completion() in bashline.h is less than 32 characters. Consequently, it 

cannot clash with bash_groupname_completion_funct on any compliant platform. 

/* In bashline.h */ 

extern char * bash_groupname_completion(const char *, int); 

/* In a.c */ 

#include "bashline.h" 

void f(const char *s, int i) { 

bash_groupname_completion(s, i); 

} 

/* In b.c */ 

int bash_groupname_completion_funct; 



DCL40-C Low Unlikely Medium P2 L3 



MISRA C:2012 

Declaring the same function or object in incompatible 

ways [funcdecl] 







[Hatton 1995] Section 2.8.3 

[ISO/IEC 9899:2011] 

6.7.6.3, “Function Declarators (including Prototypes)” 

J.2, “Undefined Behavior” 




Declarations and Initialization (DCL) - DCL41-C. Do not declare variables inside a switch statement before the first case 

label 

3.8 DCL41-C. Do not declare variables inside a switch statement before 

the first case label 

According to the C Standard, 6.8.4.2, paragraph 4 [ISO/IEC 9899:2011], 

A switch statement causes control to jump to, into, or past the statement that is the 

switch body, depending on the value of a controlling expression, and on the presence of 

a default label and the values of any case labels on or in the switch body. 

If a programmer declares variables, initializes them before the first case statement, and then tries 

to use them inside any of the case statements, those variables will have scope inside the switch 

block but will not be initialized and will consequently contain indeterminate values. 


This noncompliant code example declares variables and contains executable statements before the 

first case label within the switch statement: 

#include 

extern void f(int i); 

void func(int expr) { 

switch (expr) { 

int i = 4; 

f(i); 

case 0: 

i = 17; 

/* Falls through into default code */ 

default: 

printf("%d\n" , i); 

} 

} 


When the preceding example is executed on GCC 4.8.1, the variable i is instantiated with automatic 

storage duration within the block, but it is not initialized. Consequently, if the controlling 

expression expr has a nonzero value, the call to printf() will access an indeterminate value of 

i. Similarly, the call to f() is not executed. 

Value of expr 

0 17 

Nonzero 

Output 

Indeterminate 




Declarations and Initialization (DCL) - DCL41-C. Do not declare variables inside a switch statement before the first case 

label 


In this compliant solution, the statements before the first case label occur before the switch statement: 

#include 

extern void f(int i); 

int func(int expr) { 

/* 

* Move the code outside the switch block; now the statements 

* will get executed. 

*/ 

int i = 4; 

f(i); 

} 

switch (expr) { 

case 0: 

i = 17; 

/* Falls through into default code */ 

default: 

printf(" 

} 

return 0; 

œ%d \n" 

, i); 


Using test conditions or initializing variables before the first case statement in a switch block 

can result in unexpected behavior and undefined behavior. 


DCL41-C Medium Unlikely Medium P4 L3 


MISRA C:2012 



[ISO/IEC 9899:2011] 

6.8.4.2, “The switch Statement” 




Expressions (EXP) - EXP30-C. Do not depend on the order of evaluation for side effects 

4 Expressions (EXP) 

4.1 EXP30-C. Do not depend on the order of evaluation for side effects 

Evaluation of an expression may produce side effects. At specific points during execution, known 

as sequence points, all side effects of previous evaluations are complete, and no side effects of 

subsequent evaluations have yet taken place. Do not depend on the order of evaluation for side effects 

unless there is an intervening sequence point. 

The C Standard, 6.5, paragraph 2 [ISO/IEC 9899:2011], states 

If a side effect on a scalar object is unsequenced relative to either a different side effect 

on the same scalar object or a value computation using the value of the same scalar object, 

the behavior is undefined. If there are multiple allowable orderings of the subexpressions 

of an expression, the behavior is undefined if such an unsequenced side effect 

occurs in any of the orderings. 

This requirement must be met for each allowable ordering of the subexpressions of a full expression; 

otherwise, the behavior is undefined. (See undefined behavior 35.) 

The following sequence points are defined in the C Standard, Annex C [ISO/IEC 9899:2011]: 

• Between the evaluations of the function designator and actual arguments in a function call 

and the actual call 

• Between the evaluations of the first and second operands of the following operators: 

− Logical AND: && 

− Logical OR: || 

− Comma: , 

• Between the evaluations of the first operand of the conditional ?: operator and whichever of 

the second and third operands is evaluated 

• The end of a full declarator 

• Between the evaluation of a full expression and the next full expression to be evaluated; the 

following are full expressions: 

− An initializer that is not part of a compound literal 

− The expression in an expression statement 

− The controlling expression of a selection statement (if or switch) 

− The controlling expression of a while or do statement 

− Each of the (optional) expressions of a for statement 

− The (optional) expression in a return statement 

• Immediately before a library function returns 





• After the actions associated with each formatted input/output function conversion specifier 

• Immediately before and immediately after each call to a comparison function, and also between 

any call to a comparison function and any movement of the objects passed as arguments 

to that call 

This rule means that statements such as 

i = i + 1; 

a[i] = i; 

have defined behavior, and statements such as the following do not: 

/* i is modified twice between sequence points */ 

i = ++i + 1; 

/* i is read other than to determine the value to be stored */ 

a[i++] = i; 

Not all instances of a comma in C code denote a usage of the comma operator. For example, the 

comma between arguments in a function call is not a sequence point. However, according to the C 

Standard, 6.5.2.2, paragraph 10 [ISO/IEC 9899:2011] 

Every evaluation in the calling function (including other function calls) that is not otherwise 

specifically sequenced before or after the execution of the body of the called function 

is indeterminately sequenced with respect to the execution of the called function. 

This rule means that the order of evaluation for function call arguments is unspecified and can 

happen in any order. 


Programs cannot safely rely on the order of evaluation of operands between sequence points. In 

this noncompliant code example, i is evaluated twice without an intervening sequence point so 

the behavior of the expression is undefined: 

#include 

void func(int i, int *b) { 

int a = i + b[++i]; 

printf("%d, %d", a, i); 

} 






These examples are independent of the order of evaluation of the operands and can be interpreted 

in only one way: 

#include 


int a; 

++i; 

a = i + b[i]; 


} 

Alternatively: 

#include 


int a = i + b[i + 1]; 

++i; 


} 


The call to func() in this noncompliant code example has undefined behavior because there is 

no sequence point between the argument expressions: 

extern void func(int i, int j); 

void f(int i) { 

func(i++, i); 

} 

The first (left) argument expression reads the value of i (to determine the value to be stored) and 

then modifies i. The second (right) argument expression reads the value of i between the same 

pair of sequence points as the first argument, but not to determine the value to be stored in i. This 

additional attempt to read the value of i has undefined behavior. 


This compliant solution is appropriate when the programmer intends for both arguments to 

func() to be equivalent: 







i++; 

func(i, i); 

} 

This compliant solution is appropriate when the programmer intends the second argument to be 1 

greater than the first: 



int j = i++; 

func(j, i); 

} 


The order of evaluation for function arguments is unspecified. This noncompliant code example 

exhibits unspecified behavior but not undefined behavior: 

extern void c(int i, int j); 

int glob; 

int a(void) { 

return glob + 10; 

} 

int b(void) { 

glob = 42; 

return glob; 

} 


c(a(), b()); 

} 

It is unspecified what order a() and b() are called in; the only guarantee is that both a() and 

b() will be called before c() is called. If a() or b() rely on shared state when calculating their 

return value, as they do in this example, the resulting arguments passed to c() may differ between 

compilers or architectures. 






In this compliant solution, the order of evaluation for a() and b() is fixed, and so no unspecified 

behavior occurs: 

extern void c(int i, int j); 

int glob; 

int a(void) { 

return glob + 10; 

} 

int b(void) { 

glob = 42; 

return glob; 

} 


int a_val, b_val; 

a_val = a(); 

b_val = b(); 

} 

c(a_val, b_val); 


Attempting to modify an object multiple times between sequence points may cause that object to 

take on an unexpected value, which can lead to unexpected program behavior. 


EXP30-C Medium Probable Medium P8 L2 



EXP50-CPP. Do not depend on the order of 

evaluation for side effects 

CERT Oracle Secure Coding Standard for Java EXP05-J. Do not follow a write by a subsequent 

write or read of the same object within an 

expression 

ISO/IEC TR 24772:2013 

Operator Precedence/Order of Evaluation 

[JCW] 

Side-effects and Order of Evaluation [SAM] 

MISRA C:2012 







[ISO/IEC 9899:2011] 

[Saks 2007] 

6.5, “Expressions” 

6.5.2.2, “Function Calls” 

Annex C, “Sequence Points” 

[Summit 2005] Questions 3.1, 3.2, 3.3, 3.3b, 3.7, 3.8, 3.9, 

3.10a, 3.10b, and 3.11 




Expressions (EXP) - EXP32-C. Do not access a volatile object through a nonvolatile reference 

4.2 EXP32-C. Do not access a volatile object through a nonvolatile 

reference 

An object that has volatile-qualified type may be modified in ways unknown to the implementation 

or have other unknown side effects. Referencing a volatile object by using a non-volatile 

lvalue is undefined behavior. The C Standard, 6.7.3 [ISO/IEC 9899:2011], states 

If an attempt is made to refer to an object defined with a volatile-qualified type through 

use of an lvalue with non-volatile-qualified type, the behavior is undefined. 



In this noncompliant code example, a volatile object is accessed through a non-volatile-qualified 

reference, resulting in undefined behavior: 

#include 


static volatile int **ipp; 

static int *ip; 

static volatile int i = 0; 

printf("i = %d.\n", i); 

ipp = &ip; /* May produce a warning diagnostic */ 

ipp = (int**) &ip; /* Constraint violation; may produce a warning 

diagnostic */ 

*ipp = &i; /* Valid */ 

if (*ip != 0) { /* Valid */ 

/* ... */ 

} 

} 

The assignment ipp = &ip is not safe because it allows the valid code that follows to reference 

the value of the volatile object i through the non-volatile-qualified reference ip. In this example, 

the compiler may optimize out the entire if block because *ip != 0 must be false if the object 

to which ip points is not volatile. 


This example compiles without warning on Microsoft Visual Studio 2013 when compiled in C 

mode (/TC) but causes errors when compiled in C++ mode (/TP). 

GCC 4.8.1 generates a warning but compiles successfully. 




Expressions (EXP) - EXP32-C. Do not access a volatile object through a nonvolatile reference 


In this compliant solution, ip is declared volatile: 

#include 


static volatile int **ipp; 

static volatile int *ip; 

static volatile int i = 0; 

} 

printf("i = %d.\n", i); 

ipp = &ip; 

*ipp = &i; 

if (*ip != 0) { 

/* ... */ 

} 


Accessing an object with a volatile-qualified type through a reference with a non-volatile-qualified 

type is undefined behavior. 


EXP32-C Low Likely Medium P6 L2 


ISO/IEC TR 24772:2013 

MISRA C:2012 


Pointer Casting and Pointer Type Changes 

[HFC] 

Type System [IHN] 


EXP55-CPP. Do not access a cv-qualified object 

through a cv-unqualified type 


[ISO/IEC 9899:2011] 

6.7.3, “Type Qualifiers” 




Expressions (EXP) - EXP33-C. Do not read uninitialized memory 

4.3 EXP33-C. Do not read uninitialized memory 

Local, automatic variables assume unexpected values if they are read before they are initialized. 

The C Standard, 6.7.9, paragraph 10, specifies [ISO/IEC 9899:2011] 

If an object that has automatic storage duration is not initialized explicitly, its value is indeterminate. 


When local, automatic variables are stored on the program stack, for example, their values default 

to whichever values are currently stored in stack memory. 

Additionally, some dynamic memory allocation functions do not initialize the contents of the 

memory they allocate. 

Function 

aligned_alloc() 

calloc() 

malloc() 

realloc() 

Initialization 

Does not perform initialization 

Zero-initializes allocated memory 

Does not perform initialization 

Copies contents from original pointer; may not initialize 

all memory 

Uninitialized automatic variables or dynamically allocated memory has indeterminate values, 

which, for objects of some types, can be a trap representation. Reading such trap representations 

is undefined behavior (see undefined behavior 10 and undefined behavior 12); it can cause a program 

to behave in an unexpected manner and provide an avenue for attack. In many cases, compilers 

issue a warning diagnostic message when reading uninitialized variables. (See MSC00-C. 

Compile cleanly at high warning levels for more information.) 

4.3.1 Noncompliant Code Example (Return-by-Reference) 

In this noncompliant code example, the set_flag() function is intended to set the parameter, 

sign_flag, to the sign of number. However, the programmer neglected to account for the case 

where number is equal to 0. Because the local variable sign is uninitialized when calling 

set_flag() and is never written to by set_flag(), the comparison operation exhibits undefined 

behavior when reading sign. 

void set_flag(int number, int *sign_flag) { 

if (NULL == sign_flag) { 

return; 

} 

if (number > 0) { 

*sign_flag = 1; 

} else if (number < 0) { 





} 

} 

*sign_flag = -1; 

int is_negative(int number) { 

int sign; 

set_flag(number, &sign); 

return sign < 0; 

} 

Some compilers assume that when the address of an uninitialized variable is passed to a function, 

the variable is initialized within that function. Because compilers frequently fail to diagnose any 

resulting failure to initialize the variable, the programmer must apply additional scrutiny to ensure 

the correctness of the code. 

This defect results from a failure to consider all possible data states. (See MSC01-C. Strive for 

logical completeness for more information.) 

4.3.2 Compliant Solution (Return-by-Reference) 

This compliant solution trivially repairs the problem by accounting for the possibility that number 

can be equal to 0. 

Although compilers and static analysis tools often detect uses of uninitialized variables when they 

have access to the source code, diagnosing the problem is difficult or impossible when either the 

initialization or the use takes place in object code for which the source code is inaccessible. Unless 

doing so is prohibitive for performance reasons, an additional defense-in-depth practice worth 

considering is to initialize local variables immediately after declaration. 

void set_flag(int number, int *sign_flag) { 

if (NULL == sign_flag) { 

return; 

} 

} 

/* Account for number being 0 */ 

if (number >= 0) { 

*sign_flag = 1; 

} else { 

*sign_flag = -1; 

} 

int is_negative(int number) { 

int sign = 0; /* Initialize for defense-in-depth */ 

set_flag(number, &sign); 

return sign < 0; 

} 





4.3.3 Noncompliant Code Example (Uninitialized Local) 

In this noncompliant code example, the programmer mistakenly fails to set the local variable error_log 

to the msg argument in the report_error() function [Mercy 2006]. Because error_log 

has not been initialized, an indeterminate value is read. The sprintf() call copies data 

from the arbitrary location pointed to by the indeterminate error_log variable until a null byte is 

reached, which can result in a buffer overflow. 

#include 

/* Get username and password from user, return -1 on error */ 

extern int do_auth(void); 

enum { BUFFERSIZE = 24 }; 

void report_error(const char *msg) { 

const char *error_log; 

char buffer[BUFFERSIZE]; 

} 

sprintf(buffer, "Error: %s", error_log); 

printf("%s\n", buffer); 


if (do_auth() == -1) { 

report_error("Unable to login"); 

} 

return 0; 

} 

4.3.4 Noncompliant Code Example (Uninitialized Local) 

In this noncompliant code example, the report_error() function has been modified so that 

error_log is properly initialized: 

#include 



const char *error_log = msg; 


} 

sprintf(buffer, "Error: %s", error_log); 


This example remains problematic because a buffer overflow will occur if the null-terminated 

byte string referenced by msg is greater than 17 characters, including the null terminator. (See 

STR31-C. Guarantee that storage for strings has sufficient space for character data and the null 

terminator for more information.) 





4.3.5 Compliant Solution (Uninitialized Local) 

In this compliant solution, the buffer overflow is eliminated by calling the snprintf() function: 

#include 




} 

if (0 < snprintf(buffer, BUFFERSIZE, "Error: %s", msg)) 


else 

puts("Unknown error"); 

4.3.6 Compliant Solution (Uninitialized Local) 

A less error-prone compliant solution is to simply print the error message directly instead of using 

an intermediate buffer: 

#include 


printf("Error: %s\n", msg); 

} 

4.3.7 Noncompliant Code Example (mbstate_t) 

In this noncompliant code example, the function mbrlen() is passed the address of an automatic 

mbstate_t object that has not been properly initialized. This is undefined behavior 200 because 

mbrlen() dereferences and reads its third argument. 

#include 

#include 

void func(const char *mbs) { 

size_t len; 

mbstate_t state; 

} 

len = mbrlen(mbs, strlen(mbs), &state); 

4.3.8 Compliant Solution (mbstate_t) 

Before being passed to a multibyte conversion function, an mbstate_t object must be either initialized 

to the initial conversion state or set to a value that corresponds to the most recent shift 





state by a prior call to a multibyte conversion function. This compliant solution sets the 

mbstate_t object to the initial conversion state by setting it to all zeros: 

#include 

#include 

void func(const char *mbs) { 

size_t len; 

mbstate_t state; 

} 

memset(&state, 0, sizeof(state)); 

len = mbrlen(mbs, strlen(mbs), &state); 

4.3.9 Noncompliant Code Example (POSIX, Entropy) 

In this noncompliant code example described in “More Randomness or Less” [Wang 2012], the 

process ID, time of day, and uninitialized memory junk is used to seed a random number generator. 

This behavior is characteristic of some distributions derived from Debian Linux that use uninitialized 

memory as a source of entropy because the value stored in junk is indeterminate. However, 

because accessing an indeterminate value is undefined behavior, compilers may optimize 

out the uninitialized variable access completely, leaving only the time and process ID and resulting 

in a loss of desired entropy. 

#include 

#include 

#include 

#include 


struct timeval tv; 

unsigned long junk; 

} 

gettimeofday(&tv, NULL); 

srandom((getpid()


#include 

#include 


double cpu_time; 

struct timeval tv; 

} 

cpu_time = ((double) clock()) / CLOCKS_PER_SEC; 

gettimeofday(&tv, NULL); 

srandom((getpid()


if (0 == array) { 

/* Handle error */ 

} 

for (size_t i = 0; i < OLD_SIZE; ++i) { 

array[i] = i; 

} 

array = resize_array(array, NEW_SIZE); 

if (0 == array) { 


} 

} 

for (size_t i = 0; i < NEW_SIZE; ++i) { 

printf("%d ", array[i]); 

} 

4.3.12 Compliant Solution (realloc()) 

In this compliant solution, the resize_array() helper function takes a second parameter for the 

old size of the array so that it can initialize any newly allocated elements: 

#include 

#include 

#include 

enum { OLD_SIZE = 10, NEW_SIZE = 20 }; 

int *resize_array(int *array, size_t old_count, size_t new_count) { 

if (0 == new_count) { 

return 0; 

} 

int *ret = (int *)realloc(array, new_count * sizeof(int)); 

if (!ret) { 

free(array); 

return 0; 

} 

if (new_count > old_count) { 

memset(ret + old_count, 0, (new_count – old_count) * 

sizeof(int)); 

} 

} 

return ret; 






int *array = (int *)malloc(OLD_SIZE * sizeof(int)); 

if (0 == array) { 


} 

for (size_t i = 0; i < OLD_SIZE; ++i) { 

array[i] = i; 

} 

array = resize_array(array, OLD_SIZE, NEW_SIZE); 

if (0 == array) { 


} 

} 

for (size_t i = 0; i < NEW_SIZE; ++i) { 

printf("%d ", array[i]); 

} 


EXP33-C-EX1: Reading uninitialized memory by an lvalue of type unsigned char does not 

trigger undefined behavior. The unsigned char type is defined to not have a trap representation 

(see the C Standard, 6.2.6.1, paragraph 3), which allows for moving bytes without knowing if they 

are initialized. However, on some architectures, such as the Intel Itanium, registers have a bit to 

indicate whether or not they have been initialized. The C Standard, 6.3.2.1, paragraph 2, allows 

such implementations to cause a trap for an object that never had its address taken and is stored in 

a register if such an object is referred to in any way. 


Reading uninitialized variables is undefined behavior and can result in unexpected program behavior. 

In some cases, these security flaws may allow the execution of arbitrary code. 

Reading uninitialized variables for creating entropy is problematic because these memory accesses 

can be removed by compiler optimization. VU#925211 is an example of a vulnerability 

caused by this coding error. 


EXP33-C High Probable Medium P12 L1 


CVE-2009-1888 results from a violation of this rule. Some versions of SAMBA (up to 3.3.5) call 

a function that takes in two potentially uninitialized variables involving access rights. An attacker 

can exploit these coding errors to bypass the access control list and gain access to protected files 

[xorl 2009]. 








ISO/IEC TR 24772:2013 


MITRE CWE 


levels 

MSC01-C. Strive for logical completeness 

EXP53-CPP. Do not read uninitialized memory 

Initialization of Variables [LAV] 

Referencing uninitialized memory [uninitref] 

CWE-119, Improper Restriction of Operations 

within the Bounds of a Memory Buffer 

CWE-123, Write-what-where Condition 

CWE-125, Out-of-bounds Read 

CWE-665, Improper Initialization 


[Flake 2006] 

[ISO/IEC 9899:2011] 

[Mercy 2006] 

[VU#925211] 

[Wang 2012] 

[xorl 2009] 

Subclause 6.7.9, “Initialization” 

Subclause 6.2.6.1, “General” 

Subclause 6.3.2.1, “Lvalues, Arrays, and Function 

Designators” 

“More Randomness or Less” 

“CVE-2009-1888: SAMBA ACLs Uninitialized 

Memory Read” 




Expressions (EXP) - EXP34-C. Do not dereference null pointers 

4.4 EXP34-C. Do not dereference null pointers 

Dereferencing a null pointer is undefined behavior. 

On many platforms, dereferencing a null pointer results in abnormal program termination, but this 

is not required by the standard. See “Clever Attack Exploits Fully-Patched Linux Kernel” 

[Goodin 2009] for an example of a code execution exploit that resulted from a null pointer dereference. 


This noncompliant code example is derived from a real-world example taken from a vulnerable 

version of the libpng library as deployed on a popular ARM-based cell phone [Jack 2007]. The 

libpng library allows applications to read, create, and manipulate PNG (Portable Network 

Graphics) raster image files. The libpng library implements its own wrapper to malloc() that 

returns a null pointer on error or on being passed a 0-byte-length argument. 

This code also violates ERR33-C. Detect and handle standard library errors. 

#include /* From libpng */ 

#include 

void func(png_structp png_ptr, int length, const void *user_data) { 

png_charp chunkdata; 

chunkdata = (png_charp)png_malloc(png_ptr, length + 1); 

/* ... */ 

memcpy(chunkdata, user_data, length); 

/* ... */ 

} 

If length has the value −1, the addition yields 0, and png_malloc() subsequently returns a null 

pointer, which is assigned to chunkdata. The chunkdata pointer is later used as a destination 

argument in a call to memcpy(), resulting in user-defined data overwriting memory starting at address 

0. In the case of the ARM and XScale architectures, the 0x0 address is mapped in memory 

and serves as the exception vector table; consequently, dereferencing 0x0 did not cause an abnormal 

program termination. 


This compliant solution ensures that the pointer returned by png_malloc() is not null. It also 

uses the unsigned type size_t to pass the length parameter, ensuring that negative values are 

not passed to func(). 

#include /* From libpng */ 

#include 

void func(png_structp png_ptr, size_t length, const void 





*user_data) { 

png_charp chunkdata; 

if (length == SIZE_MAX) { 


} 

chunkdata = (png_charp)png_malloc(png_ptr, length + 1); 

if (NULL == chunkdata) { 


} 

/* ... */ 

memcpy(chunkdata, user_data, length); 

/* ... */ 

} 


In this noncompliant code example, input_str is copied into dynamically allocated memory referenced 

by c_str. If malloc() fails, it returns a null pointer that is assigned to c_str. When 

c_str is dereferenced in memcpy(), the program exhibits undefined behavior. Additionally, if 

input_str is a null pointer, the call to strlen() dereferences a null pointer, also resulting in 

undefined behavior. This code also violates ERR33-C. Detect and handle standard library errors. 

#include 

#include 

void f(const char *input_str) { 

size_t size = strlen(input_str) + 1; 

char *c_str = (char *)malloc(size); 

memcpy(c_str, input_str, size); 

/* ... */ 

free(c_str); 

c_str = NULL; 

/* ... */ 

} 


This compliant solution ensures that both input_str and the pointer returned by malloc() are 

not null: 

#include 

#include 

void f(const char *input_str) { 

size_t size; 

char *c_str; 

if (NULL == input_str) { 





} 


} 

size = strlen(input_str) + 1; 

c_str = (char *)malloc(size); 

if (NULL == c_str) { 


} 

memcpy(c_str, input_str, size); 

/* ... */ 

free(c_str); 

c_str = NULL; 

/* ... */ 


This noncompliant code example is from a version of drivers/net/tun.c and affects Linux 

Kernel 2.6.30 [Goodin 2009]: 

static unsigned int tun_chr_poll(struct file *file, poll_table 

*wait) { 

struct tun_file *tfile = file->private_data; 

struct tun_struct *tun = __tun_get(tfile); 

struct sock *sk = tun->sk; 

unsigned int mask = 0; 

if (!tun) 

return POLLERR; 

DBG(KERN_INFO "%s: tun_chr_poll\n", tun->dev->name); 

poll_wait(file, &tun->socket.wait, wait); 

if (!skb_queue_empty(&tun->readq)) 

mask |= POLLIN | POLLRDNORM; 

if (sock_writeable(sk) || 

(!test_and_set_bit(SOCK_ASYNC_NOSPACE, &sk->sk_socket->flags) 

&& 

sock_writeable(sk))) 

mask |= POLLOUT | POLLWRNORM; 

} 

if (tun->dev->reg_state != NETREG_REGISTERED) 

mask = POLLERR; 

tun_put(tun); 

return mask; 





The sk pointer is initialized to tun->sk before checking if tun is a null pointer. Because null 

pointer dereferencing is undefined behavior, the compiler (GCC in this case) can optimize away 

the if (!tun) check because it is performed after tun->sk is accessed, implying that tun is 

non-null. As a result, this noncompliant code example is vulnerable to a null pointer dereference 

exploit, because null pointer dereferencing can be permitted on several platforms, for example, by 

using mmap(2) with the MAP_FIXED flag on Linux and Mac OS X, or by using the shmat() 

POSIX function with the SHM_RND flag [Liu 2009]. 


This compliant solution eliminates the null pointer deference by initializing sk to tun->sk following 

the null pointer check: 

static unsigned int tun_chr_poll(struct file *file, poll_table 

*wait) { 

struct tun_file *tfile = file->private_data; 

struct tun_struct *tun = __tun_get(tfile); 

struct sock *sk; 

unsigned int mask = 0; 

} 

if (!tun) 

return POLLERR; 

sk = tun->sk; 

/* The remaining code is omitted because it is unchanged... */ 


Dereferencing a null pointer is undefined behavior, typically abnormal program termination. In 

some situations, however, dereferencing a null pointer can lead to the execution of arbitrary code 

[Jack 2007, van Sprundel 2006]. The indicated severity is for this more severe case; on platforms 

where it is not possible to exploit a null pointer dereference to execute arbitrary code, the actual 

severity is low. 


EXP34-C High Likely Medium P18 L1 


CERT Oracle Secure Coding Standard for Java EXP01-J. Do not use a null in a case where an 

object is required 





ISO/IEC TR 24772:2013 


MITRE CWE 


[HFC] 

Null Pointer Dereference [XYH] 

Dereferencing an out-of-domain pointer 

[nullref] 

CWE-476, NULL Pointer Dereference 


[Goodin 2009] 

[Jack 2007] 

[Liu 2009] 

[van Sprundel 2006] 

[Viega 2005] 

Section 5.2.18, “Null-Pointer Dereference” 




Expressions (EXP) - EXP35-C. Do not modify objects with temporary lifetime 

4.5 EXP35-C. Do not modify objects with temporary lifetime 

The C11 Standard [ISO/IEC 9899:2011] introduced a new term: temporary lifetime. Modifying an 

object with temporary lifetime is undefined behavior. According to subclause 6.2.4, paragraph 8 

A non-lvalue expression with structure or union type, where the structure or union contains 

a member with array type (including, recursively, members of all contained structures 

and unions) refers to an object with automatic storage duration and temporary lifetime. 

Its lifetime begins when the expression is evaluated and its initial value is the value 

of the expression. Its lifetime ends when the evaluation of the containing full expression 

or full declarator ends. Any attempt to modify an object with temporary lifetime results in 

undefined behavior. 

This definition differs from the C99 Standard (which defines modifying the result of a function 

call or accessing it after the next sequence point as undefined behavior) because a temporary object’s 

lifetime ends when the evaluation containing the full expression or full declarator ends, so 

the result of a function call can be accessed. This extension to the lifetime of a temporary also removes 

a quiet change to C90 and improves compatibility with C++. 

C functions may not return arrays; however, functions can return a pointer to an array or a 

struct or union that contains arrays. Consequently, if a function call returns by value a struct 

or union containing an array, do not modify those arrays within the expression containing the 

function call. Do not access an array returned by a function after the next sequence point or after 

the evaluation of the containing full expression or full declarator ends. 

4.5.1 Noncompliant Code Example (C99) 

This noncompliant code example conforms to the C11 Standard; however, it fails to conform to 

C99. If compiled with a C99-conforming implementation, this code has undefined behavior because 

the sequence point preceding the call to printf() comes between the call and the access 

by printf() of the string in the returned object. 

#include 

struct X { char a[8]; }; 

struct X salutation(void) { 

struct X result = { "Hello" }; 

return result; 

} 

struct X addressee(void) { 

struct X result = { "world" }; 


} 






} 

printf("%s, %s!\n", salutation().a, addressee().a); 

return 0; 


This compliant solution stores the structures returned by the call to addressee() before calling 

the printf() function. Consequently, this program conforms to both C99 and C11. 

#include 

struct X { char a[8]; }; 

struct X salutation(void) { 

struct X result = { "Hello" }; 


} 


struct X result = { "world" }; 


} 


struct X my_salutation = salutation(); 

struct X my_addressee = addressee(); 

} 

printf("%s, %s!\n", my_salutation.a, my_addressee.a); 

return 0; 


This noncompliant code example attempts to retrieve an array and increment the array’s first 

value. The array is part of a struct that is returned by a function call. Consequently, the array 

has temporary lifetime, and modifying the array is undefined behavior. 

#include 

struct X { int a[6]; }; 


struct X result = { { 1, 2, 3, 4, 5, 6 } }; 


} 


printf("%x", ++(addressee().a[0])); 





} 

return 0; 


This compliant solution stores the structure returned by the call to addressee() as my_x before 

calling the printf() function. When the array is modified, its lifetime is no longer temporary 

but matches the lifetime of the block in main(). 

#include 

struct X { int a[6]; }; 


struct X result = { { 1, 2, 3, 4, 5, 6 } }; 


} 


struct X my_x = addressee(); 

printf("%x", ++(my_x.a[0])); 

return 0; 

} 


Attempting to modify an array or access it after its lifetime expires may result in erroneous program 

behavior. 


EXP35-C Low Probable Medium P4 L3 


ISO/IEC TR 24772:2013 


Side-effects and Order of Evaluation [SAM] 


[ISO/IEC 9899:2011] 





Expressions (EXP) - EXP36-C. Do not cast pointers into more strictly aligned pointer types 

4.6 EXP36-C. Do not cast pointers into more strictly aligned pointer 

types 

Do not convert a pointer value to a pointer type that is more strictly aligned than the referenced 

type. Different alignments are possible for different types of objects. If the type-checking system 

is overridden by an explicit cast or the pointer is converted to a void pointer (void *) and then to 

a different type, the alignment of an object may be changed. 

The C Standard, 6.3.2.3, paragraph 7 [ISO/IEC 9899:2011], states 

A pointer to an object or incomplete type may be converted to a pointer to a different object 

or incomplete type. If the resulting pointer is not correctly aligned for the referenced 

type, the behavior is undefined. 


If the misaligned pointer is dereferenced, the program may terminate abnormally. On some architectures, 

the cast alone may cause a loss of information even if the value is not dereferenced if the 

types involved have differing alignment requirements. 


In this noncompliant example, the char pointer &c is converted to the more strictly aligned int 

pointer ip. On some implementations, cp will not match &c. As a result, if a pointer to one object 

type is converted to a pointer to a different object type, the second object type must not require 

stricter alignment than the first. 

#include 


char c = 'x'; 

int *ip = (int *)&c; /* This can lose information */ 

char *cp = (char *)ip; 

} 

/* Will fail on some conforming implementations */ 

assert(cp == &c); 

4.6.2 Compliant Solution (Intermediate Object) 

In this compliant solution, the char value is stored into an object of type int so that the pointer's 

value will be properly aligned: 

#include 


char c = 'x'; 





int i = c; 

int *ip = &i; 

} 

assert(ip == &i); 


The C Standard allows any object pointer to be cast to and from void *. As a result, it is possible 

to silently convert from one pointer type to another without the compiler diagnosing the problem 

by storing or casting a pointer to void * and then storing or casting it to the final type. In this 

noncompliant code example, loop_function() is passed the char pointer loop_ptr but returns 

an object of type int pointer: 

int *loop_function(void *v_pointer) { 

/* ... */ 

return v_pointer; 

} 

void func(char *loop_ptr) { 

int *int_ptr = loop_function(loop_ptr); 

} 

/* ... */ 

This example compiles without warning using GCC 4.8 on Ubuntu Linux 14.04. However, 

v_pointer can be more strictly aligned than an object of type int *. 


Because the input parameter directly influences the return value, and loop_function() returns 

an object of type int *, the formal parameter v_pointer is redeclared to accept only an object 

of type int *: 

int *loop_function(int *v_pointer) { 

/* ... */ 

return v_pointer; 

} 

void func(int *loop_ptr) { 

int *int_ptr = loop_function(loop_ptr); 

} 

/* ... */ 






Some architectures require that pointers are correctly aligned when accessing objects larger than a 

byte. However, it is common in system code that unaligned data (for example, the network stacks) 

must be copied to a properly aligned memory location, such as in this noncompliant code example: 

#include 

struct foo_header { 

int len; 

/* ... */ 

}; 

void func(char *data, size_t offset) { 

struct foo_header *tmp; 

struct foo_header header; 

tmp = (struct foo_header *)(data + offset); 

memcpy(&header, tmp, sizeof(header)); 

} 

/* ... */ 

Assigning an unaligned value to a pointer that references a type that needs to be aligned is undefined 

behavior. An implementation may notice, for example, that tmp and header must be 

aligned and use an inline memcpy() that uses instructions that assume aligned data. 


This compliant solution avoids the use of the foo_header pointer: 

#include 

struct foo_header { 

int len; 

/* ... */ 

}; 

void func(char *data, size_t offset) { 

struct foo_header header; 

memcpy(&header, data + offset, sizeof(header)); 

} 

/* ... */ 






EXP36-C-EX1: Some hardware architectures have relaxed requirements with regard to pointer 

alignment. Using a pointer that is not properly aligned is correctly handled by the architecture, although 

there might be a performance penalty. On such an architecture, improper pointer alignment 

is permitted but remains an efficiency problem. 

EXP36-C-EX2: If a pointer is known to be correctly aligned to the target type, then a cast to that 

type is permitted. There are several cases where a pointer is known to be correctly aligned to the 

target type. The pointer could point to an object declared with a suitable alignment specifier. It 

could point to an object returned by aligned_alloc(), calloc(), malloc(), or realloc(), 

as per the C standard, section 7.22.3, paragraph 1 [ISO/IEC 9899:2011]. 

This compliant solution uses the alignment specifier, which is new to C11, to declare the char 

object c with the same alignment as that of an object of type int. As a result, the two pointers 

reference equally aligned pointer types: 

#include 

#include 


/* Align c to the alignment of an int */ 

alignas(int) char c = 'x'; 

int *ip = (int *)&c; 

char *cp = (char *)ip; 

/* Both cp and &c point to equally aligned objects */ 

assert(cp == &c); 

} 


Accessing a pointer or an object that is not properly aligned can cause a program to crash or give 

erroneous information, or it can cause slow pointer accesses (if the architecture allows misaligned 

accesses). 


EXP36-C Low Probable Medium P4 L3 



ISO/IEC TR 24772:2013 


EXP56-CPP. Do not cast pointers into more 

strictly aligned pointer types 


[HFC] 

Converting pointer values to more strictly 

aligned pointer types [alignconv] 





MISRA C:2012 






[Bryant 2003] 

[ISO/IEC 9899:2011] 

6.3.2.3, “Pointers” 

[Walfridsson 2003] Aliasing, Pointer Casts and GCC 3.3 




Expressions (EXP) - EXP37-C. Call functions with the correct number and type of arguments 

4.7 EXP37-C. Call functions with the correct number and type of 

arguments 

Do not call a function with the wrong number or type of arguments. 

The C Standard identifies five distinct situations in which undefined behavior (UB) may arise as a 

result of invoking a function using a declaration that is incompatible with its definition or by supplying 

incorrect types or numbers of arguments: 

UB 

Description 

26 A pointer is used to call a function whose type is not compatible with the referenced type 

(6.3.2.3). 

38 For a call to a function without a function prototype in scope, the number of arguments does 

not equal the number of parameters (6.5.2.2). 

39 For a call to a function without a function prototype in scope where the function is defined 

with a function prototype, either the prototype ends with an ellipsis or the types of the arguments 

after promotion are not compatible with the types of the parameters (6.5.2.2). 

40 For a call to a function without a function prototype in scope where the function is not defined 

with a function prototype, the types of the arguments after promotion are not compatible 

with those of the parameters after promotion (with certain exceptions) (6.5.2.2). 

41 A function is defined with a type that is not compatible with the type (of the expression) 

pointed to by the expression that denotes the called function (6.5.2.2). 

Functions that are appropriately declared (as in DCL40-C. Do not create incompatible declarations 

of the same function or object) will typically generate a compiler diagnostic message if they 

are supplied with the wrong number or types of arguments. However, there are cases in which 

supplying the incorrect arguments to a function will, at best, generate compiler warnings. Although 

such warnings should be resolved, they do not prevent program compilation. (See MSC00- 

C. Compile cleanly at high warning levels.) 


The header provides type-generic macros for math functions. Although most functions 

from the header have a complex counterpart in , several functions 

do not. Calling any of the following type-generic functions with complex values is undefined behavior. 

Functions That Should Not Be Called with Complex Values 

atan2() erf() fdim() fmin() ilogb() 

llround() logb() nextafter() rint() tgamma() 

cbrt() erfc() floor() fmod() ldexp() 

log10() lrint() nexttoward() round() trunc() 

ceil() exp2() fma() frexp() lgamma() 

log1p() lround() remainder() scalbn() copysign() 

expm1() fmax() hypot() llrint() log2() 

nearbyint() remquo() scalbln() 





This noncompliant code example attempts to take the base-2 logarithm of a complex number, resulting 

in undefined behavior: 

#include 


double complex c = 2.0 + 4.0 * I; 

double complex result = log2(c); 

} 

4.7.2 Compliant Solution (Complex Number) 

If the clog2() function is not available for an implementation as an extension, the programmer 

can take the base-2 logarithm of a complex number, using log() instead of log2(), because 

log() can be used on complex arguments, as shown in this compliant solution: 

#include 



double complex result = log(c)/log(2); 

} 

4.7.3 Compliant Solution (Real Number) 

The programmer can use this compliant solution if the intent is to take the base-2 logarithm of the 

real part of the complex number: 

#include 



double complex result = log2(creal(c)); 

} 


In this noncompliant example, the C standard library function strchr() is called through the 

function pointer fp declared with a prototype with incorrectly typed arguments. According to the 

C Standard, 6.3.2.3, paragraph 8 [ISO/IEC 9899:2011], 

A pointer to a function of one type may be converted to a pointer to a function of another 

type and back again; the result shall compare equal to the original pointer. If a converted 

pointer is used to call a function whose type is not compatible with the referenced type, 

the behavior is undefined. 






#include 

#include 

char *(*fp)(); 


const char *c; 

fp = strchr; 

c = fp('e', "Hello"); 

printf("%s\n", c); 

return 0; 

} 


In this compliant solution, the function pointer fp, which points to the C standard library function 

strchr(), is declared with the correct parameters and is invoked with the correct number and 

type of arguments: 

#include 

#include 

char *(*fp)(const char *, int); 


const char *c; 

fp = strchr; 

c = fp("Hello",'e'); 

printf("%s\n", c); 

return 0; 

} 


In this noncompliant example, the function f() is defined to take an argument of type long but 

f() is called from another file with an argument of type int: 

/* In another source file */ 

long f(long x) { 

return x < 0 ? -x : x; 

} 

/* In this source file, no f prototype in scope */ 

long f(); 

long g(int x) { 

return f(x); 

} 






In this compliant solution, the prototype for the function f() is included in the source file in the 

scope of where it is called, and the function f() is correctly called with an argument of type 

long: 

/* In another source file */ 

long f(long x) { 

return x < 0 ? -x : x; 

} 

/* f prototype in scope in this source file */ 

long f(long x); 

long g(int x) { 

return f((long)x); 

} 

4.7.8 Noncompliant Code Example (POSIX) 

The POSIX function open() [IEEE Std 1003.1:2013] is a variadic function with the following 

prototype: 

int open(const char *path, int oflag, ... ); 

The open() function accepts a third argument to determine a newly created file’s access mode. If 

open() is used to create a new file and the third argument is omitted, the file may be created with 

unintended access permissions. (See FIO06-C. Create files with appropriate access permissions.) 

In this noncompliant code example from a vulnerability in the useradd() function of the 

shadow-utils package CVE-2006-1174, the third argument to open() is accidentally omitted: 

fd = open(ms, O_CREAT | O_EXCL | O_WRONLY | O_TRUNC); 

Technically, it is incorrect to pass a third argument to open() when not creating a new file (that 

is, with the O_CREAT flag not set). 

4.7.9 Compliant Solution (POSIX) 

In this compliant solution, a third argument is specified in the call to open(): 

#include 

void func(const char *ms, mode_t perms) { 

/* ... */ 

int fd; 





} 

fd = open(ms, O_CREAT | O_EXCL | O_WRONLY | O_TRUNC, perms); 

if (fd == -1) { 


} 


Calling a function with incorrect arguments can result in unexpected or unintended program behavior. 


EXP37-C Medium Probable High P4 L3 



ISO/IEC TR 24772:2013 


MISRA C:2012 

MITRE CWE 

DCL07-C. Include the appropriate type information 

in function declarators 


levels 

FIO06-C. Create files with appropriate access 

permissions 

Subprogram Signature Mismatch [OTR] 

Calling functions with incorrect arguments [argcomp] 


Rule 17.3 (mandatory) 

CWE-628, Function Call with Incorrectly 

Specified Arguments 

CWE-686, Function Call with Incorrect Argument 

Type 


[CVE] 

[ISO/IEC 9899:2011] 

[IEEE Std 1003.1:2013] 

[Spinellis 2006] 

CVE-2006-1174 


6.5.2.2, “Function Calls” 

open() 

Section 2.6.1, “Incorrect Routine or Arguments” 




Expressions (EXP) - EXP39-C. Do not access a variable through a pointer of an incompatible type 

4.8 EXP39-C. Do not access a variable through a pointer of an 

incompatible type 

Modifying a variable through a pointer of an incompatible type (other than unsigned char) can 

lead to unpredictable results. Subclause 6.2.7 of the C Standard states that two types may be distinct 

yet compatible and addresses precisely when two distinct types are compatible. 

This problem is often caused by a violation of aliasing rules. The C Standard, 6.5, paragraph 7 

[ISO/IEC 9899:2011], specifies those circumstances in which an object may or may not be aliased. 

An object shall have its stored value accessed only by an lvalue expression that has 

one of the following types: 

• a type compatible with the effective type of the object, 

• a qualified version of a type compatible with the effective type of the object, 

• a type that is the signed or unsigned type corresponding to the effective type of the 

object, 

• a type that is the signed or unsigned type corresponding to a qualified version of 

the effective type of the object, 

• an aggregate or union type that includes one of the aforementioned types among 

its members (including, recursively, a member of a subaggregate or contained union), 

or 

• a character type. 

Accessing an object by means of any other lvalue expression (other than unsigned char) is undefined 

behavior 37. 


In this noncompliant example, an object of type float is incremented through an int *. The 

programmer can use the unit in the last place to get the next representable value for a floatingpoint 

type. However, accessing an object through a pointer of an incompatible type is undefined 

behavior. 

#include 

void f(void) { 

if (sizeof(int) == sizeof(float)) { 

float f = 0.0f; 

int *ip = (int *)&f; 

(*ip)++; 

printf("float is %f\n", f); 

} 

} 






In this compliant solution, the standard C function nextafterf() is used to round toward the 

highest representable floating-point value: 

#include 

#include 

#include 


float f = 0.0f; 

f = nextafterf(f, FLT_MAX); 

printf("float is %f\n", f); 

} 


In this noncompliant code example, an array of two values of type short is treated as an integer 

and assigned an integer value. The resulting values are indeterminate. 

#include 


short a[2]; 

a[0]=0x1111; 

a[1]=0x1111; 

*(int *)a = 0x22222222; 

} 

printf("%x %x\n", a[0], a[1]); 

When translating this code, an implementation can assume that no access through an integer 

pointer can change the array a, consisting of shorts. Consequently, printf() may be called with 

the original values of a[0] and a[1]. 


Recent versions of GCC turn on the option -fstrict-aliasing (which allows alias-based optimizations) 

by default with -O2. Some architectures then print “1111 1111” as a result. Without 

optimization, the executable generates the expected output “2222 2222.” 

To disable optimizations based on alias analysis for faulty legacy code, the option -fnostrict-aliasing 

can be used as a workaround. The option -Wstrict-aliasing (which is 

included in -Wall) warns about some, but not all, violations of aliasing rules when -fstrictaliasing 

is active. 





When GCC 3.4.6 compiles this code with optimization, the assignment through the aliased pointer 

is effectively eliminated. 


This compliant solution uses a union type that includes a type compatible with the effective type 

of the object: 

#include 


union { 

short a[2]; 

int i; 

} u; 

u.a[0]=0x1111; 

u.a[1]=0x1111; 

u.i = 0x22222222; 

printf("%x %x\n", u.a[0], u.a[1]); 

} 

/* ... */ 

The printf() behavior in this compliant solution is unspecified, but it is commonly accepted as 

an implementation extension. (See unspecified behavior 11.) 

This function typically outputs “2222 2222.” However, there is no guarantee that this will be true, 

even on implementations that defined the unspecified behavior; values of type short need not 

have the same representation as values of type int. 


In this noncompliant code example, a gadget object is allocated, then realloc() is called to 

create a widget object using the memory from the gadget object. Although reusing memory to 

change types is acceptable, accessing the memory copied from the original object is undefined behavior. 

#include 

struct gadget { 

int i; 

double d; 

char *p; 

}; 

struct widget { 





char *q; 

int j; 

double e; 

}; 


struct gadget *gp; 

struct widget *wp; 

} 

gp = (struct gadget *)malloc(sizeof(struct gadget)); 

if (!gp) { 


} 

/* ... Initialize gadget ... */ 

wp = (struct widget *)realloc(gp, sizeof(struct widget)); 

if (!wp) { 

free(gp); 


} 

if (wp->j == 12) { 

/* ... */ 

} 


This compliant solution reuses the memory from the gadget object but reinitializes the memory 

to a consistent state before reading from it: 

#include 

#include 

struct gadget { 

int i; 

double d; 

char *p; 

}; 

struct widget { 

char *q; 

int j; 

double e; 

}; 


struct gadget *gp; 

struct widget *wp; 

gp = (struct gadget *)malloc(sizeof (struct gadget)); 

if (!gp) { 






} 

/* ... */ 

wp = (struct widget *)realloc(gp, sizeof(struct widget)); 

if (!wp) { 

free(gp); 


} 

memset(wp, 0, sizeof(struct widget)); 

/* ... Initialize widget ... */ 

} 

if (wp->j == 12) { 

/* ... */ 

} 


According to the C Standard, 6.7.6.2 [ISO/IEC 9899:2011], using two or more incompatible arrays 

in an expression is undefined behavior. (See also undefined behavior 76.) 

For two array types to be compatible, both should have compatible underlying element types, and 

both size specifiers should have the same constant value. If either of these properties is violated, 

the resulting behavior is undefined. 

In this noncompliant code example, the two arrays a and b fail to satisfy the equal size specifier 

criterion for array compatibility. Because a and b are not equal, writing to what is believed to be a 

valid member of a might exceed its defined memory boundary, resulting in an arbitrary memory 

overwrite. 

enum { ROWS = 10, COLS = 15 }; 


int a[ROWS][COLS]; 

int (*b)[ROWS] = a; 

} 

Most compilers will produce a warning diagnostic if the two array types used in an expression are 

incompatible. 


In this compliant solution, b is declared to point to an array with the same number of elements as 

a, satisfying the size specifier criterion for array compatibility: 

enum { ROWS = 10, COLS = 15 }; 






} 

int a[ROWS][COLS]; 

int (*b)[COLS] = a; 


Optimizing for performance can lead to aliasing errors that can be quite difficult to detect. Furthermore, 

as in the preceding example, unexpected results can lead to buffer overflow attacks, bypassing 

security checks, or unexpected execution. 

Recommendation Severity Likelihood Remediation Cost Priority Level 

EXP39-C Medium Unlikely High P2 L3 



MITRE CWE 

Accessing an object through a pointer to an incompatible 

type [ptrcomp] 






[Acton 2006] 

“Understanding Strict Aliasing” 

GCC Known Bugs 

“C Bugs, Aliasing Issues while Casting to Incompatible 

Types” 

[ISO/IEC 9899:2011] 

6.5, “Expressions” 

6.7.6.2, “Array Declarators” 

[Walfridsson 2003] Aliasing, Pointer Casts and GCC 3.3 




Expressions (EXP) - EXP40-C. Do not modify constant objects 

4.9 EXP40-C. Do not modify constant objects 

The C Standard, 6.7.3, paragraph 6 [ISO/IEC 9899:2011], states 

If an attempt is made to modify an object defined with a const-qualified type through 

use of an lvalue with non-const-qualified type, the behavior is undefined. 


There are existing compiler implementations that allow const-qualified objects to be modified 

without generating a warning message. 

Avoid casting away const qualification because doing so makes it possible to modify constqualified 

objects without issuing diagnostics. (See EXP05-C. Do not cast away a const qualification 

and STR30-C. Do not attempt to modify string literals for more details.) 


This noncompliant code example allows a constant object to be modified: 

const int **ipp; 

int *ip; 

const int i = 42; 


ipp = &ip; /* Constraint violation */ 

*ipp = &i; /* Valid */ 

*ip = 0; /* Modifies constant i (was 42) */ 

} 

The first assignment is unsafe because it allows the code that follows it to attempt to change the 

value of the const object i. 


If ipp, ip, and i are declared as automatic variables, this example compiles without warning with 

Microsoft Visual Studio 2013 when compiled in C mode (/TC) and the resulting program changes 

the value of i. GCC 4.8.1 generates a warning but compiles, and the resulting program changes 

the value of i. 

If ipp, ip, and i are declared with static storage duration, this program compiles without warning 

and terminates abnormally with Microsoft Visual Studio 2013, and compiles with warning and 

terminates abnormally with GCC 4.8.1. 




Expressions (EXP) - EXP40-C. Do not modify constant objects 


The compliant solution depends on the intent of the programmer. If the intent is that the value of i 

is modifiable, then it should not be declared as a constant, as in this compliant solution: 

int **ipp; 

int *ip; 

int i = 42; 


ipp = &ip; /* Valid */ 

*ipp = &i; /* Valid */ 

*ip = 0; /* Valid */ 

} 

If the intent is that the value of i is not meant to change, then do not write noncompliant code that 

attempts to modify it. 


Modifying constant objects through nonconstant references is undefined behavior. 


EXP40-C Low Unlikely Medium P2 L3 



EXP05-C. Do not cast away a const qualification 

STR30-C. Do not attempt to modify string literals 


[ISO/IEC 9899:2011] 

Subclause 6.7.3, “Type Qualifiers” 




Expressions (EXP) - EXP42-C. Do not compare padding data 

4.10 EXP42-C. Do not compare padding data 

The C Standard, 6.7.2.1 [ISO/IEC 9899:2011], states 

There may be unnamed padding within a structure object, but not at its beginning. . . . 

There may be unnamed padding at the end of a structure or union. 

Subclause 6.7.9, paragraph 9, states that 

unnamed members of objects of structure and union type do not participate in initialization. 

Unnamed members of structure objects have indeterminate value even after initialization. 

The only exception is that padding bits are set to zero when a static or thread-local object is implicitly 

initialized (paragraph10): 

If an object that has automatic storage duration is not initialized explicitly, its value is indeterminate. 

If an object that has static or thread storage duration is not initialized explicitly, 

then: 

• if it is an aggregate, every member is initialized (recursively) according to these rules, 

and any padding is initialized to zero bits; 

• if it is a union, the first named member is initialized (recursively) according to these 

rules, and any padding is initialized to zero bits; 

Because these padding values are unspecified, attempting a byte-by-byte comparison between 

structures can lead to incorrect results [Summit 1995]. 


In this noncompliant code example, memcmp() is used to compare the contents of two structures, 

including any padding bytes: 

#include 

struct s { 

char c; 

int i; 

char buffer[13]; 

}; 

void compare(const struct s *left, const struct s *right) { 

if (0 == memcmp(left, right, sizeof(struct s))) { 

/* ... */ 

} 

} 






In this compliant solution, all of the fields are compared manually to avoid comparing any padding 

bytes: 

#include 

struct s { 

char c; 

int i; 


}; 


if ((left && right) && 

(left->c == right->c) && 

(left->i == right->i) && 

(0 == memcmp(left->buffer, right->buffer, 13))) { 

/* ... */ 

} 

} 


EXP42-C-EX1: A structure can be defined such that the members are aligned properly or the 

structure is packed using implementation-specific packing instructions. This is true only when the 

members’ data types have no padding bits of their own and when their object representations are 

the same as their value representations. This frequently is not true for the _Bool type or floatingpoint 

types and need not be true for pointers. In such cases, the compiler does not insert padding, 

and use of functions such as memcmp() is acceptable. 

This compliant example uses the #pragma pack compiler extension from Microsoft Visual Studio 

to ensure the structure members are packed as tightly as possible: 

#include 

#pragma pack(push, 1) 

struct s { 

char c; 

int i; 


}; 

#pragma pack(pop) 


if (0 == memcmp(left, right, sizeof(struct s))) { 

/* ... */ 





} 

} 


Comparing padding bytes, when present, can lead to unexpected program behavior. 


EXP42-C Medium Probable Medium P8 L2 




Comparison of padding data [padcomp] 

EXP62-CPP. Do not access the bits of an object 

representation that are not part of the object’s 

value representation 


[ISO/IEC 9899:2011] 

[Summit 1995] Question 2.8 

Question 2.12 


6.7.9, “Initialization” 




Expressions (EXP) - EXP43-C. Avoid undefined behavior when using restrict-qualified pointers 

4.11 EXP43-C. Avoid undefined behavior when using restrict-qualified 

pointers 

An object that is accessed through a restrict-qualified pointer has a special association with 

that pointer. This association requires that all accesses to that object use, directly or indirectly, the 

value of that particular pointer. The intended use of the restrict qualifier is to promote optimization, 

and deleting all instances of the qualifier from a program does not change its meaning (that 

is, observable behavior). In the absence of this qualifier, other pointers can alias this object. Caching 

the value in an object designated through a restrict-qualified pointer is safe at the beginning 

of the block in which the pointer is declared because no preexisting aliases may also be used 

to reference that object. The cached value must be restored to the object by the end of the block, 

where preexisting aliases again become available. New aliases may be formed within the block, 

but these must all depend on the value of the restrict-qualified pointer so that they can be identified 

and adjusted to refer to the cached value. For a restrict-qualified pointer at file scope, 

the block is the body of each function in the file [Walls 2006]. Developers should be aware that 

C++ does not support the restrict qualifier, but some C++ compiler implementations support 

an equivalent qualifier as an extension. 

The C Standard [ISO/IEC 9899:2011] identifies the following undefined behavior: 

A restrict-qualified pointer is assigned a value based on another restricted pointer whose 

associated block neither began execution before the block associated with this pointer, 

nor ended before the assignment (6.7.3.1). 

This is an oversimplification, however, and it is important to review the formal definition of restrict 

in subclause 6.7.3.1 of the C Standard to properly understand undefined behaviors associated 

with the use of restrict-qualified pointers. 

4.11.1 Overlapping Objects 

The restrict qualifier requires that the pointers do not reference overlapping objects. If the objects 

referenced by arguments to functions overlap (meaning the objects share some common 

memory addresses), the behavior is undefined. 

4.11.1.1 Noncompliant Code Example 

This code example is noncompliant because an assignment is made between two restrict-qualified 

pointers in the same scope: 

int *restrict a; 

int *restrict b; 

extern int c[]; 


c[0] = 17; 

c[1] = 18; 





} 

a = &c[0]; 

b = &c[1]; 

a = b; /* Undefined behavior */ 

/* ... */ 

Note that undefined behavior occurs only when a is assigned to b. It is valid for a and b to point 

into the same array object, provided the range of elements accessed through one of the pointers 

does not overlap with the range of elements accessed through the other pointer. 

4.11.1.2 Compliant Solution 

One way to eliminate the undefined behavior is simply to remove the restrict-qualification 

from the affected pointers: 

int *a; 

int *b; 

extern int c[]; 


c[0] = 17; 

c[1] = 18; 

a = &c[0]; 

b = &c[1]; 

a = b; /* Defined behavior */ 

/* ... */ 

} 

4.11.2 restrict-Qualified Function Parameters 

When calling functions that have restrict-qualified function parameters, it is important that the 

pointer arguments do not reference overlapping objects if one or more of the pointers are used to 

modify memory. Consequently, it is important to understand the semantics of the function being 

called. 


In this noncompliant code example, the function f() accepts three parameters. The function copies 

n integers from the int array referenced by the restrict-qualified pointer p to the int array 

referenced by the restrict-qualified pointer q. Because the destination array is modified during 

each execution of the function (for which n is nonzero), if the array is accessed through one of the 





pointer parameters, it cannot also be accessed through the other. Declaring these function parameters 

as restrict-qualified pointers allows aggressive optimization by the compiler but can also 

result in undefined behavior if these pointers refer to overlapping objects. 

#include 

void f(size_t n, int *restrict p, const int *restrict q) { 

while (n-- > 0) { 

*p++ = *q++; 

} 

} 

void g(void) { 

extern int d[100]; 

/* ... */ 

f(50, d + 1, d); /* Undefined behavior */ 

} 

The function g() declares an array d consisting of 100 int values and then invokes f() to copy 

memory from one area of the array to another. This call has undefined behavior because each of 

d[1]through d[49] is accessed through both p and q. 


In this compliant solution, the function f() is unchanged but the programmer has ensured that 

none of the calls to f() result in undefined behavior. The call to f() in g() is valid because the 

storage allocated to d is effectively divided into two disjoint objects. 

#include 

void f(size_t n, int *restrict p, const int *restrict q) { 

while (n-- > 0) { 

*p++ = *q++; 

} 

} 


extern int d[100]; 

/* ... */ 

f(50, d + 50, d); /* Defined behavior */ 

} 


In this noncompliant code example, the function add() adds the integer array referenced by the 

restrict-qualified pointers lhs to the integer array referenced by the restrict-qualified 

pointer rhs and stores the result in the restrict-qualified pointer referenced by res. The function 

f() declares an array a consisting of 100 int values and then invokes add() to copy 





memory from one area of the array to another. The call add(100, a, a, a)has undefined behavior 

because the object modified by res is accessed by lhs and rhs. 

#include 

void add(size_t n, int *restrict res, const int *restrict lhs, 

const int *restrict rhs) { 

for (size_t i = 0; i < n; ++i) { 

res[i] = lhs[i] + rhs[i]; 

} 

} 


int a[100]; 

add(100, a, a, a); /* Undefined behavior */ 

} 


In this compliant solution, an unmodified object is aliased through two restricted pointers. Because 

a and b are disjoint arrays, a call of the form add(100, a, b, b) has defined behavior, 

because array b is not modified within function add. 

#include 

void add(size_t n, int *restrict res, const int *restrict lhs, 

const int *restrict rhs) { 

for (size_t i = 0; i < n; ++i) { 

res[i] = lhs[i] + rhs[i]; 

} 

} 


int a[100]; 

int b[100]; 

add(100, a, b, b); /* Defined behavior */ 

} 

4.11.3 Invoking Library Functions with restrict-Qualified Pointers 

Ensure that restrict-qualified source and destination pointers do not reference overlapping objects 

when invoking library functions. For example, the following table lists C standard library 

functions that copy memory from a source object referenced by a restrict-qualified pointer to a 

destination object that is also referenced by a restrict-qualified pointer: 

Standard C 

strcpy() 

strncpy() 

Annex K 

strcpy_s() 

strncpy_s() 






strcat() 

strncat() 

memcpy() 

Annex K 

strcat_s() 

strncat_s() 

memcpy_s() 

strtok_s() 

If the objects referenced by arguments to functions overlap (meaning the objects share some common 

memory addresses), the behavior is undefined. (See also undefined behavior 68.) The result 

of the functions is unknown, and data may be corrupted. As a result, these functions must never 

be passed pointers to overlapping objects. If data must be copied between objects that share common 

memory addresses, a copy function guaranteed to work on overlapping memory, such as 

memmove(), should be used. 


In this noncompliant code example, the values of objects referenced by ptr1 and ptr2 become 

unpredictable after the call to memcpy() because their memory areas overlap: 

#include 


char c_str[]= "test string"; 

char *ptr1 = c_str; 

char *ptr2; 

} 

ptr2 = ptr1 + 3; 

/* Undefined behavior because of overlapping objects */ 

memcpy(ptr2, ptr1, 6); 

/* ... */ 


In this compliant solution, the call to memcpy() is replaced with a call to memmove(). The 

memmove() function performs the same operation as memcpy() when the memory regions do not 

overlap. When the memory regions do overlap, the n characters from the object pointed to by the 

source (ptr1) are first copied into a temporary array of n characters that does not overlap the objects 

pointed to by the destination (ptr2) or the source. The n characters from the temporary array 

are then copied into the object pointed to by the destination. 

#include 


char c_str[]= "test string"; 

char *ptr1 = c_str; 

char *ptr2; 





} 

ptr2 = ptr1 + 3; 

memmove(ptr2, ptr1, 6); /* Replace call to memcpy() */ 

/* ... */ 

Similar solutions using memmove() can replace the string functions as long as care is taken regarding 

the byte size of the characters and proper null-termination of the copied string. 

4.11.4 Calling Functions with restrict-Qualified Pointer to a const-Qualified 

Type 

Ensure that functions that accept a restrict-qualified pointer to a const-qualified type do not 

modify the object referenced by that pointer. Formatted input and output standard library functions 

frequently fit this description. The following table lists of some of the common functions for 

which the format argument is a restrict-qualified pointer to a const-qualified type. 


printf() 

scanf() 

sprintf() 

snprintf() 

Annex K 

printf_s() 

scanf_s() 

sprintf_s() 

snprintf_s() 

For formatted output functions such as printf(), it is unlikely that a programmer would modify 

the format string. However, an attacker may attempt to do so if a program violates FIO30-C. Exclude 

user input from format strings and passes tainted values as part of the format string. 


In this noncompliant code example, the programmer is attempting to overwrite the format string 

with a string value read in from stdin such as “%d%f 1 3.3” and use the resulting modified 

string of “%s%d%f” to input the subsequent values of 1 and 3.3: 

#include 


int i; 

float x; 

char format[100] = "%s"; 

/* Undefined behavior */ 

int n = scanf(format, format + 2, &i, &x); 

/* ... */ 

} 






The intended results are achieved by this compliant solution: 

#include 


int i; 

float x 

int n = scanf("%d%f", &i, &x); /* Defined behavior */ 

/* ... */ 

} 

4.11.5 Outer-to-Inner Assignments between Restricted Pointers 

The assignment between restrict-qualified pointers declared in an inner nested block from an 

outer block has defined behavior. 


The assignment of restrict-qualified pointers to other restrict-qualified pointers within the 

same block has undefined behavior: 


int *restrict p1; 

int *restrict q1; 

int *restrict p2 = p1; /* Undefined behavior */ 

int *restrict q2 = q1; /* Undefined behavior */ 

} 


The intended results can be achieved using an inner nested block, as shown in this compliant solution: 


int *restrict p1; 

int *restrict q1; 

{ /* Added inner block */ 

int *restrict p2 = p1; /* Valid, well-defined behavior */ 

int *restrict q2 = q1; /* Valid, well-defined behavior */ 

} 

} 


The incorrect use of restrict-qualified pointers can result in undefined behavior that might be 

exploited to cause data integrity violations. 






EXP43-C Medium Probable High P4 L3 



ISO/IEC TR 24772:2013 


FIO30-C. Exclude user input from format 

strings 

MISRA C:2012 Rule 8.14 (required) 1 

Passing Parameters and Return Values [CSJ] 

Passing pointers into the same object as arguments 

to different restrict-qualified parameters 

[restrict] 


[ISO/IEC 9899:2011] 

[Walls 2006] 

6.7.3.1, “Formal Definition of restrict” 

______________________ 

1. MISRA Rule 8.14 prohibits the use of the restrict keyword except in C standard library functions. 




Expressions (EXP) - EXP44-C. Do not rely on side effects in operands to sizeof, _Alignof, or _Generic 

4.12 EXP44-C. Do not rely on side effects in operands to sizeof, 

_Alignof, or _Generic 

Some operators do not evaluate their operands beyond the type information the operands provide. 

When using one of these operators, do not pass an operand that would otherwise yield a side effect 

since the side effect will not be generated. 

The sizeof operator yields the size (in bytes) of its operand, which may be an expression or the 

parenthesized name of a type. In most cases, the operand is not evaluated. A possible exception 

is when the type of the operand is a variable length array type (VLA); then the expression is evaluated. 

When part of the operand of the sizeof operator is a VLA type and when changing the 

value of the VLA’s size expression would not affect the result of the operator, it is unspecified 

whether or not the size expression is evaluated. (See unspecified behavior 22.) 

The operand passed to_Alignof is never evaluated, despite not being an expression. For instance, 

if the operand is a VLA type and the VLA’s size expression contains a side effect, that 

side effect is never evaluated. 

The operand used in the controlling expression of a _Generic selection expression is never evaluated. 

Providing an expression that appears to produce side effects may be misleading to programmers 

who are not aware that these expressions are not evaluated, and in the case of a VLA used in 

sizeof, have unspecified results. As a result, programmers may make invalid assumptions about 

program state, leading to errors and possible software vulnerabilities. 

This rule is similar to PRE31-C. Avoid side effects in arguments to unsafe macros. 

4.12.1 Noncompliant Code Example (sizeof) 

In this noncompliant code example, the expression a++ is not evaluated: 

#include 


int a = 14; 

int b = sizeof(a++); 

printf("%d, %d\n", a, b); 

} 

Consequently, the value of a after b has been initialized is 14. 





4.12.2 Compliant Solution (sizeof) 

In this compliant solution, the variable a is incremented outside of the sizeof operation: 

#include 


int a = 14; 

int b = sizeof(a); 

++a; 


} 

4.12.3 Noncompliant Code Example (sizeof, VLA) 

In this noncompliant code example, the expression ++n in the initialization expression of a must 

be evaluated because its value affects the size of the VLA operand of the sizeof operator. However, 

in the initialization expression of b, the expression ++n % 1 evaluates to 0. This means that 

the value of n does not affect the result of the sizeof operator. Consequently, it is unspecified 

whether or not n will be incremented when initializing b. 

#include 

#include 

void f(size_t n) { 

/* n must be incremented */ 

size_t a = sizeof(int[++n]); 

/* n need not be incremented */ 

size_t b = sizeof(int[++n % 1 + 1]); 

} 

printf("%zu, %zu, %zu\n", a, b, n); 

/* ... */ 

4.12.4 Compliant Solution (sizeof, VLA) 

This compliant solution avoids changing the value of the variable n used in each sizeof expression 

and instead increments n safely afterwards: 

#include 

#include 

void f(size_t n) { 

size_t a = sizeof(int[n + 1]); 

++n; 

size_t b = sizeof(int[n % 1 + 1]); 

++n; 





} 

printf("%zu, %zu, %zu\n", a, b, n); 

/* ... */ 

4.12.5 Noncompliant Code Example (_Generic) 

This noncompliant code example attempts to modify a variable’s value as part of the _Generic 

selection control expression. The programmer may expect that a is incremented, but because 

_Generic does not evaluate its control expression, the value of a is not modified. 

#include 

#define S(val) _Generic(val, int : 2, \ 

short : 3, \ 

default : 1) 


int a = 0; 

int b = S(a++); 


} 

4.12.6 Compliant Solution (_Generic) 

In this compliant solution, a is incremented outside of the _Generic selection expression: 

#include 

#define S(val) _Generic(val, int : 2, \ 

short : 3, \ 

default : 1) 


int a = 0; 

int b = S(a); 

++a; 


} 

4.12.7 Noncompliant Code Example (_Alignof) 

This noncompliant code example attempts to modify a variable while getting its default alignment 

value. The user may have expected val to be incremented as part of the _Alignof expression, 

but because _Alignof does not evaluate its operand, val is unchanged. 

#include 


int val = 0; 

/* ... */ 





} 

size_t align = _Alignof(int[++val]); 

printf("%zu, %d\n", align, val); 

/* ... */ 

4.12.8 Compliant Solution (_Alignof) 

This compliant solution moves the expression out of the _Alignof operator: 

#include 


int val = 0; 

/* ... */ 

++val; 

size_t align = _Alignof(int[val]); 

printf("%zu, %d\n, align, val); 

/* ... */ 

} 


EXP44-C-EX1: Reading a volatile-qualified value is a side-effecting operation. However, accessing 

a value through a volatile-qualified type does not guarantee side effects will happen on 

the read of the value unless the underlying object is also volatile-qualified. Idiomatic reads of a 

volatile-qualified object are permissible as an operand to a sizeof(), _Alignof(), or _Generic 

expression, as in the following example: 


int * volatile v; 

(void)sizeof(*v); 

} 


If expressions that appear to produce side effects are supplied to an operator that does not evaluate 

its operands, the results may be different than expected. Depending on how this result is used, it 

can lead to unintended program behavior. 


EXP44-C Low Unlikely Low P3 L3 



EXP52-CPP. Do not rely on side effects in unevaluated 

operands 




Expressions (EXP) - EXP45-C. Do not perform assignments in selection statements 

4.13 EXP45-C. Do not perform assignments in selection statements 

Do not use the assignment operator in the contexts listed in the following table because doing so 

typically indicates programmer error and can result in unexpected behavior. 

Operator 

if 

while 

do ... while 

for 

Context 

Controlling expression 



Second operand 

?: First operand 

?: Second or third operands, where the ternary expression 

is used in any of these contexts 

&& 

Either operand 

|| either operand 

, Second operand, when the comma expression is 

used in any of these contexts 


In this noncompliant code example, an assignment expression is the outermost expression in an 

if statement: 

if (a = b) { 

/* ... */ 

} 

Although the intent of the code may be to assign b to a and test the value of the result for equality 

to 0, it is frequently a case of the programmer mistakenly using the assignment operator = instead 

of the equals operator ==. Consequently, many compilers will warn about this condition, making 

this coding error detectable by adhering to MSC00-C. Compile cleanly at high warning levels. 

4.13.2 Compliant Solution (Unintentional Assignment) 

When the assignment of b to a is not intended, the conditional block is now executed when a is 

equal to b: 

if (a == b) { 

/* ... */ 

} 





4.13.3 Compliant Solution (Intentional Assignment) 

When the assignment is intended, this compliant solution explicitly uses inequality as the outermost 

expression while performing the assignment in the inner expression: 

if ((a = b) != 0) { 

/* ... */ 

} 

It is less desirable in general, depending on what was intended, because it mixes the assignment in 

the condition, but it is clear that the programmer intended the assignment to occur. 


In this noncompliant code example, the expression x = y is used as the controlling expression of 

the while statement: 

do { /* ... */ } while (foo(), x = y); 

The same result can be obtained using the for statement, which is specifically designed to evaluate 

an expression on each iteration of the loop, just before performing the test in its controlling expression: 

for (; x; foo(), x = y) { /* ... */ } 

4.13.5 Compliant Solution (Unintentional Assignment) 

When the assignment of y to x is not intended, the conditional block should be executed only 

when x is equal to y, as in this compliant solution: 

do { /* ... */ } while (foo(), x == y); 

4.13.6 Compliant Solution (Intentional Assignment) 

When the assignment is intended, this compliant solution can be used: 

do { /* ... */ } while (foo(), (x = y) != 0); 


In this noncompliant example, the expression p = q is used as the controlling expression of the 

while statement: 

do { /* ... */ } while (x = y, p = q); 






In this compliant solution, the expression x = y is not used as the controlling expression of the 

while statement: 

do { /* ... */ } while (x = y, p == q); 


This noncompliant code example has a typo that results in an assignment rather than a comparison. 

while (ch = '\t' && ch == ' ' && ch == '\n') { 

/* ... */ 

} 

Many compilers will warn about this condition. This coding error would typically be eliminated 

by adherence to MSC00-C. Compile cleanly at high warning levels. Although this code compiles, 

it will cause unexpected behavior to an unsuspecting programmer. If the intent was to verify a 

string such as a password, user name, or group user ID, the code may produce significant vulnerabilities 

and require significant debugging. 

4.13.10 Compliant Solution (RHS Variable) 

When comparisons are made between a variable and a literal or const-qualified variable, placing 

the variable on the right of the comparison operation can prevent a spurious assignment. 

In this code example, the literals are placed on the left-hand side of each comparison. If the programmer 

were to inadvertently use an assignment operator, the statement would assign ch to 

'\t', which is invalid and produces a diagnostic message. 

while ('\t' = ch && ' ' == ch && '\n' == ch) { 

/* ... */ 

} 

Due to the diagnostic, the typo will be easily spotted and fixed. 

while ('\t' == ch && ' ' == ch && '\n' == ch) { 

/* ... */ 

} 

As a result, any mistaken use of the assignment operator that could otherwise create a vulnerability 

for operations such as string verification will result in a compiler diagnostic regardless of compiler, 

warning level, or implementation. 






EXP45-C-EX1: Assignment can be used where the result of the assignment is itself an operand to 

a comparison expression or relational expression. In this compliant example, the expression x = 

y is itself an operand to a comparison operation: 

if ((x = y) != 0) { /* ... */ } 

EXP45-C-EX2: Assignment can be used where the expression consists of a single primary expression. 

The following code is compliant because the expression x = y is a single primary expression: 

if ((x = y)) { /* ... */ } 

The following controlling expression is noncompliant because && is not a comparison or relational 

operator and the entire expression is not primary: 

if ((v = w) && flag) { /* ... */ } 

When the assignment of v to w is not intended, the following controlling expression can be used to 

execute the conditional block when v is equal to w: 

if ((v == w) && flag) { /* ... */ }; 

When the assignment is intended, the following controlling expression can be used: 

if (((v = w) != 0) && flag) { /* ... */ }; 

EXP45-C-EX3: Assignment can be used in a function argument or array index. In this compliant 

solution, the expression x = y is used in a function argument: 

if (foo(x = y)) { /* ... */ } 


Errors of omission can result in unintended program flow. 


EXP45-C Low Likely Medium P6 L2 







EXP19-CPP. Do not perform assignments in 

conditional expressions 

CERT Oracle Secure Coding Standard for Java EXP51-J. Do not perform assignments in conditional 

expressions 

ISO/IEC TR 24772:2013 


MITRE CWE 

Likely Incorrect Expression [KOA] 

No assignment in conditional expressions 

[boolasgn] 

CWE-480, Use of Incorrect Operator 


[Dutta 03] “Best Practices for Programming in C” 

[Hatton 1995] 

Section 2.7.2, “Errors of Omission and Addition” 




Expressions (EXP) - EXP46-C. Do not use a bitwise operator with a Boolean-like operand 

4.14 EXP46-C. Do not use a bitwise operator with a Boolean-like 

operand 

Mixing bitwise and relational operators in the same full expression can be a sign of a logic error 

in the expression where a logical operator is usually the intended operator. Do not use the bitwise 

AND (&), bitwise OR (|), or bitwise XOR (^) operators with an operand of type _Bool, or the result 

of a relational-expression or equality-expression. If the bitwise operator is intended, it should 

be indicated with use of a parenthesized expression. 


In this noncompliant code example, a bitwise & operator is used with the results of an equalityexpression: 

if (!(getuid() & geteuid() == 0)) { 

/* ... */ 

} 


This compliant solution uses the && operator for the logical operation within the conditional expression: 

if (!(getuid() && geteuid() == 0)) { 

/* ... */ 

} 



EXP46-C Low Likely Low P9 L2 


ISO/IEC TR 24772:2013 

MITRE CWE 

Likely Incorrect Expression [KOA] 

CWE-480, Use of incorrect operator 


[Hatton 1995] 

Section 2.7.2, “Errors of Omission and Addition” 




Integers (INT) - INT30-C. Ensure that unsigned integer operations do not wrap 

5 Integers (INT) 

5.1 INT30-C. Ensure that unsigned integer operations do not wrap 


A computation involving unsigned operands can never overflow, because a result that 

cannot be represented by the resulting unsigned integer type is reduced modulo the 

number that is one greater than the largest value that can be represented by the resulting 

type. 

This behavior is more informally called unsigned integer wrapping. Unsigned integer operations 

can wrap if the resulting value cannot be represented by the underlying representation of the integer. 

The following table indicates which operators can result in wrapping: 

Operator Wrap Operator Wrap Operator Wrap Operator Wrap 

+ Yes -= Yes > No > No 

* Yes /= No & No >= No 

/ No %= No | No


• Function arguments of type size_t or rsize_t (for example, an argument to a memory allocation 

function) 

• In security-critical code 

The C Standard defines arithmetic on atomic integer types as read-modify-write operations with 

the same representation as regular integer types. As a result, wrapping of atomic unsigned integers 

is identical to regular unsigned integers and should also be prevented or detected. 

5.1.1 Addition 

Addition is between two operands of arithmetic type or between a pointer to an object type and an 

integer type. This rule applies only to addition between two operands of arithmetic type. (See 

ARR37-C. Do not add or subtract an integer to a pointer to a non-array object and ARR30-C. Do 

not form or use out-of-bounds pointers or array subscripts.) 

Incrementing is equivalent to adding 1. 


This noncompliant code example can result in an unsigned integer wrap during the addition of the 

unsigned operands ui_a and ui_b. If this behavior is unexpected, the resulting value may be 

used to allocate insufficient memory for a subsequent operation or in some other manner that can 

lead to an exploitable vulnerability. 

void func(unsigned int ui_a, unsigned int ui_b) { 

unsigned int usum = ui_a + ui_b; 

/* ... */ 

} 

5.1.1.2 Compliant Solution (Precondition Test) 

This compliant solution performs a precondition test of the operands of the addition to guarantee 

there is no possibility of unsigned wrap: 

#include 


unsigned int usum; 

if (UINT_MAX - ui_a < ui_b) { 


} else { 

usum = ui_a + ui_b; 

} 

/* ... */ 

} 





5.1.1.3 Compliant Solution (Postcondition Test) 

This compliant solution performs a postcondition test to ensure that the result of the unsigned addition 

operation usum is not less than the first operand: 


unsigned int usum = ui_a + ui_b; 

if (usum < ui_a) { 


} 

/* ... */ 

} 

5.1.2 Subtraction 

Subtraction is between two operands of arithmetic type, two pointers to qualified or unqualified 

versions of compatible object types, or a pointer to an object type and an integer type. This rule 

applies only to subtraction between two operands of arithmetic type. (See ARR36-C. Do not subtract 

or compare two pointers that do not refer to the same array, ARR37-C. Do not add or subtract 

an integer to a pointer to a non-array object, and ARR30-C. Do not form or use out-ofbounds 

pointers or array subscripts for information about pointer subtraction.) 

Decrementing is equivalent to subtracting 1. 


This noncompliant code example can result in an unsigned integer wrap during the subtraction of 

the unsigned operands ui_a and ui_b. If this behavior is unanticipated, it may lead to an exploitable 

vulnerability. 


unsigned int udiff = ui_a - ui_b; 

/* ... */ 

} 

5.1.2.2 Compliant Solution (Precondition Test) 

This compliant solution performs a precondition test of the unsigned operands of the subtraction 

operation to guarantee there is no possibility of unsigned wrap: 


unsigned int udiff; 

if (ui_a < ui_b){ 


} else { 

udiff = ui_a - ui_b; 

} 





} 

/* ... */ 

5.1.2.3 Compliant Solution (Postcondition Test) 

This compliant solution performs a postcondition test that the result of the unsigned subtraction 

operation udiff is not greater than the minuend: 


unsigned int udiff = ui_a - ui_b; 

if (udiff > ui_a) { 


} 

/* ... */ 

} 

5.1.3 Multiplication 

Multiplication is between two operands of arithmetic type. 


The Mozilla Foundation Security Advisory 2007-01 describes a heap buffer overflow vulnerability 

in the Mozilla Scalable Vector Graphics (SVG) viewer resulting from an unsigned integer 

wrap during the multiplication of the signed int value pen->num_vertices and the size_t 

value sizeof(cairo_pen_vertex_t) [VU#551436]. The signed int operand is converted 

to size_t prior to the multiplication operation so that the multiplication takes place between two 

size_t integers, which are unsigned. (See INT02-C. Understand integer conversion rules.) 

pen->num_vertices = _cairo_pen_vertices_needed( 

gstate->tolerance, radius, &gstate->ctm 

); 

pen->vertices = malloc( 

pen->num_vertices * sizeof(cairo_pen_vertex_t) 

); 

The unsigned integer wrap can result in allocating memory of insufficient size. 


This compliant solution tests the operands of the multiplication to guarantee that there is no unsigned 

integer wrap: 

pen->num_vertices = _cairo_pen_vertices_needed( 

gstate->tolerance, radius, &gstate->ctm 

); 

if (pen->num_vertices > SIZE_MAX / sizeof(cairo_pen_vertex_t)) { 






} 

pen->vertices = malloc( 

pen->num_vertices * sizeof(cairo_pen_vertex_t) 

); 


INT30-C-EX1: Unsigned integers can exhibit modulo behavior (wrapping) when necessary for 

the proper execution of the program. It is recommended that the variable declaration be clearly 

commented as supporting modulo behavior and that each operation on that integer also be clearly 

commented as supporting modulo behavior. 

INT30-C-EX2: Checks for wraparound can be omitted when it can be determined at compile time 

that wraparound will not occur. As such, the following operations on unsigned integers require no 

validation: 

• Operations on two compile-time constants 

• Operations on a variable and 0 (except division or remainder by 0) 

• Subtracting any variable from its type’s maximum; for example, any unsigned int may 

safely be subtracted from UINT_MAX 

• Multiplying any variable by 1 

• Division or remainder, as long as the divisor is nonzero 

• Right-shifting any type maximum by any number no larger than the type precision; for example, 

UINT_MAX >> x is valid as long as 0



CVE-2009-1385 results from a violation of this rule. The value performs an unchecked subtraction 

on the length of a buffer and then adds those many bytes of data to another buffer [xorl 

2009]. This can cause a buffer overflow, which allows an attacker to execute arbitrary code. 

A Linux Kernel vmsplice exploit, described by Rafal Wojtczuk [Wojtczuk 2008], documents a 

vulnerability and exploit arising from a buffer overflow (caused by unsigned integer wrapping). 

Don Bailey [Bailey 2014] describes an unsigned integer wrap vulnerability in the LZO compression 

algorithm, which can be exploited in some implementations. 

CVE-2014-4377 describes a vulnerability in iOS 7.1 resulting from a multiplication operation that 

wraps, producing an insufficiently small value to pass to a memory allocation routine, which is 

subsequently overflowed. 



ISO/IEC TR 24772:2013 

MITRE CWE 

INT02-C. Understand integer conversion rules 

ARR30-C. Do not form or use out-of-bounds 

pointers or array subscripts 

ARR36-C. Do not subtract or compare two 

pointers that do not refer to the same array 

ARR37-C. Do not add or subtract an integer to 

a pointer to a non-array object 

CON08-C. Do not assume that a group of calls 

to independently atomic methods is atomic 

Arithmetic Wrap-Around Error [FIF] 

CWE-190, Integer Overflow or Wraparound 


[Bailey 2014] 

[Dowd 2006] 

[ISO/IEC 9899:2011] 

[Seacord 2013b] 

[Viega 2005] 

[VU#551436] 

[Warren 2002] 

[Wojtczuk 2008] 

[xorl 2009] 

Raising Lazarus - The 20 Year Old Bug that 

Went to Mars 

Chapter 6, “C Language Issues” (“Arithmetic 

Boundary Conditions,” pp. 211–223) 

Subclause 6.2.5, “Types” 

Chapter 5, “Integer Security” 

Section 5.2.7, “Integer Overflow” 

Chapter 2, “Basics” 

“CVE-2009-1385: Linux Kernel E1000 Integer 

Underflow” 




Integers (INT) - INT31-C. Ensure that integer conversions do not result in lost or misinterpreted data 

5.2 INT31-C. Ensure that integer conversions do not result in lost or 

misinterpreted data 

Integer conversions, both implicit and explicit (using a cast), must be guaranteed not to result in 

lost or misinterpreted data. This rule is particularly true for integer values that originate from untrusted 

sources and are used in any of the following ways: 

• Integer operands of any pointer arithmetic, including array indexing 

• The assignment expression for the declaration of a variable length array 

• The postfix expression preceding square brackets [] or the expression in square brackets [] 

of a subscripted designation of an element of an array object 


function) 

This rule also applies to arguments passed to the following library functions that are converted to 

unsigned char: 

• memset() 

• memset_s() 

• fprintf() and related functions (For the length modifier c, if no l length modifier is present, 

the int argument is converted to an unsigned char, and the resulting character is 

written.) 

• fputc() 

• ungetc() 

• memchr() 

and to arguments to the following library functions that are converted to char: 

• strchr() 

• strrchr() 

• All of the functions listed in 

The only integer type conversions that are guaranteed to be safe for all data values and all possible 

conforming implementations are conversions of an integral value to a wider type of the same signedness. 

The C Standard, subclause 6.3.1.3 [ISO/IEC 9899:2011], says 

When a value with integer type is converted to another integer type other than _Bool, if 

the value can be represented by the new type, it is unchanged. 

Otherwise, if the new type is unsigned, the value is converted by repeatedly adding or 

subtracting one more than the maximum value that can be represented in the new type 

until the value is in the range of the new type. 

Otherwise, the new type is signed and the value cannot be represented in it; either the 

result is implementation-defined or an implementation-defined signal is raised. 





Typically, converting an integer to a smaller type results in truncation of the high-order bits. 

5.2.1 Noncompliant Code Example (Unsigned to Signed) 

Type range errors, including loss of data (truncation) and loss of sign (sign errors), can occur 

when converting from a value of an unsigned integer type to a value of a signed integer type. This 

noncompliant code example results in a truncation error on most implementations: 

#include 


unsigned long int u_a = ULONG_MAX; 

signed char sc; 

sc = (signed char)u_a; /* Cast eliminates warning */ 

/* ... */ 

} 

5.2.2 Compliant Solution (Unsigned to Signed) 

Validate ranges when converting from an unsigned type to a signed type. This compliant solution 

can be used to convert a value of unsigned long int type to a value of signed char type: 

#include 




if (u_a


} 

/* ... */ 

5.2.4 Compliant Solution (Signed to Unsigned) 

Validate ranges when converting from a signed type to an unsigned type. This compliant solution 

converts a value of a signed int type to a value of an unsigned int type: 

#include 


signed int si = INT_MIN; 

unsigned int ui; 

if (si < 0) { 


} else { 

ui = (unsigned int)si; /* Cast eliminates warning */ 

} 

/* ... */ 

} 

Subclause 6.2.5, paragraph 9, of the C Standard [ISO/IEC 9899:2011] provides the necessary 

guarantees to ensure this solution works on a conforming implementation: 

The range of nonnegative values of a signed integer type is a subrange of the corresponding 

unsigned integer type, and the representation of the same value in each type 

is the same. 

5.2.5 Noncompliant Code Example (Signed, Loss of Precision) 

A loss of data (truncation) can occur when converting from a value of a signed integer type to a 

value of a signed type with less precision. This noncompliant code example results in a truncation 

error on most implementations: 

#include 


signed long int s_a = LONG_MAX; 

signed char sc = (signed char)s_a; /* Cast eliminates warning */ 

/* ... */ 

} 





5.2.6 Compliant Solution (Signed, Loss of Precision) 

Validate ranges when converting from a signed type to a signed type with less precision. This 

compliant solution converts a value of a signed long int type to a value of a signed char 

type: 

#include 


signed long int s_a = LONG_MAX; 


if ((s_a < SCHAR_MIN) || (s_a > SCHAR_MAX)) { 


} else { 

sc = (signed char)s_a; /* Use cast to eliminate warning */ 

} 

/* ... */ 

} 

Conversions from a value of a signed integer type to a value of a signed integer type with less precision 

requires that both the upper and lower bounds are checked. 

5.2.7 Noncompliant Code Example (Unsigned, Loss of Precision) 

A loss of data (truncation) can occur when converting from a value of an unsigned integer type to 

a value of an unsigned type with less precision. This noncompliant code example results in a truncation 

error on most implementations: 

#include 



unsigned char uc = (unsigned char)u_a; /* Cast eliminates warning 

*/ 

/* ... */ 

} 

5.2.8 Compliant Solution (Unsigned, Loss of Precision) 

Validate ranges when converting a value of an unsigned integer type to a value of an unsigned integer 

type with less precision. This compliant solution converts a value of an unsigned long 

int type to a value of an unsigned char type: 

#include 



unsigned char uc; 





} 

if (u_a > UCHAR_MAX) { 


} else { 

uc = (unsigned char)u_a; /* Cast eliminates warning */ 

} 

/* ... */ 

Conversions from unsigned types with greater precision to unsigned types with less precision require 

only the upper bounds to be checked. 

5.2.9 Noncompliant Code Example (time_t Return Value) 

The time() function returns the value (time_t)(-1) to indicate that the calendar time is not 

available. The C Standard requires that the time_t type is only a real type capable of representing 

time. (The integer and real floating types are collectively called real types.) It is left to the implementor 

to decide the best real type to use to represent time. If time_t is implemented as an 

unsigned integer type with less precision than a signed int, the return value of time() will never 

compare equal to the integer literal -1. 

#include 


time_t now = time(NULL); 

if (now != -1) { 

/* Continue processing */ 

} 

} 

5.2.10 Compliant Solution (time_t Return Value) 

To ensure the comparison is properly performed, the return value of time() should be compared 

against -1 cast to type time_t: 

#include 


time_t now = time(NULL); 

if (now != (time_t)-1) { 

/* Continue processing */ 

} 

} 

This solution is in accordance with INT18-C. Evaluate integer expressions in a larger size before 

comparing or assigning to that size. 





5.2.11 Noncompliant Code Example (memset()) 

For historical reasons, certain C Standard functions accept an argument of type int and convert it 

to either unsigned char or plain char. This conversion can result in unexpected behavior if the 

value cannot be represented in the smaller type. This noncompliant solution unexpectedly clears 

the array: 

#include 

#include 

int *init_memory(int *array, size_t n) { 

return memset(array, 4096, n); 

} 

5.2.12 Compliant Solution (memset()) 

In general, the memset() function should not be used to initialize an integer array unless it is to 

set or clear all the bits, as in this compliant solution: 

#include 

#include 

int *init_memory(int *array, size_t n) { 

return memset(array, 0, n); 

} 


INT31-C-EX1: The C Standard defines minimum ranges for standard integer types. For example, 

the minimum range for an object of type unsigned short int is 0 to 65,535, whereas the minimum 

range for int is −32,767 to +32,767. Consequently, it is not always possible to represent all 

possible values of an unsigned short int as an int. However, on the IA-32 architecture, for 

example, the actual integer range is from −2,147,483,648 to +2,147,483,647, meaning that it is 

quite possible to represent all the values of an unsigned short int as an int for this architecture. 

As a result, it is not necessary to provide a test for this conversion on IA-32. It is not possible 

to make assumptions about conversions without knowing the precision of the underlying types. If 

these tests are not provided, assumptions concerning precision must be clearly documented, as the 

resulting code cannot be safely ported to a system where these assumptions are invalid. A good 

way to document these assumptions is to use static assertions. (See DCL03-C. Use a static assertion 

to test the value of a constant expression.) 

INT31-C-EX2: Conversion from any integer type with a value between SCHAR_MIN and 

UCHAR_MAX to a character type is permitted provided the value represents a character and not an 

integer. 





Conversions to unsigned character types are well defined by C to have modular behavior. A character’s 

value is not misinterpreted by the loss of sign or conversion to a negative number. For example, 

the Euro symbol € is sometimes represented by bit pattern 0x80 which can have the numerical 

value 128 or −127 depending on the signedness of the type. 

Conversions to signed character types are more problematic. The C Standard, subclause 6.3.1.3, 

paragraph 3 [ISO/IEC 9899:2011], says, regarding conversions 

Otherwise, the new type is signed and the value cannot be represented in it; either the 

result is implementation-defined or an implementation-defined signal is raised. 

Furthermore, subclause 6.2.6.2, paragraph 2, says, regarding integer modifications 

If the sign bit is one, the value shall be modified in one of the following ways: 

• the corresponding value with sign bit 0 is negated (sign and magnitude) 

• the sign bit has the value −(2M ) (two’s complement); 

• the sign bit has the value −(2M − 1) (ones’ complement). 

Which of these applies is implementation-defined, as is whether the value with sign bit 1 

and all value bits zero (for the first two), or with sign bit and all value bits 1 (for ones’ complement), 

is a trap representation or a normal value. [See note.] 

NOTE: Two’s complement is shorthand for “radix complement in radix 2.” Ones’ complement is 

shorthand for “diminished radix complement in radix 2.” 

Consequently, the standard allows for this code to trap: 

int i = 128; /* 1000 0000 in binary */ 

assert(SCHAR_MAX == 127); 

signed char c = i; /* can trap */ 

However, platforms where this code traps or produces an unexpected value are rare. According to 

The New C Standard: An Economic and Cultural Commentary by Derek Jones [Jones 2008] 

Implementations with such trap representations are thought to have existed in the past. 

Your author was unable to locate any documents describing such processors. 


Integer truncation errors can lead to buffer overflows and the execution of arbitrary code by an 

attacker. 


INT31-C High Probable High P6 L2 






CVE-2009-1376 results from a violation of this rule. In version 2.5.5 of Pidgin, a size_t offset is 

set to the value of a 64-bit unsigned integer, which can lead to truncation [xorl 2009] on platforms 

where a size_t is implemented as a 32-bit unsigned integer. An attacker can execute arbitrary 

code by carefully choosing this value and causing a buffer overflow. 



DCL03-C. Use a static assertion to test the 

value of a constant expression 

INT18-C. Evaluate integer expressions in a 

larger size before comparing or assigning to 

that size 

FIO34-C. Distinguish between characters read 

from a file and EOF or WEOF 

CERT Oracle Secure Coding Standard for Java NUM12-J. Ensure conversions of numeric 

types to narrower types do not result in lost or 


ISO/IEC TR 24772:2013 

MISRA C:2012 

MITRE CWE 

Numeric Conversion Errors [FLC] 






CWE-192, Integer Coercion Error 

CWE-197, Numeric Truncation Error 

CWE-681, Incorrect Conversion between Numeric 

Types 


[Dowd 2006] 

[ISO/IEC 9899:2011] 

[Jones 2008] 


[Viega 2005] 

Chapter 6, “C Language Issues” (“Type Conversions,” 

pp. 223–270) 

6.3.1.3, “Signed and Unsigned Integers” 

Section 6.2.6.2, “Integer Types” 


Section 5.2.9, “Truncation Error” 

Section 5.2.10, “Sign Extension Error” 

Section 5.2.11, “Signed to Unsigned Conversion 

Error” 

Section 5.2.12, “Unsigned to Signed Conversion 

Error” 





[Warren 2002] 

[xorl 2009] 


“CVE-2009-1376: Pidgin MSN SLP Integer 

Truncation” 




Integers (INT) - INT32-C. Ensure that operations on signed integers do not result in overflow 

5.3 INT32-C. Ensure that operations on signed integers do not result in 

overflow 

Signed integer overflow is undefined behavior 36. Consequently, implementations have considerable 

latitude in how they deal with signed integer overflow. (See MSC15-C. Do not depend on 

undefined behavior.) An implementation that defines signed integer types as being modulo, for 

example, need not detect integer overflow. Implementations may also trap on signed arithmetic 

overflows, or simply assume that overflows will never happen and generate object code accordingly. 

It is also possible for the same conforming implementation to emit code that exhibits different 

behavior in different contexts. For example, an implementation may determine that a signed 

integer loop control variable declared in a local scope cannot overflow and may emit efficient 

code on the basis of that determination, while the same implementation may determine that a 

global variable used in a similar context will wrap. 

For these reasons, it is important to ensure that operations on signed integers do not result in overflow. 

Of particular importance are operations on signed integer values that originate from a 

tainted source and are used as 

• Integer operands of any pointer arithmetic, including array indexing 

• The assignment expression for the declaration of a variable length array 

• The postfix expression preceding square brackets [] or the expression in square brackets [] 

of a subscripted designation of an element of an array object 


function) 

Integer operations will overflow if the resulting value cannot be represented by the underlying 

representation of the integer. The following table indicates which operations can result in overflow. 

Operator Overflow Operator Overflow Operator Overflow Operator Overflow 

+ Yes -= Yes > No > No 

* Yes /= Yes & No >= No 

/ Yes %= Yes | No


rules before trying to implement secure arithmetic operations. (See INT02-C. Understand integer 

conversion rules.) 

5.3.1 Implementation Details 

GNU GCC invoked with the -fwrapv command-line option defines the same modulo arithmetic 

for both unsigned and signed integers. 

GNU GCC invoked with the -ftrapv command-line option causes a trap to be generated when a 

signed integer overflows, which will most likely abnormally exit. On a UNIX system, the result of 

such an event may be a signal sent to the process. 

GNU GCC invoked without either the -fwrapv or the -ftrapv option may simply assume that 

signed integers never overflow and may generate object code accordingly. 

5.3.2 Atomic Integers 

The C Standard defines the behavior of arithmetic on atomic signed integer types to use two’s 

complement representation with silent wraparound on overflow; there are no undefined results. 

Although defined, these results may be unexpected and therefore carry similar risks to unsigned 

integer wrapping. (See INT30-C. Ensure that unsigned integer operations do not wrap.) Consequently, 

signed integer overflow of atomic integer types should also be prevented or detected. 

5.3.3 Addition 

Addition is between two operands of arithmetic type or between a pointer to an object type and an 

integer type. This rule applies only to addition between two operands of arithmetic type. (See 

ARR37-C. Do not add or subtract an integer to a pointer to a non-array object and ARR30-C. Do 

not form or use out-of-bounds pointers or array subscripts.) 

Incrementing is equivalent to adding 1. 


This noncompliant code example can result in a signed integer overflow during the addition of the 

signed operands si_a and si_b: 

void func(signed int si_a, signed int si_b) { 

signed int sum = si_a + si_b; 

/* ... */ 

} 






This compliant solution ensures that the addition operation cannot overflow, regardless of representation: 

#include 

void f(signed int si_a, signed int si_b) { 

signed int sum; 

if (((si_b > 0) && (si_a > (INT_MAX - si_b))) || 

((si_b < 0) && (si_a < (INT_MIN - si_b)))) { 


} else { 

sum = si_a + si_b; 

} 

/* ... */ 

} 

5.3.4 Subtraction 

Subtraction is between two operands of arithmetic type, two pointers to qualified or unqualified 

versions of compatible object types, or a pointer to an object type and an integer type. This rule 

applies only to subtraction between two operands of arithmetic type. (See ARR36-C. Do not subtract 

or compare two pointers that do not refer to the same array, ARR37-C. Do not add or subtract 

an integer to a pointer to a non-array object, and ARR30-C. Do not form or use out-ofbounds 

pointers or array subscripts for information about pointer subtraction.) 

Decrementing is equivalent to subtracting 1. 


This noncompliant code example can result in a signed integer overflow during the subtraction of 

the signed operands si_a and si_b: 


signed int diff = si_a - si_b; 

/* ... */ 

} 


This compliant solution tests the operands of the subtraction to guarantee there is no possibility of 

signed overflow, regardless of representation: 

#include 


signed int diff; 





if ((si_b > 0 && si_a < INT_MIN + si_b) || 

(si_b < 0 && si_a > INT_MAX + si_b)) { 


} else { 

diff = si_a - si_b; 

} 

} 

/* ... */ 

5.3.5 Multiplication 

Multiplication is between two operands of arithmetic type. 


This noncompliant code example can result in a signed integer overflow during the multiplication 

of the signed operands si_a and si_b: 


signed int result = si_a * si_b; 

/* ... */ 

} 


The product of two operands can always be represented using twice the number of bits than exist 

in the precision of the larger of the two operands. This compliant solution eliminates signed overflow 

on systems where long is at least twice the precision of int: 

#include 

#include 

#include 

#include 

extern size_t popcount(uintmax_t); 

#define PRECISION(umax_value) popcount(umax_value) 


signed int result; 

signed long tmp; 

assert(PRECISION(ULLONG_MAX) >= 2 * PRECISION(UINT_MAX)); 

tmp = (signed long long)si_a * (signed long long)si_b; 

/* 

* If the product cannot be represented as a 32-bit integer, 

* handle as an error condition. 

*/ 

if ((tmp > INT_MAX) || (tmp < INT_MIN)) { 





} 


} else { 

result = (int)tmp; 

} 

/* ... */ 

The assertion fails if long long has less than twice the precision of int. The PRECISION() 

macro and popcount()function provide the correct precision for any integer type. (See INT35- 

C. Use correct integer precisions.) 


The following portable compliant solution can be used with any conforming implementation, including 

those that do not have an integer type that is at least twice the precision of int: 

#include 


signed int result; 

if (si_a > 0) { /* si_a is positive */ 

if (si_b > 0) { /* si_a and si_b are positive */ 

if (si_a > (INT_MAX / si_b)) { 


} 

} else { /* si_a positive, si_b nonpositive */ 

if (si_b < (INT_MIN / si_a)) { 


} 

} /* si_a positive, si_b nonpositive */ 

} else { /* si_a is nonpositive */ 

if (si_b > 0) { /* si_a is nonpositive, si_b is positive */ 

if (si_a < (INT_MIN / si_b)) { 


} 

} else { /* si_a and si_b are nonpositive */ 

if ( (si_a != 0) && (si_b < (INT_MAX / si_a))) { 


} 

} /* End if si_a and si_b are nonpositive */ 

} /* End if si_a is nonpositive */ 

} 

result = si_a * si_b; 





5.3.6 Division 

Division is between two operands of arithmetic type. Overflow can occur during two’s complement 

signed integer division when the dividend is equal to the minimum (negative) value for the 

signed integer type and the divisor is equal to −1. Division operations are also susceptible to divide-by-zero 

errors. (See INT33-C. Ensure that division and remainder operations do not result in 

divide-by-zero errors.) 


This noncompliant code example prevents divide-by-zero errors in compliance with INT33-C. 

Ensure that division and remainder operations do not result in divide-by-zero errors but does not 

prevent a signed integer overflow error in two’s-complement. 

void func(signed long s_a, signed long s_b) { 

signed long result; 

if (s_b == 0) { 


} else { 

result = s_a / s_b; 

} 

/* ... */ 

} 


On the x86-32 architecture, overflow results in a fault, which can be exploited as a denial-of-service 

attack. 


This compliant solution eliminates the possibility of divide-by-zero errors or signed overflow: 

#include 



if ((s_b == 0) || ((s_a == LONG_MIN) && (s_b == -1))) { 


} else { 


} 

/* ... */ 

} 

5.3.7 Remainder 

The remainder operator provides the remainder when two operands of integer type are divided. 

Because many platforms implement remainder and division in the same instruction, the remainder 





operator is also susceptible to arithmetic overflow and division by zero. (See INT33-C. Ensure 

that division and remainder operations do not result in divide-by-zero errors.) 


Many hardware architectures implement remainder as part of the division operator, which can 

overflow. Overflow can occur during a remainder operation when the dividend is equal to the 

minimum (negative) value for the signed integer type and the divisor is equal to −1. It occurs even 

though the result of such a remainder operation is mathematically 0. This noncompliant code example 

prevents divide-by-zero errors in compliance with INT33-C. Ensure that division and remainder 

operations do not result in divide-by-zero errors but does not prevent integer overflow: 



if (s_b == 0) { 


} else { 

result = s_a % s_b; 

} 

/* ... */ 

} 


On x86-32 platforms, the remainder operator for signed integers is implemented by the idiv instruction 

code, along with the divide operator. Because LONG_MIN / −1 overflows, it results in a 

software exception with LONG_MIN % −1 as well. 


This compliant solution also tests the remainder operands to guarantee there is no possibility of an 

overflow: 

#include 



if ((s_b == 0 ) || ((s_a == LONG_MIN) && (s_b == -1))) { 


} else { 


} 

/* ... */ 

} 





5.3.8 Left-Shift Operator 

The left-shift operator takes two integer operands. The result of E1 = PRECISION(ULONG_MAX)) { 


} else { 

result = si_a




void func(signed long si_a, signed long si_b) { 


if ((si_a < 0) || (si_b < 0) || 

(si_b >= PRECISION(ULONG_MAX)) || 

(si_a > (LONG_MAX >> si_b))) { 


} else { 

result = si_a



Integer overflow can lead to buffer overflows and the execution of arbitrary code by an attacker. 


INT32-C High Likely High P9 L2 



INT02-C. Understand integer conversion rules 

INT35-C. Use correct integer precisions 

INT33-C. Ensure that division and remainder 

operations do not result in divide-by-zero errors 

INT34-C. Do not shift an expression by a negative 

number of bits or by greater than or equal 

to the number of bits that exist in the operand 



ARR36-C. Do not subtract or compare two 

pointers that do not refer to the same array 



MSC15-C. Do not depend on undefined behavior 

CON08-C. Do not assume that a group of calls 

to independently atomic methods is atomic 

CERT Oracle Secure Coding Standard for Java INT00-J. Perform explicit range checking to 

avoid integer overflow 

ISO/IEC TR 24772:2013 


MITRE CWE 


Overflowing signed integers [intoflow] 

CWE-129, Improper Validation of Array Index 



[Dowd 2006] 

[ISO/IEC 9899:2011] 


[Viega 2005] 

[Warren 2002] 

Chapter 6, “C Language Issues” (“Arithmetic 

Boundary Conditions,” pp. 211–223) 

Subclause 6.5.5, “Multiplicative Operators” 







Integers (INT) - INT33-C. Ensure that division and remainder operations do not result in divide-by-zero errors 

5.4 INT33-C. Ensure that division and remainder operations do not 

result in divide-by-zero errors 

The C Standard identifies the following condition under which division and remainder operations 

result in undefined behavior (UB): 

UB 

Description 

45 The value of the second operand of the / or % operator 

is zero (6.5.5). 

Ensure that division and remainder operations do not result in divide-by-zero errors. 

5.4.1 Division 

The result of the / operator is the quotient from the division of the first arithmetic operand by the 

second arithmetic operand. Division operations are susceptible to divide-by-zero errors. Overflow 

can also occur during two’s complement signed integer division when the dividend is equal to the 

minimum (most negative) value for the signed integer type and the divisor is equal to −1. (See 

INT32-C. Ensure that operations on signed integers do not result in overflow.) 


This noncompliant code example prevents signed integer overflow in compliance with INT32-C. 

Ensure that operations on signed integers do not result in overflow but fails to prevent a divideby-zero 

error during the division of the signed operands s_a and s_b: 

#include 



if ((s_a == LONG_MIN) && (s_b == -1)) { 


} else { 


} 

/* ... */ 

} 


This compliant solution tests the division operation to guarantee there is no possibility of divideby-zero 

errors or signed overflow: 

#include 



if ((s_b == 0) || ((s_a == LONG_MIN) && (s_b == -1))) { 






} 

} else { 


} 

/* ... */ 

5.4.2 Remainder 

The remainder operator provides the remainder when two operands of integer type are divided. 


This noncompliant code example prevents signed integer overflow in compliance with INT32-C. 

Ensure that operations on signed integers do not result in overflow but fails to prevent a divideby-zero 

error during the remainder operation on the signed operands s_a and s_b: 

#include 



if ((s_a == LONG_MIN) && (s_b == -1)) { 


} else { 


} 

/* ... */ 

} 


This compliant solution tests the remainder operand to guarantee there is no possibility of a divide-by-zero 

error or an overflow error: 

#include 



if ((s_b == 0 ) || ((s_a == LONG_MIN) && (s_b == -1))) { 


} else { 


} 

/* ... */ 

} 






A divide-by-zero error can result in abnormal program termination and denial of service. 


INT33-C Low Likely Medium P6 L2 



INT32-C. Ensure that operations on signed integers 

do not result in overflow 

CERT Oracle Secure Coding Standard for Java NUM02-J. Ensure that division and remainder 

operations do not result in divide-by-zero errors 


MITRE CWE 

Integer division errors [diverr] 

CWE-369, Divide By Zero 



[Warren 2002] 






Integers (INT) - INT34-C. Do not shift an expression by a negative number of bits or by greater than or equal to the number 

of bits that exist in the operand 

5.5 INT34-C. Do not shift an expression by a negative number of bits or 

by greater than or equal to the number of bits that exist in the 

operand 

Bitwise shifts include left-shift operations of the form shift-expressionadditive-expression. The standard integer promotions 

are first performed on the operands, each of which has an integer type. The type of the 

result is that of the promoted left operand. If the value of the right operand is negative or is greater 

than or equal to the width of the promoted left operand, the behavior is undefined. (See undefined 

behavior 51.) 

Do not shift an expression by a negative number of bits or by a number greater than or equal to 

the precision of the promoted left operand. The precision of an integer type is the number of bits it 

uses to represent values, excluding any sign and padding bits. For unsigned integer types, the 

width and the precision are the same; whereas for signed integer types, the width is one greater 

than the precision. This rule uses precision instead of width because, in almost every case, an attempt 

to shift by a number of bits greater than or equal to the precision of the operand indicates a 

bug (logic error). A logic error is different from overflow, in which there is simply a representational 

deficiency. In general, shifts should be performed only on unsigned operands. (See INT13- 

C. Use bitwise operators only on unsigned operands.) 

5.5.1 Noncompliant Code Example (Left Shift, Unsigned Type) 

The result of E1



} 

/* ... */ 

5.5.2 Compliant Solution (Left Shift, Unsigned Type) 

This compliant solution eliminates the possibility of shifting by greater than or equal to the number 

of bits that exist in the precision of the left operand: 

#include 

#include 

#include 


#define PRECISION(x) popcount(x) 


unsigned int uresult = 0; 

if (ui_b >= PRECISION(UINT_MAX)) { 


} else { 

uresult = ui_a



} 

if (si_a > (LONG_MAX >> si_b)) { 


} else { 

result = si_a = PRECISION(ULONG_MAX)) || 

(si_a > (LONG_MAX >> si_b))) { 


} else { 

result = si_a > E2 is E1 right-shifted E2 bit positions. If E1 has an unsigned type or if E1 

has a signed type and a nonnegative value, the value of the result is the integral part of the quotient 

of E1 / 2E2. If E1 has a signed type and a negative value, the resulting value is implementation-defined 

and can be either an arithmetic (signed) shift 






or a logical (unsigned) shift 

This noncompliant code example fails to test whether the right operand is greater than or equal to 

the precision of the promoted left operand, allowing undefined behavior: 


unsigned int uresult = ui_a >> ui_b; 

/* ... */ 

} 

When working with signed operands, making assumptions about whether a right shift is implemented 

as an arithmetic (signed) shift or a logical (unsigned) shift can also lead to vulnerabilities. 

(See INT13-C. Use bitwise operators only on unsigned operands.) 

5.5.6 Compliant Solution (Right Shift) 

This compliant solution eliminates the possibility of shifting by greater than or equal to the number 

of bits that exist in the precision of the left operand: 

#include 

#include 

#include 


#define PRECISION(x) popcount(x) 







unsigned int uresult = 0; 

if (ui_b >= PRECISION(UINT_MAX)) { 


} else { 

uresult = ui_a >> ui_b; 

} 

/* ... */ 

} 


GCC has no options to handle shifts by negative amounts or by amounts outside the width of the 

type predictably or to trap on them; they are always treated as undefined. Processors may reduce 

the shift amount modulo the width of the type. For example, 32-bit right shifts are implemented 

using the following instruction on x86-32: 

sarl 

%cl, %eax 

The sarl instruction takes a bit mask of the least significant 5 bits from %cl to produce a value 

in the range [0, 31] and then shift %eax that many bits: 

// 64-bit right shifts on IA-32 platforms become 

shrdl %edx, %eax 

sarl %cl, %edx 

where %eax stores the least significant bits in the doubleword to be shifted, and %edx stores the 

most significant bits. 


Although shifting a negative number of bits or shifting a number of bits greater than or equal to 

the width of the promoted left operand is undefined behavior in C, the risk is generally low because 

processors frequently reduce the shift amount modulo the width of the type. 


INT34-C Low Unlikely Medium P2 L3 



ISO/IEC TR 24772:2013 

INT13-C. Use bitwise operators only on unsigned 

operands 

INT35-C. Use correct integer precisions 










[C99 Rationale 2003] 6.5.7, “Bitwise Shift Operators” 

[Dowd 2006] 

Chapter 6, “C Language Issues” 



[Viega 2005] 





Integers (INT) - INT35-C. Use correct integer precisions 

5.6 INT35-C. Use correct integer precisions 

Integer types in C have both a size and a precision. The size indicates the number of bytes used by 

an object and can be retrieved for any object or type using the sizeof operator. The precision of 

an integer type is the number of bits it uses to represent values, excluding any sign and padding 

bits. 

Padding bits contribute to the integer’s size, but not to its precision. Consequently, inferring the 

precision of an integer type from its size may result in too large a value, which can then lead to 

incorrect assumptions about the numeric range of these types. Programmers should use correct 

integer precisions in their code, and in particular, should not use the sizeof operator to compute 

the precision of an integer type on architectures that use padding bits or in strictly conforming 

(that is, portable) programs. 


This noncompliant code example illustrates a function that produces 2 raised to the power of the 

function argument. To prevent undefined behavior in compliance with INT34-C. Do not shift an 

expression by a negative number of bits or by greater than or equal to the number of bits that exist 

in the operand, the function ensures that the argument is less than the number of bits used to store 

a value of type unsigned int. 

#include 

unsigned int pow2(unsigned int exp) { 

if (exp >= sizeof(unsigned int) * CHAR_BIT) { 


} 

return 1


size_t precision = 0; 

while (num != 0) { 

if (num % 2 == 1) { 

precision++; 

} 

num >>= 1; 

} 

return precision; 

} 


Implementations can replace the PRECISION() macro with a type-generic macro that returns an 

integer constant expression that is the precision of the specified type for that implementation. This 

return value can then be used anywhere an integer constant expression can be used, such as in a 

static assertion. (See DCL03-C. Use a static assertion to test the value of a constant expression.) 

The following type generic macro, for example, might be used for a specific implementation targeting 

the IA-32 architecture: 

#define PRECISION(value) _Generic(value, \ 

unsigned char : 8, \ 

unsigned short: 16, \ 

unsigned int : 32, \ 

unsigned long : 32, \ 

unsigned long long : 64, \ 

signed char : 7, \ 

signed short : 15, \ 

signed int : 31, \ 

signed long : 31, \ 

signed long long : 63) 

The revised version of the pow2() function uses the PRECISION() macro to determine the precision 

of the unsigned type: 

#include 

#include 

#include 



unsigned int pow2(unsigned int exp) { 

if (exp >= PRECISION(UINT_MAX)) { 


} 

return 1



Some platforms, such as the Cray Linux Environment (CLE; supported on Cray XT CNL compute 

nodes), provide a _popcnt instruction that can substitute for the popcount() function. 

#define PRECISION(umax_value) _popcnt(umax_value) 


Mistaking an integer’s size for its precision can permit invalid precision arguments to operations 

such as bitwise shifts, resulting in undefined behavior. 


INT35-C Low Unlikely Medium P2 L3 


MITRE CWE 



[Dowd 2006] 


[C99 Rationale 2003] 6.5.7, “Bitwise Shift Operators” 




Integers (INT) - INT36-C. Converting a pointer to integer or integer to pointer 

5.7 INT36-C. Converting a pointer to integer or integer to pointer 

Although programmers often use integers and pointers interchangeably in C, pointer-to-integer 

and integer-to-pointer conversions are implementation-defined. 

Conversions between integers and pointers can have undesired consequences depending on the 

implementation. According to the C Standard, subclause 6.3.2.3 [ISO/IEC 9899:2011], 

An integer may be converted to any pointer type. Except as previously specified, the result 

is implementation-defined, might not be correctly aligned, might not point to an entity 

of the referenced type, and might be a trap representation. 

Any pointer type may be converted to an integer type. Except as previously specified, 

the result is implementation-defined. If the result cannot be represented in the integer 

type, the behavior is undefined. The result need not be in the range of values of any integer 

type. 

Do not convert an integer type to a pointer type if the resulting pointer is incorrectly aligned, does 

not point to an entity of the referenced type, or is a trap representation. 

Do not convert a pointer type to an integer type if the result cannot be represented in the integer 

type. (See undefined behavior 24.) 

The mapping between pointers and integers must be consistent with the addressing structure of 

the execution environment. Issues may arise, for example, on architectures that have a segmented 

memory model. 


The size of a pointer can be greater than the size of an integer, such as in an implementation 

where pointers are 64 bits and unsigned integers are 32 bits. This code example is noncompliant 

on such implementations because the result of converting the 64-bit ptr cannot be represented in 

the 32-bit integer type: 


char *ptr; 

/* ... */ 

unsigned int number = (unsigned int)ptr; 

/* ... */ 

} 


Any valid pointer to void can be converted to intptr_t or uintptr_t and back with no 

change in value. (See INT36-EX2.) The C Standard guarantees that a pointer to void may be 

converted to or from a pointer to any object type and back again and that the result must compare 





equal to the original pointer. Consequently, converting directly from a char * pointer to a 

uintptr_t, as in this compliant solution, is allowed on implementations that support the 

uintptr_t type. 

#include 


char *ptr; 

/* ... */ 

uintptr_t number = (uintptr_t)ptr; 

/* ... */ 

} 


In this noncompliant code example, the pointer ptr is converted to an integer value. The highorder 

9 bits of the number are used to hold a flag value, and the result is converted back into a 

pointer. This example is noncompliant on an implementation where pointers are 64 bits and unsigned 

integers are 32 bits because the result of converting the 64-bit ptr cannot be represented in 

the 32-bit integer type. 

void func(unsigned int flag) { 

char *ptr; 

/* ... */ 

unsigned int number = (unsigned int)ptr; 

number = (number & 0x7fffff) | (flag


} 

ptrflag.flag = flag; 


It is sometimes necessary to access memory at a specific location, requiring a literal integer to 

pointer conversion. In this noncompliant code, a pointer is set directly to an integer constant, 

where it is unknown whether the result will be as intended: 

unsigned int *g(void) { 

unsigned int *ptr = 0xdeadbeef; 

/* ... */ 

return ptr; 

} 

The result of this assignment is implementation-defined, might not be correctly aligned, might not 

point to an entity of the referenced type, and might be a trap representation. 


Adding an explicit cast may help the compiler convert the integer value into a valid pointer. A 

common technique is to assign the integer to a volatile-qualified object of type intptr_t or 

uintptr_t and then assign the integer value to the pointer: 

unsigned int *g(void) { 

volatile uintptr_t iptr = 0xdeadbeef; 

unsigned int *ptr = (unsigned int *)iptr; 

/* ... */ 

return ptr; 

} 


INT36-C-EX1: A null pointer can be converted to an integer; it takes on the value 0. Likewise, 

the integer value 0 can be converted to a pointer; it becomes the null pointer. 

INT36-C-EX2: Any valid pointer to void can be converted to intptr_t or uintptr_t or their 

underlying types and back again with no change in value. Use of underlying types instead of 

intptr_t or uintptr_t is discouraged, however, because it limits portability. 

#include 

#include 

void h(void) { 

intptr_t i = (intptr_t)(void *)&i; 

uintptr_t j = (uintptr_t)(void *)&j; 





void *ip = (void *)i; 

void *jp = (void *)j; 

} 

assert(ip == &i); 

assert(jp == &j); 


Converting from pointer to integer or vice versa results in code that is not portable and may create 

unexpected pointers to invalid memory locations. 


INT36-C Low Probable High P2 L3 



ISO/IEC TR 24772:2013 

ISO/IEC TS 17961:2013 

MITRE CWE 

INT11-CPP. Take care when converting from 

pointer to integer or integer to pointer 


[HFC] 

Converting a pointer to integer or integer to 

pointer [intptrconv] 

CWE-466, Return of Pointer Value Outside of 

Expected Range 

CWE-587, Assignment of a Fixed Address to a 

Pointer 


[ISO/IEC 9899:2011] 





Floating Point (FLP) - FLP30-C. Do not use floating-point variables as loop counters 

6 Floating Point (FLP) 

6.1 FLP30-C. Do not use floating-point variables as loop counters 

Because floating-point numbers represent real numbers, it is often mistakenly assumed that they 

can represent any simple fraction exactly. Floating-point numbers are subject to representational 

limitations just as integers are, and binary floating-point numbers cannot represent all real numbers 

exactly, even if they can be represented in a small number of decimal digits. 

In addition, because floating-point numbers can represent large values, it is often mistakenly assumed 

that they can represent all significant digits of those values. To gain a large dynamic range, 

floating-point numbers maintain a fixed number of precision bits (also called the significand) and 

an exponent, which limit the number of significant digits they can represent. 

Different implementations have different precision limitations, and to keep code portable, floating-point 

variables must not be used as the loop induction variable. 


In this noncompliant code example, a floating-point variable is used as a loop counter. The decimal 

number 0.1 is a repeating fraction in binary and cannot be exactly represented as a binary 

floating-point number. Depending on the implementation, the loop may iterate 9 or 10 times. 


for (float x = 0.1f; x


} 

} 


In this noncompliant code example, a floating-point loop counter is incremented by an amount 

that is too small to change its value given its precision: 


for (float x = 100000001.0f; x



[Lockheed Martin 05] AV Rule 197 




Floating Point (FLP) - FLP32-C. Prevent or detect domain and range errors in math functions 

6.2 FLP32-C. Prevent or detect domain and range errors in math 

functions 

The C Standard, 7.12.1 [ISO/IEC 9899:2011], defines three types of errors that relate specifically 

to math functions in . Paragraph 2 states 

A domain error occurs if an input argument is outside the domain over which the mathematical 

function is defined. 

Paragraph 3 states 

A pole error (also known as a singularity or infinitary) occurs if the mathematical function 

has an exact infinite result as the finite input argument(s) are approached in the limit. 

Paragraph 4 states 

A range error occurs if the mathematical result of the function cannot be represented in 

an object of the specified type, due to extreme magnitude. 

An example of a domain error is the square root of a negative number, such as sqrt(-1.0), 

which has no meaning in real arithmetic. Contrastingly, 10 raised to the 1-millionth power, 

pow(10., 1e6), cannot be represented in many floating-point implementations because of the 

limited range of the type double and consequently constitutes a range error. In both cases, the 

function will return some value, but the value returned is not the correct result of the computation. 

An example of a pole error is log(0.0), which results in negative infinity. 

Programmers can prevent domain and pole errors by carefully bounds-checking the arguments before 

calling mathematical functions and taking alternative action if the bounds are violated. 

Range errors usually cannot be prevented because they are dependent on the implementation of 

floating-point numbers as well as on the function being applied. Instead of preventing range errors, 

programmers should attempt to detect them and take alternative action if a range error occurs. 

The following table lists the double forms of standard mathematical functions, along with checks 

that should be performed to ensure a proper input domain, and indicates whether they can also result 

in range or pole errors, as reported by the C Standard. Both float and long double forms 

of these functions also exist but are omitted from the table for brevity. If a function has a specific 

domain over which it is defined, the programmer must check its input values. The programmer 

must also check for range errors where they might occur. The standard math functions not listed 

in this table, such as fabs(), have no domain restrictions and cannot result in range or pole errors. 





Function Domain Range Pole 

acos(x) -1 0 || (x == 0 && y > 0) || 

(x < 0 && y is an integer) 

sqrt(x) x >= 0 No No 

erf(x) None Yes No 

erfc(x) None Yes No 

lgamma(x), 

tgamma(x) 

Yes 

Yes 

x != 0 &&! (x < 0 && x is an integer) Yes Yes 

lrint(x), lround(x) None Yes No 

fmod(x, y), 

remainder(x, y), 

remquo(x, y, quo) 

nextafter(x, y), 

nexttoward(x, y) 

y != 0 Yes No 

None Yes No 

fdim(x,y) None Yes No 

fma(x,y,z) None Yes No 





6.2.1 Domain and Pole Checking 

The most reliable way to handle domain and pole errors is to prevent them by checking arguments 

beforehand, as in the following exemplar: 

double safe_sqrt(double x) { 

if (x < 0) { 

fprintf(stderr, "sqrt requires a nonnegative argument"); 

/* Handle domain / pole error */ 

} 

return sqrt (x); 

} 

6.2.2 Range Checking 

Programmers usually cannot prevent range errors, so the most reliable way to handle them is to 

detect when they have occurred and act accordingly. 

The exact treatment of error conditions from math functions is tedious. The C Standard, 7.12.1 

[ISO/IEC 9899:2011], defines the following behavior for floating-point overflow: 

A floating result overflows if the magnitude of the mathematical result is finite but so 

large that the mathematical result cannot be represented without extraordinary roundoff 

error in an object of the specified type. If a floating result overflows and default rounding 

is in effect, then the function returns the value of the macro HUGE_VAL, HUGE_VALF, or 

HUGE_VALL according to the return type, with the same sign as the correct value of the 

function; if the integer expression math_errhandling & MATH_ERRNO is nonzero, the 

integer expression errno acquires the value ERANGE; if the integer expression 

math_errhandling & MATH_ERREXCEPT is nonzero, the “overflow” floating-point exception 

is raised. 

It is preferable not to check for errors by comparing the returned value against HUGE_VAL or 0 for 

several reasons: 

• These are, in general, valid (albeit unlikely) data values. 

• Making such tests requires detailed knowledge of the various error returns for each math 

function. 

• Multiple results aside from HUGE_VAL and 0 are possible, and programmers must know 

which are possible in each case. 

• Different versions of the library have varied in their error-return behavior. 

It can be unreliable to check for math errors using errno because an implementation might not 

set errno. For real functions, the programmer determines if the implementation sets errno by 

checking whether math_errhandling & MATH_ERRNO is nonzero. For complex functions, the 





C Standard, 7.3.2, paragraph 1, simply states that “an implementation may set errno but is not 

required to” [ISO/IEC 9899:2011]. 

The obsolete System V Interface Definition (SVID3) [UNIX 1992] provides more control over the 

treatment of errors in the math library. The programmer can define a function named matherr() 

that is invoked if errors occur in a math function. This function can print diagnostics, terminate 

the execution, or specify the desired return value. The matherr() function has not been adopted 

by C or POSIX, so it is not generally portable. 

The following error-handing template uses C Standard functions for floating-point errors when 

the C macro math_errhandling is defined and indicates that they should be used; otherwise, it 

examines errno: 

#include 

#include 

#include 

/* ... */ 

/* Use to call a math function and check errors */ 

{ 

#pragma STDC FENV_ACCESS ON 

if (math_errhandling & MATH_ERREXCEPT) { 

feclearexcept(FE_ALL_EXCEPT); 

} 

errno = 0; 

/* Call the math function */ 

} 

if ((math_errhandling & MATH_ERRNO) && errno != 0) { 

/* Handle range error */ 

} else if ((math_errhandling & MATH_ERREXCEPT) && 

fetestexcept(FE_INVALID | FE_DIVBYZERO | 

FE_OVERFLOW | FE_UNDERFLOW) != 0) { 


} 

See FLP03-C. Detect and handle floating-point errors for more details on how to detect floatingpoint 

errors. 

6.2.3 Subnormal Numbers 

A subnormal number is a nonzero number that does not use all of its precision bits [IEEE 754 

2006]. These numbers can be used to represent values that are closer to 0 than the smallest normal 

number (one that uses all of its precision bits). However, the asin(), asinh(), atan(), 

atanh(), and erf() functions may produce range errors, specifically when passed a subnormal 





number. When evaluated with a subnormal number, these functions can produce an inexact, subnormal 

value, which is an underflow error. The C Standard, 7.12.1, paragraph 6 [ISO/IEC 

9899:2011], defines the following behavior for floating-point underflow: 

The result underflows if the magnitude of the mathematical result is so small that the 

mathematical result cannot be represented, without extraordinary roundoff error, in an 

object of the specified type. If the result underflows, the function returns an implementation-defined 

value whose magnitude is no greater than the smallest normalized positive 

number in the specified type; if the integer expression math_errhandling & 

MATH_ERRNO is nonzero, whether errno acquires the value ERANGE is implementation-defined; 

if the integer expression math_errhandling & MATH_ERREXCEPT is nonzero, 

whether the ‘‘underflow’’ floating-point exception is raised is implementation-defined. 

Implementations that support floating-point arithmetic but do not support subnormal numbers, 

such as IBM S/360 hex floating-point or nonconforming IEEE-754 implementations that skip subnormals 

(or support them by flushing them to zero), can return a range error when calling one of 

the following families of functions with the following arguments: 

• fmod((min+subnorm), min) 

• remainder((min+subnorm), min) 

• remquo((min+subnorm), min, quo) 

where min is the minimum value for the corresponding floating point type and subnorm is a subnormal 

value. 

If Annex F is supported and subnormal results are supported, the returned value is exact and a 

range error cannot occur. The C Standard, F.10.7.1 [ISO/IEC 9899:2011], specifies the following 

for the fmod(), remainder(), and remquo() functions: 

When subnormal results are supported, the returned value is exact and is independent 

of the current rounding direction mode. 

Annex F, subclause F.10.7.2, paragraph 2, and subclause F.10.7.3, paragraph 2, of the C Standard 

identify when subnormal results are supported. 

6.2.4 Noncompliant Code Example (sqrt()) 

This noncompliant code example determines the square root of x: 

#include 

void func(double x) { 

double result; 





} 

result = sqrt(x); 

However, this code may produce a domain error if x is negative. 

6.2.5 Compliant Solution (sqrt()) 

Because this function has domain errors but no range errors, bounds checking can be used to prevent 

domain errors: 

#include 



if (isless(x, 0.0)) { 

/* Handle domain error */ 

} 

} 

result = sqrt(x); 

6.2.6 Noncompliant Code Example (sinh(), Range Errors) 

This noncompliant code example determines the hyperbolic sine of x: 

#include 



result = sinh(x); 

} 

This code may produce a range error if x has a very large magnitude. 

6.2.7 Compliant Solution (sinh(), Range Errors) 

Because this function has no domain errors but may have range errors, the programmer must detect 

a range error and act accordingly: 

#include 

#include 

#include 



{ 








} 

errno = 0; 

result = sinh(x); 

} 







} 

} 

/* Use result... */ 

6.2.8 Noncompliant Code Example (pow()) 

This noncompliant code example raises x to the power of y: 

#include 

void func(double x, double y) { 


result = pow(x, y); 

} 

This code may produce a domain error if x is negative and y is not an integer value or if x is 0 and 

y is 0. A domain error or pole error may occur if x is 0 and y is negative, and a range error may 

occur if the result cannot be represented as a double. 

6.2.9 Compliant Solution (pow()) 

Because the pow() function can produce domain errors, pole errors, and range errors, the programmer 

must first check that x and y lie within the proper domain and do not generate a pole error 

and then detect whether a range error occurs and act accordingly: 

#include 

#include 

#include 

void func(double x, double y) { 


if (((x == 0.0f) && islessequal(y, 0.0)) || isless(x, 0.0)) { 

/* Handle domain or pole error */ 





} 

{ 




} 

errno = 0; 

result = pow(x, y); 

} 







} 

} 


6.2.10 Noncompliant Code Example (asin(), Subnormal Number) 

This noncompliant code example determines the inverse sine of x: 

#include 

void func(float x) { 

float result = asin(x); 

/* ... */ 

} 

6.2.11 Compliant Solution (asin(), Subnormal Number) 

Because this function has no domain errors but may have range errors, the programmer must detect 

a range error and act accordingly: 

#include 

#include 

#include 

void func(float x) { 

float result; 

{ 








} 

errno = 0; 

result = asin(x); 

} 







} 

} 



Failure to prevent or detect domain and range errors in math functions may cause unexpected results. 


FLP32-C Medium Probable Medium P8 L2 



MITRE CWE 

FLP03-C. Detect and handle floating-point errors 

CWE-682, Incorrect Calculation 


[ISO/IEC 9899:2011] 

7.3.2, “Conventions” 

7.12.1, “Treatment of Error Conditions” 

F.10.7, “Remainder Functions” 

[IEEE 754 2006] 

[Plum 1985] Rule 2-2 

[Plum 1989] 

Topic 2.10, “conv—Conversions and Overflow” 

[UNIX 1992] 

System V Interface Definition (SVID3) 




Floating Point (FLP) - FLP34-C. Ensure that floating-point conversions are within range of the new type 

6.3 FLP34-C. Ensure that floating-point conversions are within range 

of the new type 

If a floating-point value is to be converted to a floating-point value of a smaller range and precision 

or to an integer type, or if an integer type is to be converted to a floating-point type, the value 

must be representable in the destination type. 

The C Standard, 6.3.1.4, paragraph 1 [ISO/IEC 9899:2011], says, 

When a finite value of real floating type is converted to an integer type other than _Bool, 

the fractional part is discarded (i.e., the value is truncated toward zero). If the value of 

the integral part cannot be represented by the integer type, the behavior is undefined. 

Paragraph 2 of the same subclause says, 

When a value of integer type is converted to a real floating type, if the value being converted 

can be represented exactly in the new type, it is unchanged. If the value being 

converted is in the range of values that can be represented but cannot be represented 

exactly, the result is either the nearest higher or nearest lower representable value, chosen 

in an implementation-defined manner. If the value being converted is outside the 

range of values that can be represented, the behavior is undefined. 

And subclause 6.3.1.5, paragraph 1, says, 

When a value of real floating type is converted to a real floating type, if the value being 

converted can be represented exactly in the new type, it is unchanged. If the value being 




range of values that can be represented, the behavior is undefined. 

See undefined behaviors 17 and 18. 

This rule does not apply to demotions of floating-point types on implementations that support 

signed infinity, such as IEEE 754, as all values are within range. 

6.3.1 Noncompliant Code Example (float to int) 

This noncompliant code example leads to undefined behavior if the integral part of f_a cannot be 

represented as an integer: 

void func(float f_a) { 

int i_a; 

/* Undefined if the integral part of f_a cannot be represented. 

*/ 





} 

i_a = f_a; 

6.3.2 Compliant Solution (float to int) 

This compliant solution tests to ensure that the float value will fit within the int variable before 

performing the assignment. 

#include 

#include 

#include 

#include 

#include 

extern size_t popcount(uintmax_t); /* See INT35-C */ 


void func(float f_a) { 

int i_a; 

} 

if (PRECISION(INT_MAX) < log2f(fabsf(f_a)) || 

(f_a != 0.0F && fabsf(f_a) < FLT_MIN)) { 


} else { 

i_a = f_a; 

} 

6.3.3 Noncompliant Code Example (Narrowing Conversion) 

This noncompliant code example attempts to perform conversions that may result in truncating 

values outside the range of the destination types: 

void func(double d_a, long double big_d) { 

double d_b = (float)big_d; 

float f_a = (float)d_a; 

float f_b = (float)big_d; 

} 

As a result of these conversions, it is possible that d_a is outside the range of values that can be 

represented by a float or that big_d is outside the range of values that can be represented as either 

a float or a double. If this is the case, the result is undefined on implementations that do not 

support Annex F, “IEC 60559 Floating-Point Arithmetic.” 





6.3.4 Compliant Solution (Narrowing Conversion) 

This compliant solution checks whether the values to be stored can be represented in the new 

type: 

#include 

#include 

void func(double d_a, long double big_d) { 

double d_b; 

float f_a; 

float f_b; 

} 

if (isgreater(fabs(d_a), FLT_MAX) || 

isless(fabs(d_a), FLT_MIN)) { 


} else { 

f_a = (float)d_a; 

} 

if (isgreater(fabsl(big_d), FLT_MAX) || 

isless(fabsl(big_d), FLT_MIN)) { 


} else { 

f_b = (float)big_d; 

} 

if (isgreater(fabsl(big_d), DBL_MAX) || 

isless(fabsl(big_d), DBL_MIN)) { 


} else { 

d_b = (double)big_d; 

} 


Converting a floating-point value to a floating-point value of a smaller range and precision or to 

an integer type, or converting an integer type to a floating-point type, can result in a value that is 

not representable in the destination type and is undefined behavior on implementations that do not 

support Annex F. 


FLP34-C Low Unlikely Low P3 L3 






CERT Oracle Secure Coding Standard for Java NUM12-J. Ensure conversions of numeric 

types to narrower types do not result in lost or 


ISO/IEC TR 24772:2013 

MITRE CWE 

Numeric Conversion Errors [FLC] 

CWE-681, Incorrect Conversion between Numeric 

Types 


[IEEE 754 2006] 

[ISO/IEC 9899:2011] 

Subclause 6.3.1.4, “Real Floating and Integer” 

Subclause 6.3.1.5, “Real Floating Types” 




Floating Point (FLP) - FLP36-C. Preserve precision when converting integral values to floating-point type 

6.4 FLP36-C. Preserve precision when converting integral values to 

floating-point type 

Narrower arithmetic types can be cast to wider types without any effect on the magnitude of numeric 

values. However, whereas integer types represent exact values, floating-point types have 

limited precision. The C Standard, 6.3.1.4 paragraph 2 [ISO/IEC 9899:2011], states 

When a value of integer type is converted to a real floating type, if the value being converted 

can be represented exactly in the new type, it is unchanged. If the value being 




range of values that can be represented, the behavior is undefined. Results of some implicit 

conversions may be represented in greater range and precision than that required 

by the new type (see 6.3.1.8 and 6.8.6.4). 

Conversion from integral types to floating-point types without sufficient precision can lead to loss 

of precision (loss of least significant bits). No runtime exception occurs despite the loss. 


In this noncompliant example, a large value of type long int is converted to a value of type 

float without ensuring it is representable in the type: 

#include 


long int big = 1234567890; 

float approx = big; 

printf("%ld\n", (big - (long int)approx)); 

return 0; 

} 

For most floating-point hardware, the value closest to 1234567890 that is representable in type 

float is 1234567844; consequently, this program prints the value -46. 


This compliant solution replaces the type float with a double. Furthermore, it uses an assertion 

to guarantee that the double type can represent any long int without loss of precision. (See 

INT35-C. Use correct integer precisions and MSC11-C. Incorporate diagnostic tests using assertions.) 

#include 

#include 

#include 

#include 




Floating Point (FLP) - FLP36-C. Preserve precision when converting integral values to floating-point type 

#include 

#include 

extern size_t popcount(uintmax_t); /* See INT35-C */ 



assert(PRECISION(LONG_MAX)

Floating Point (FLP) - FLP37-C. Do not use object representations to compare floating-point values 

6.5 FLP37-C. Do not use object representations to compare floatingpoint 

values 

The object representation for floating-point values is implementation defined. However, an implementation 

that defines the __STDC_IEC_559__ macro shall conform to the IEC 60559 floatingpoint 

standard and uses what is frequently referred to as IEEE 754 floating-point arithmetic 

[ISO/IEC 9899:2011]. The floating-point object representation used by IEC 60559 is one of the 

most common floating-point object representations in use today. 

All floating-point object representations use specific bit patterns to encode the value of the floating-point 

number being represented. However, equivalence of floating-point values is not encoded 

solely by the bit pattern used to represent the value. For instance, if the floating-point format supports 

negative zero values (as IEC 60559 does), the values -0.0 and 0.0 are equivalent and will 

compare as equal, but the bit patterns used in the object representation are not identical. Similarly, 

if two floating-point values are both (the same) NaN, they will not compare as equal, despite the 

bit patterns being identical, because they are not equivalent. 

Do not compare floating-point object representations directly, such as by calling memcmp() or its 

moral equivalents. Instead, the equality operators (== and !=) should be used to determine if two 

floating-point values are equivalent. 


In this noncompliant code example, memcmp() is used to compare two structures for equality. 

However, since the structure contains a floating-point object, this code may not behave as the programmer 

intended. 

#include 

#include 

struct S { 

int i; 

float f; 

}; 

bool are_equal(const struct S *s1, const struct S *s2) { 

if (!s1 && !s2) 

return true; 

else if (!s1 || !s2) 

return false; 

return 0 == memcmp(s1, s2, sizeof(struct S)); 

} 




Floating Point (FLP) - FLP37-C. Do not use object representations to compare floating-point values 


In this compliant solution, the structure members are compared individually: 

#include 

#include 

struct S { 

int i; 

float f; 

}; 

bool are_equal(const struct S *s1, const struct S *s2) { 

if (!s1 && !s2) 

return true; 

else if (!s1 || !s2) 

return false; 

return s1->i == s2->i && 

s1->f == s2->f; 

} 


Using the object representation of a floating-point value for comparisons can lead to incorrect 

equality results, which can lead to unexpected behavior. 


FLP37-C Low Unlikely Medium P2 L3 



[ISO/IEC 9899:2011] 

Annex F, “IEC 60559 floating-point arithmetic” 




Array (ARR) - ARR30-C. Do not form or use out-of-bounds pointers or array subscripts 

7 Array (ARR) 

7.1 ARR30-C. Do not form or use out-of-bounds pointers or array 

subscripts 

The C Standard identifies the following distinct situations in which undefined behavior (UB) can 

arise as a result of invalid pointer operations: 

UB Description Example Code 

46 Addition or subtraction of a pointer 

into, or just beyond, an array object 

and an integer type produces 

a result that does not point into, or 

just beyond, the same array object. 

47 Addition or subtraction of a pointer 

into, or just beyond, an array object 

and an integer type produces 

a result that points just beyond the 

array object and is used as the operand 

of a unary * operator that is 

evaluated. 

49 An array subscript is out of range, 

even if an object is apparently accessible 

with the given subscript, 

for example, in the lvalue expression 

a [1] [7] given the declaration 

int a [4] [5]). 

62 An attempt is made to access, or 

generate a pointer to just past, a 

flexible array member of a structure 

when the referenced object 

provides no elements for that array. 

Forming Out-of-Bounds Pointer, 

Null Pointer Arithmetic 

Dereferencing Past the End 

Pointer, Using Past the End Index 

Apparently Accessible Out-of- 

Range Index 

Pointer Past Flexible Array Member 

7.1.1 Noncompliant Code Example (Forming Out-of-Bounds Pointer) 

In this noncompliant code example, the function f() attempts to validate the index before using 

it as an offset to the statically allocated table of integers. However, the function fails to reject 

negative index values. When index is less than zero, the behavior of the addition expression in 

the return statement of the function is undefined behavior 46. On some implementations, the addition 

alone can trigger a hardware trap. On other implementations, the addition may produce a result 

that when dereferenced triggers a hardware trap. Other implementations still may produce a 





dereferenceable pointer that points to an object distinct from table. Using such a pointer to access 

the object may lead to information exposure or cause the wrong object to be modified. 

enum { TABLESIZE = 100 }; 

static int table[TABLESIZE]; 

int *f(int index) { 

if (index < TABLESIZE) { 

return table + index; 

} 

return NULL; 

} 


One compliant solution is to detect and reject invalid values of index if using them in pointer 

arithmetic would result in an invalid pointer: 



int *f(int index) { 

if (index >= 0 && index < TABLESIZE) { 


} 

return NULL; 

} 


Another slightly simpler and potentially more efficient compliant solution is to use an unsigned 

type to avoid having to check for negative values while still rejecting out-of-bounds positive values 

of index: 

#include 



int *f(size_t index) { 

if (index < TABLESIZE) { 


} 

return NULL; 

} 





7.1.4 Noncompliant Code Example (Dereferencing Past-the-End Pointer) 

This noncompliant code example shows the flawed logic in the Windows Distributed Component 

Object Model (DCOM) Remote Procedure Call (RPC) interface that was exploited by the 

W32.Blaster.Worm. The error is that the while loop in the GetMachineName() function (used 

to extract the host name from a longer string) is not sufficiently bounded. When the character array 

pointed to by pwszTemp does not contain the backslash character among the first 

MAX_COMPUTERNAME_LENGTH_FQDN + 1 elements, the final valid iteration of the loop will 

dereference past the end pointer, resulting in exploitable undefined behavior 47. In this case, the 

actual exploit allowed the attacker to inject executable code into a running program. Economic 

damage from the Blaster worm has been estimated to be at least $525 million [Pethia 2003]. 

For a discussion of this programming error in the Common Weakness Enumeration database, see 

CWE-119, “Improper Restriction of Operations within the Bounds of a Memory Buffer,” and 

CWE-121, “Stack-based Buffer Overflow” [MITRE 2013]. 

error_status_t _RemoteActivation( 

/* ... */, WCHAR *pwszObjectName, ... ) { 

*phr = GetServerPath( 

pwszObjectName, &pwszObjectName); 

/* ... */ 

} 

HRESULT GetServerPath( 

WCHAR *pwszPath, WCHAR **pwszServerPath ){ 

WCHAR *pwszFinalPath = pwszPath; 

WCHAR wszMachineName[MAX_COMPUTERNAME_LENGTH_FQDN+1]; 

hr = GetMachineName(pwszPath, wszMachineName); 

*pwszServerPath = pwszFinalPath; 

} 

HRESULT GetMachineName( 

WCHAR *pwszPath, 

WCHAR wszMachineName[MAX_COMPUTERNAME_LENGTH_FQDN+1]) 

{ 

pwszServerName = wszMachineName; 

LPWSTR pwszTemp = pwszPath + 2; 

while (*pwszTemp != L'\\') 

*pwszServerName++ = *pwszTemp++; 

/* ... */ 

} 


In this compliant solution, the while loop in the GetMachineName() function is bounded so 

that the loop terminates when a backslash character is found, the null-termination character 





(L'\0') is discovered, or the end of the buffer is reached. This code does not result in a buffer 

overflow even if no backslash character is found in wszMachineName. 

HRESULT GetMachineName( 

wchar_t *pwszPath, 

wchar_t wszMachineName[MAX_COMPUTERNAME_LENGTH_FQDN+1]) 

{ 

wchar_t *pwszServerName = wszMachineName; 

wchar_t *pwszTemp = pwszPath + 2; 

wchar_t *end_addr 

= pwszServerName + MAX_COMPUTERNAME_LENGTH_FQDN; 

while ( (*pwszTemp != L'\\') 

&& ((*pwszTemp != L'\0')) 

&& (pwszServerName < end_addr) ) 

{ 

*pwszServerName++ = *pwszTemp++; 

} 

} 

/* ... */ 

This compliant solution is for illustrative purposes and is not necessarily the solution implemented 

by Microsoft. This particular solution may not be correct because there is no guarantee that a 

backslash is found. 

7.1.6 Noncompliant Code Example (Using Past-the-End Index) 

Similar to the dereferencing-past-the-end-pointer error, the function insert_in_table() in this 

noncompliant code example uses an otherwise valid index to attempt to store a value in an element 

just past the end of an array. 

First, the function incorrectly validates the index pos against the size of the buffer. When pos is 

initially equal to size, the function attempts to store value in a memory location just past the 

end of the buffer. 

Second, when the index is greater than size, the function modifies size before growing the size 

of the buffer. If the call to realloc() fails to increase the size of the buffer, the next call to the 

function with a value of pos equal to or greater than the original value of size will again attempt 

to store value in a memory location just past the end of the buffer or beyond. 

Third, the function violates INT30-C. Ensure that unsigned integer operations do not wrap, which 

could lead to wrapping when 1 is added to pos or when size is multiplied by the size of int. 





For a discussion of this programming error in the Common Weakness Enumeration database, see 

CWE-122, “Heap-based Buffer Overflow,” and CWE-129, “Improper Validation of Array Index” 

[MITRE 2013]. 

#include 

static int *table = NULL; 

static size_t size = 0; 

int insert_in_table(size_t pos, int value) { 

if (size < pos) { 

int *tmp; 

size = pos + 1; 

tmp = (int *)realloc(table, sizeof(*table) * size); 

if (tmp == NULL) { 

return -1; /* Failure */ 

} 

table = tmp; 

} 

} 

table[pos] = value; 

return 0; 


This compliant solution correctly validates the index pos by using the


} 

table = tmp; 

} 

table[pos] = value; 

return 0; 

7.1.8 Noncompliant Code Example (Apparently Accessible Out-of-Range Index) 

This noncompliant code example declares matrix to consist of 7 rows and 5 columns in row-major 

order. The function init_matrix iterates over all 35 elements in an attempt to initialize each 

to the value given by the function argument x. However, because multidimensional arrays are declared 

in C in row-major order, the function iterates over the elements in column-major order, and 

when the value of j reaches the value COLS during the first iteration of the outer loop, the function 

attempts to access element matrix[0][5]. Because the type of matrix is int[7][5], the 

j subscript is out of range, and the access has undefined behavior 49. 

#include 

#define COLS 5 

#define ROWS 7 

static int matrix[ROWS][COLS]; 

void init_matrix(int x) { 

for (size_t i = 0; i < COLS; i++) { 

for (size_t j = 0; j < ROWS; j++) { 

matrix[i][j] = x; 

} 

} 

} 


This compliant solution avoids using out-of-range indices by initializing matrix elements in the 

same row-major order as multidimensional objects are declared in C: 

#include 

#define COLS 5 

#define ROWS 7 

static int matrix[ROWS][COLS]; 

void init_matrix(int x) { 

for (size_t i = 0; i < ROWS; i++) { 

for (size_t j = 0; j < COLS; j++) { 

matrix[i][j] = x; 

} 

} 

} 





7.1.10 Noncompliant Code Example (Pointer Past Flexible Array Member) 

In this noncompliant code example, the function find() attempts to iterate over the elements of 

the flexible array member buf, starting with the second element. However, because function g() 

does not allocate any storage for the member, the expression first++ in find() attempts to 

form a pointer just past the end of buf when there are no elements. This attempt is undefined behavior 

62. (See MSC21-C. Use robust loop termination conditions for more information.) 

#include 

struct S { 

size_t len; 

char buf[]; /* Flexible array member */ 

}; 

const char *find(const struct S *s, int c) { 

const char *first = s->buf; 

const char *last = s->buf + s->len; 

} 

while (first++ != last) { /* Undefined behavior */ 

if (*first == (unsigned char)c) { 

return first; 

} 

} 

return NULL; 


struct S *s = (struct S *)malloc(sizeof(struct S)); 

if (s == NULL) { 


} 

s->len = 0; 

find(s, 'a'); 

} 


This compliant solution avoids incrementing the pointer unless a value past the pointer's current 

value is known to exist: 

#include 

struct S { 

size_t len; 

char buf[]; /* Flexible array member */ 

}; 

const char *find(const struct S *s, int c) { 





const char *first = s->buf; 

const char *last = s->buf + s->len; 

} 

while (first != last) { /* Avoid incrementing here */ 

if (*++first == (unsigned char)c) { 

return first; 

} 

} 

return NULL; 


struct S *s = (struct S *)malloc(sizeof(struct S)); 



} 

s->len = 0; 

find(s, 'a'); 

} 

7.1.12 Noncompliant Code Example (Null Pointer Arithmetic) 

This noncompliant code example is similar to an Adobe Flash Player vulnerability that was first 

exploited in 2008. This code allocates a block of memory and initializes it with some data. The 

data does not belong at the beginning of the block, which is left uninitialized. Instead, it is placed 

offset bytes within the block. The function ensures that the data fits within the allocated block. 

#include 

#include 

char *init_block(size_t block_size, size_t offset, 

char *data, size_t data_size) { 

char *buffer = malloc(block_size); 

if (data_size > block_size || block_size - data_size < offset) { 

/* Data won't fit in buffer, handle error */ 

} 

memcpy(buffer + offset, data, data_size); 

return buffer; 

} 

This function fails to check if the allocation succeeds, which is a violation of ERR33-C. Detect 

and handle standard library errors. If the allocation fails, then malloc() returns a null pointer. 

The null pointer is added to offset and passed as the destination argument to memcpy(). Because 

a null pointer does not point to a valid object, the result of the pointer arithmetic is undefined 

behavior 46. 

An attacker who can supply the arguments to this function can exploit it to execute arbitrary code. 

This can be accomplished by providing an overly large value for block_size, which causes 





malloc() to fail and return a null pointer. The offset argument will then serve as the destination 

address to the call to memcpy(). The attacker can specify the data and data_size arguments 

to provide the address and length of the address, respectively, that the attacker wishes to 

write into the memory referenced by offset. The overall result is that the call to memcpy() can 

be exploited by an attacker to overwrite an arbitrary memory location with an attacker-supplied 

address, typically resulting in arbitrary code execution. 

7.1.13 Compliant Solution (Null Pointer Arithmetic) 

This compliant solution ensures that the call to malloc() succeeds: 

#include 

#include 

char *init_block(size_t block_size, size_t offset, 

char *data, size_t data_size) { 

char *buffer = malloc(block_size); 

if (NULL == buffer) { 


} 

if (data_size > block_size || block_size - data_size < offset) { 

/* Data won't fit in buffer, handle error */ 

} 

memcpy(buffer + offset, data, data_size); 

return buffer; 

} 


Writing to out-of-range pointers or array subscripts can result in a buffer overflow and the execution 

of arbitrary code with the permissions of the vulnerable process. Reading from out-of-range 

pointers or array subscripts can result in unintended information disclosure. 


ARR30-C High Likely High P9 L2 


CVE-2008-1517 results from a violation of this rule. Before Mac OSX version 10.5.7, the XNU 

kernel accessed an array at an unverified user-input index, allowing an attacker to execute arbitrary 

code by passing an index greater than the length of the array and therefore accessing outside 

memory [xorl 2009]. 


ISO/IEC TR 24772:2013 


Unchecked Array Indexing [XYZ] 






MITRE CWE 

MISRA C:2012 

Forming or using out-of-bounds pointers or array 

subscripts [invptr] 



CWE-122, Heap-based Buffer Overflow 



CWE-129, Improper Validation of Array Index 

CWE-788, Access of Memory Location after 

End of Buffer 



[Finlay 2003] 

[Microsoft 2003] 

[Pethia 2003] 


[Viega 2005] 

[xorl 2009 ] 

Chapter 1, “Running with Scissors” 

Section 5.2.13, “Unchecked Array Indexing” 

“CVE-2008-1517: Apple Mac OS X (XNU) 

Missing Array Index Validation” 




Array (ARR) - ARR32-C. Ensure size arguments for variable length arrays are in a valid range 

7.2 ARR32-C. Ensure size arguments for variable length arrays are in a 

valid range 

Variable length arrays (VLAs), a conditionally supported language feature, are essentially the 

same as traditional C arrays except that they are declared with a size that is not a constant integer 

expression and can be declared only at block scope or function prototype scope and no linkage. 

When supported, a variable length array can be declared 

{ /* Block scope */ 

char vla[size]; 

} 

where the integer expression size and the declaration of vla are both evaluated at runtime. If the 

size argument supplied to a variable length array is not a positive integer value, the behavior is 

undefined. (See undefined behavior 75.) Additionally, if the magnitude of the argument is excessive, 

the program may behave in an unexpected way. An attacker may be able to leverage this behavior 

to overwrite critical program data [Griffiths 2006]. The programmer must ensure that size 

arguments to variable length arrays, especially those derived from untrusted data, are in a valid 

range. 

Because variable length arrays are a conditionally supported feature of C11, their use in portable 

code should be guarded by testing the value of the macro __STDC_NO_VLA__. Implementations 

that do not support variable length arrays indicate it by setting __STDC_NO_VLA__ to the integer 

constant 1. 


In this noncompliant code example, a variable length array of size is declared. The size is declared 

as size_t in compliance with INT01-C. Use rsize_t or size_t for all integer values representing 

the size of an object. 

#include 

extern void do_work(int *array, size_t size); 

void func(size_t size) { 

int vla[size]; 

do_work(vla, size); 

} 

However, the value of size may be zero or excessive, potentially giving rise to a security vulnerability. 






This compliant solution ensures the size argument used to allocate vla is in a valid range (between 

1 and a programmer-defined maximum); otherwise, it uses an algorithm that relies on dynamic 

memory allocation. The solution also avoids unsigned integer wrapping that, given a sufficiently 

large value of size, would cause malloc to allocate insufficient storage for the array. 

#include 

#include 

enum { MAX_ARRAY = 1024 }; 

extern void do_work(int *array, size_t size); 

void func(size_t size) { 

if (0 == size || SIZE_MAX / sizeof(int) < size) { 


return; 

} 

if (size < MAX_ARRAY) { 

int vla[size]; 

do_work(vla, size); 

} else { 

int *array = (int *)malloc(size * sizeof(int)); 

if (array == NULL) { 


} 

do_work(array, size); 

free(array); 

} 

} 

7.2.3 Noncompliant Code Example (sizeof) 

The following noncompliant code example defines A to be a variable length array and then uses 

the sizeof operator to compute its size at runtime. When the function is called with an argument 

greater than SIZE_MAX / (N1 * sizeof (int)), the runtime sizeof expression may wrap 

around, yielding a result that is smaller than the mathematical product N1 * n2 * sizeof 

(int). The call to malloc(), when successful, will then allocate storage for fewer than n2 elements 

of the array, causing one or more of the final memset() calls in the for loop to write past 

the end of that storage. 

#include 

#include 

enum { N1 = 4096 }; 

void *func(size_t n2) { 

typedef int A[n2][N1]; 





A *array = malloc(sizeof(A)); 

if (!array) { 


return NULL; 

} 

for (size_t i = 0; i != n2; ++i) { 

memset(array[i], 0, N1 * sizeof(int)); 

} 

} 

return array; 

7.2.4 Compliant Solution (sizeof) 

This compliant solution prevents sizeof wrapping by detecting the condition before it occurs 

and avoiding the subsequent computation when the condition is detected. 

#include 

#include 

#include 

enum { N1 = 4096 }; 

void *func(size_t n2) { 

if (n2 > SIZE_MAX / (N1 * sizeof(int))) { 

/* Prevent sizeof wrapping */ 

return NULL; 

} 

typedef int A[n2][N1]; 

A *array = malloc(sizeof(A)); 

if (!array) { 


return NULL; 

} 

} 

for (size_t i = 0; i != n2; ++i) { 

memset(array[i], 0, N1 * sizeof(int)); 

} 

return array; 


7.2.4.1.1 Microsoft 

Variable length arrays are not supported by Microsoft compilers. 






Failure to properly specify the size of a variable length array may allow arbitrary code execution 

or result in stack exhaustion. 


ARR32-C High Probable High P6 L2 



ISO/IEC TR 24772:2013 

ISO/IEC TS 17961:2013 

INT01-C. Use rsize_t or size_t for all integer 

values representing the size of an object 

Unchecked Array Indexing [XYZ] 

Tainted, potentially mutilated, or out-of-domain 

integer values are used in a restricted sink 

[taintsink] 


[Griffiths 2006] 




Array (ARR) - ARR36-C. Do not subtract or compare two pointers that do not refer to the same array 

7.3 ARR36-C. Do not subtract or compare two pointers that do not 

refer to the same array 

When two pointers are subtracted, both must point to elements of the same array object or just one 

past the last element of the array object (C Standard, 6.5.6 [ISO/IEC 9899:2011]); the result is the 

difference of the subscripts of the two array elements. Otherwise, the operation is undefined behavior. 

(See undefined behavior 48.) 

Similarly, comparing pointers using the relational operators gives the positions of 

the pointers relative to each other. Subtracting or comparing pointers that do not refer to the same 

array is undefined behavior. (See undefined behavior 48 and undefined behavior 53.) 

Comparing pointers using the equality operators == and != has well-defined semantics regardless 

of whether or not either of the pointers is null, points into the same object, or points one past the 

last element of an array object or function. 


In this noncompliant code example, pointer subtraction is used to determine how many free elements 

are left in the nums array: 

#include 

enum { SIZE = 32 }; 


int nums[SIZE]; 

int end; 

int *next_num_ptr = nums; 

size_t free_elements; 

/* Increment next_num_ptr as array fills */ 

} 

free_elements = &end - next_num_ptr; 

This program incorrectly assumes that the nums array is adjacent to the end variable in memory. 

A compiler is permitted to insert padding bits between these two variables or even reorder them in 

memory. 




Array (ARR) - ARR36-C. Do not subtract or compare two pointers that do not refer to the same array 


In this compliant solution, the number of free elements is computed by subtracting 

next_num_ptr from the address of the pointer past the nums array. While this pointer may not 

be dereferenced, it may be used in pointer arithmetic. 

#include 

enum { SIZE = 32 }; 


int nums[SIZE]; 

int *next_num_ptr = nums; 

size_t free_elements; 

/* Increment next_num_ptr as array fills */ 

} 

free_elements = &(nums[SIZE]) - next_num_ptr; 


ARR36-C-EX1: Comparing two pointers to distinct members of the same struct object is allowed. 

Pointers to structure members declared later in the structure compare greater-than pointers 

to members declared earlier in the structure. 



ARR36-C Medium Probable Medium P8 L2 




MITRE CWE 

CTR54-CPP. Do not subtract iterators that do 

not refer to the same container 

Subtracting or comparing two pointers that do 

not refer to the same array [ptrobj] 

CWE-469, Use of Pointer Subtraction to Determine 

Size 



[ISO/IEC 9899:2011] 

Section 5.3, “Pointers” 

Section 5.7, “Expressions Involving Pointers” 





Array (ARR) - ARR37-C. Do not add or subtract an integer to a pointer to a non-array object 

7.4 ARR37-C. Do not add or subtract an integer to a pointer to a nonarray 

object 

Pointer arithmetic must be performed only on pointers that reference elements of array objects. 

The C Standard, 6.5.6 [ISO/IEC 9899:2011], states the following about pointer arithmetic: 

When an expression that has integer type is added to or subtracted from a pointer, the 

result has the type of the pointer operand. If the pointer operand points to an element of 

an array object, and the array is large enough, the result points to an element offset from 

the original element such that the difference of the subscripts of the resulting and original 

array elements equals the integer expression. 


This noncompliant code example attempts to access structure members using pointer arithmetic. 

This practice is dangerous because structure members are not guaranteed to be contiguous. 

struct numbers { 

short num_a, num_b, num_c; 

}; 

int sum_numbers(const struct numbers *numb){ 

int total = 0; 

const short *numb_ptr; 

for (numb_ptr = &numb->num_a; 

numb_ptr num_c; 

numb_ptr++) { 

total += *(numb_ptr); 

} 

} 

return total; 


struct numbers my_numbers = { 1, 2, 3 }; 

sum_numbers(&my_numbers); 

return 0; 

} 


It is possible to use the -> operator to dereference each structure member: 

total = numb->num_a + numb->num_b + numb->num_c; 





However, this solution results in code that is hard to write and hard to maintain (especially if there 

are many more structure members), which is exactly what the author of the noncompliant code 

example was likely trying to avoid. 


A better solution is to define the structure to contain an array member to store the numbers in an 

array rather than a structure, as in this compliant solution: 

#include 

struct numbers { 

short a[3]; 

}; 

int sum_numbers(const short *numb, size_t dim) { 

int total = 0; 

for (size_t i = 0; i < dim; ++i) { 

total += numb[i]; 

} 

} 

return total; 


struct numbers my_numbers = { .a[0]= 1, .a[1]= 2, .a[2]= 3}; 

sum_numbers( 

my_numbers.a, 

sizeof(my_numbers.a)/sizeof(my_numbers.a[0]) 

); 

return 0; 

} 

Array elements are guaranteed to be contiguous in memory, so this solution is completely portable. 


ARR37-C-EX1: Any non-array object in memory can be considered an array consisting of one 

element. Adding one to a pointer to such an object yields a pointer one element past the array, and 

subtracting one from that pointer yields the original pointer. This allows for code such as the following: 

#include 

#include 

struct s { 

char *c_str; 





/* Other members */ 

}; 

struct s *create_s(const char *c_str) { 

struct s *ret; 

size_t len = strlen(c_str) + 1; 

} 

ret = (struct s *)malloc(sizeof(struct s) + len); 

if (ret != NULL) { 

ret->c_str = (char *)(ret + 1); 

memcpy(ret + 1, c_str, len); 

} 

return ret; 

A more general and safer solution to this problem is to use a flexible array member that guarantees 

the array that follows the structure is properly aligned by inserting padding, if necessary, between 

it and the member that immediately precedes it. 



ARR37-C Medium Probable Medium P8 L2 


MITRE CWE 

CWE-469, Use of Pointer Subtraction to Determine 

Size 



[ISO/IEC 9899:2011] 

[VU#162289] 

Section 5.3, “Pointers” 

Section 5.7, “Expressions Involving Pointers” 





Array (ARR) - ARR38-C. Guarantee that library functions do not form invalid pointers 

7.5 ARR38-C. Guarantee that library functions do not form invalid 

pointers 

C library functions that make changes to arrays or objects take at least two arguments: a pointer to 

the array or object and an integer indicating the number of elements or bytes to be manipulated. 

For the purposes of this rule, the element count of a pointer is the size of the object to which it 

points, expressed by the number of elements that are valid to access. Supplying arguments to such 

a function might cause the function to form a pointer that does not point into or just past the end 

of the object, resulting in undefined behavior. 

Annex J of the C Standard [ISO/IEC 9899:2011] states that it is undefined behavior if the “pointer 

passed to a library function array parameter does not have a value such that all address computations 

and object accesses are valid.” (See undefined behavior 109.) 

In the following code, 

int arr[5]; 

int *p = arr; 

unsigned char *p2 = (unsigned char *)arr; 

unsigned char *p3 = arr + 2; 

void *p4 = arr; 

the element count of the pointer p is sizeof(arr) / sizeof(arr[0]), that is, 5. The element 

count of the pointer p2 is sizeof(arr), that is, 20, on implementations where sizeof(int) 

== 4. The element count of the pointer p3 is 12 on implementations where sizeof(int) == 4, 

because p3 points two elements past the start of the array arr. The element count of p4 is treated 

as though it were unsigned char * instead of void *, so it is the same as p2. 

7.5.1 Pointer + Integer 

The following standard library functions take a pointer argument and a size argument, with the 

constraint that the pointer must point to a valid memory object of at least the number of elements 

indicated by the size argument. 

fgets() fgetws() mbstowcs() 2 wcstombs() 2 

mbrtoc16() 3 mbrtoc32() 3 mbsrtowcs() 2 wcsrtombs() 2 

mbtowc() 3 mbrtowc() 2 mblen() mbrlen() 

memchr() wmemchr() memset() wmemset() 

strftime() wcsftime() strxfrm() 2 wcsxfrm() 2 

______________________ 

2 

Takes two pointers and an integer, but the integer specifies the element count only of the output buffer, not of 

the input buffer. 

3 

Takes two pointers and an integer, but the integer specifies the element count only of the input buffer, not of the 

output buffer. 





strncat() 3 wcsncat() 3 snprintf() vsnprintf() 

swprintf() vswprintf() setvbuf() tmpnam_s() 

snprintf_s() sprintf_s() vsnprintf_s() vsprintf_s() 

gets_s() getenv_s() wctomb_s() mbstowcs_s() 4 

wcstombs_s() 4 memcpy_s() 4 memmove_s() 4 strncpy_s() 4 

strncat_s() 4 strtok_s() 3 strerror_s() strnlen_s() 

asctime_s() ctime_s() snwprintf_s() swprintf_s() 

vsnwprintf_s() vswprintf_s() wcsncpy_s() 4 wmemcpy_s() 4 

wmemmove_s() 4 wcsncat_s() 4 wcstok_s() 3 wcsnlen_s() 

wcrtomb_s() mbsrtowcs_s() 4 wcsrtombs_s() 4 memset_s() 5 

For calls that take a pointer and an integer size, the given size should not be greater than the element 

count of the pointer. 

7.5.1.1 Noncompliant Code Example (Element Count) 

In this noncompliant code example, the incorrect element count is used in a call to wmemcpy(). 

The sizeof operator returns the size expressed in bytes, but wmemcpy() uses an element count 

based on wchar_t *. 

#include 

#include 

static const char str[] = "Hello world"; 

static const wchar_t w_str[] = L"Hello world"; 



wchar_t w_buffer[32]; 

memcpy(buffer, str, sizeof(str)); /* Compliant */ 

wmemcpy(w_buffer, w_str, sizeof(w_str)); /* Noncompliant */ 

} 

7.5.1.2 Compliant Solution (Element Count) 

When using functions that operate on pointed-to regions, programmers must always express the 

integer size in terms of the element count expected by the function. For example, memcpy() expects 

the element count expressed in terms of void *, but wmemcpy() expects the element count 

expressed in terms of wchar_t *. Instead of the sizeof operator, functions that return the number 

of elements in the string are called, which matches the expected element count for the copy 

______________________ 

4 

Takes two pointers and two integers; each integer corresponds to the element count of one of the pointers. 

5 

Takes a pointer and two size-related integers; the first size-related integer parameter specifies the number of 

bytes available in the buffer; the second size-related integer parameter specifies the number of bytes to write 

within the buffer. 





functions. In the case of this compliant solution, where the argument is an array A of type T, the 

expression sizeof(A) / sizeof(T), or equivalently sizeof(A) / sizeof(*A), can be 

used to compute the number of elements in the array. 

#include 

#include 

static const char str[] = "Hello world"; 

static const wchar_t w_str[] = L"Hello world"; 



wchar_t w_buffer[32]; 

memcpy(buffer, str, strlen(str) + 1); 

wmemcpy(w_buffer, w_str, wcslen(w_str) + 1); 

} 

7.5.1.3 Noncompliant Code Example (Pointer + Integer) 

This noncompliant code example assigns a value greater than the number of bytes of available 

memory to n, which is then passed to memset(): 

#include 

#include 

void f1(size_t nchars) { 

char *p = (char *)malloc(nchars); 

/* ... */ 

const size_t n = nchars + 1; 

/* ... */ 

memset(p, 0, n); 

} 

7.5.1.4 Compliant Solution (Pointer + Integer) 

This compliant solution ensures that the value of n is not greater than the number of bytes of the 

dynamic memory pointed to by the pointer p: 

#include 

#include 

void f1(size_t nchars) { 

char *p = (char *)malloc(nchars); 

/* ... */ 

const size_t n = nchars; 

/* ... */ 


} 





7.5.1.5 Noncompliant Code Example (Pointer + Integer) 

In this noncompliant code example, the element count of the array a is ARR_SIZE elements. Because 

memset() expects a byte count, the size of the array is scaled incorrectly by sizeof(int) 

instead of sizeof(long), which can form an invalid pointer on architectures where 

sizeof(int) != sizeof(long). 

#include 

void f2(void) { 

const size_t ARR_SIZE = 4; 

long a[ARR_SIZE]; 

const size_t n = sizeof(int) * ARR_SIZE; 

void *p = a; 

} 


7.5.1.6 Compliant Solution (Pointer + Integer) 

In this compliant solution, the element count required by memset() is properly calculated without 

resorting to scaling: 

#include 


const size_t ARR_SIZE = 4; 

long a[ARR_SIZE]; 

const size_t n = sizeof(a); 

void *p = a; 

} 


7.5.2 Two Pointers + One Integer 

The following standard library functions take two pointer arguments and a size argument, with the 

constraint that both pointers must point to valid memory objects of at least the number of elements 

indicated by the size argument. 

memcpy() wmemcpy() memmove() wmemmove() 

strncpy() wcsncpy() memcmp() wmemcmp() 

strncmp() wcsncmp() strcpy_s() wcscpy_s() 

strcat_s() 

wcscat_s() 

For calls that take two pointers and an integer size, the given size should not be greater than the 

element count of either pointer. 





7.5.2.1 Noncompliant Code Example (Two Pointers + One Integer) 

In this noncompliant code example, the value of n is incorrectly computed, allowing a read past 

the end of the object referenced by q: 

#include 

void f4() { 

char p[40]; 

const char *q = "Too short"; 

size_t n = sizeof(p); 

memcpy(p, q, n); 

} 

7.5.2.2 Compliant Solution (Two Pointers + One Integer) 

This compliant solution ensures that n is equal to the size of the character array: 

#include 

void f4() { 

char p[40]; 

const char *q = "Too short"; 

size_t n = sizeof(p) < strlen(q) + 1 ? sizeof(p) : strlen(q) + 1; 

memcpy(p, q, n); 

} 

7.5.3 One Pointer + Two Integers 

The following standard library functions take a pointer argument and two size arguments, with the 

constraint that the pointer must point to a valid memory object containing at least as many bytes 

as the product of the two size arguments. 

bsearch() bsearch_s() qsort() qsort_s() 

fread() 

fwrite() 

For calls that take a pointer and two integers, one integer represents the number of bytes required 

for an individual object, and a second integer represents the number of elements in the array. The 

resulting product of the two integers should not be greater than the element count of the pointer 

were it expressed as an unsigned char *. 

7.5.3.1 Noncompliant Code Example (One Pointer + Two Integers) 

This noncompliant code example allocates a variable number of objects of type struct obj. 

The function checks that num_objs is small enough to prevent wrapping, in compliance with 

INT30-C. Ensure that unsigned integer operations do not wrap. The size of struct obj is assumed 

to be 16 bytes to account for padding to achieve the assumed alignment of long long. 





However, the padding typically depends on the target architecture, so this object size may be incorrect, 

resulting in an incorrect element count. 

#include 

#include 

struct obj { 

char c; 

long long i; 

}; 

void func(FILE *f, struct obj *objs, size_t num_objs) { 

const size_t obj_size = 16; 

if (num_objs > (SIZE_MAX / obj_size) || 

num_objs != fwrite(objs, obj_size, num_objs, f)) { 


} 

} 

7.5.3.2 Compliant Solution (One Pointer + Two Integers) 

This compliant solution uses the sizeof operator to correctly provide the object size and 

num_objs to provide the element count: 

#include 

#include 

struct obj { 

char c; 

long long i; 

}; 

void func(FILE *f, struct obj *objs, size_t num_objs) { 

const size_t obj_size = sizeof *objs; 

if (num_objs > (SIZE_MAX / obj_size) || 

num_objs != fwrite(objs, obj_size, num_objs, f)) { 


} 

} 

7.5.3.3 Noncompliant Code Example (One Pointer + Two Integers) 

In this noncompliant code example, the function f() calls fread() to read nitems of type 

wchar_t, each size bytes in size, into an array of BUFFER_SIZE elements, wbuf. However, the 

expression used to compute the value of nitems fails to account for the fact that, unlike the size 

of char, the size of wchar_t may be greater than 1. Consequently, fread() could attempt to 

form pointers past the end of wbuf and use them to assign values to nonexistent elements of the 

array. Such an attempt is undefined behavior. (See undefined behavior 109.) A likely consequence 





of this undefined behavior is a buffer overflow. For a discussion of this programming error in the 

Common Weakness Enumeration database, see CWE-121, “Stack-based Buffer Overflow,” and 

CWE-805, “Buffer Access with Incorrect Length Value.” 

#include 

#include 

void f(FILE *file) { 

enum { BUFFER_SIZE = 1024 }; 

wchar_t wbuf[BUFFER_SIZE]; 

const size_t size = sizeof(*wbuf); 

const size_t nitems = sizeof(wbuf); 

} 

size_t nread = fread(wbuf, size, nitems, file); 

/* ... */ 

7.5.3.4 Compliant Solution (One Pointer + Two Integers) 

This compliant solution correctly computes the maximum number of items for fread() to read 

from the file: 

#include 

#include 

void f(FILE *file) { 


wchar_t wbuf[BUFFER_SIZE]; 

const size_t size = sizeof(*wbuf); 

const size_t nitems = sizeof(wbuf) / size; 

} 

size_t nread = fread(wbuf, size, nitems, file); 

/* ... */ 

7.5.3.5 Noncompliant Code Example (Heartbleed) 

CERT vulnerability 720951 describes a vulnerability in OpenSSL versions 1.0.1 through 1.0.1f, 

popularly known as “Heartbleed.” This vulnerability allows an attacker to steal information that 

under normal conditions would be protected by Secure Socket Layer/Transport Layer Security 

(SSL/TLS) encryption. 





Despite the seriousness of the vulnerability, Heartbleed is the result of a common programming 

error and an apparent lack of awareness of secure coding principles. Following is the vulnerable 

code: 

int dtls1_process_heartbeat(SSL *s) { 

unsigned char *p = &s->s3->rrec.data[0], *pl; 

unsigned short hbtype; 

unsigned int payload; 

unsigned int padding = 16; /* Use minimum padding */ 

/* Read type and payload length first */ 

hbtype = *p++; 

n2s(p, payload); 

pl = p; 

/* ... More code ... */ 

if (hbtype == TLS1_HB_REQUEST) { 

unsigned char *buffer, *bp; 

int r; 

/* 

* Allocate memory for the response; size is 1 byte 

* message type, plus 2 bytes payload length, plus 

* payload, plus padding. 

*/ 

buffer = OPENSSL_malloc(1 + 2 + payload + padding); 

bp = buffer; 

/* Enter response type, length, and copy payload */ 

*bp++ = TLS1_HB_RESPONSE; 

s2n(payload, bp); 

memcpy(bp, pl, payload); 

} 

/* ... More code ... */ 

} 

/* ... More code ... */ 

This code processes a “heartbeat” packet from a client. As specified in RFC 6520, when the program 

receives a heartbeat packet, it must echo the packet’s data back to the client. In addition to 

the data, the packet contains a length field that conventionally indicates the number of bytes in the 

packet data, but there is nothing to prevent a malicious packet from lying about its data length. 

The p pointer, along with payload and p1, contains data from a packet. The code allocates a 

buffer sufficient to contain payload bytes, with some overhead, then copies payload bytes 

starting at p1 into this buffer and sends it to the client. Notably absent from this code are any 

checks that the payload integer variable extracted from the heartbeat packet corresponds to the 





size of the packet data. Because the client can specify an arbitrary value of payload, an attacker 

can cause the server to read and return the contents of memory beyond the end of the packet data, 

which violates INT04-C. Enforce limits on integer values originating from tainted sources. The 

resulting call to memcpy() can then copy the contents of memory past the end of the packet data 

and the packet itself, potentially exposing sensitive data to the attacker. This call to memcpy() violates 

cba927fd45b4. A version of ARR38-C also appears in ISO/IEC TS 17961:2013, “Forming 

invalid pointers by library functions [libptr].” This rule would require a conforming analyzer to 

diagnose the Heartbleed vulnerability. 

7.5.3.6 Compliant Solution (Heartbleed) 

OpenSSL version 1.0.1g contains the following patch, which guarantees that payload is within a 

valid range. The range is limited by the size of the input record. 

int dtls1_process_heartbeat(SSL *s) { 

unsigned char *p = &s->s3->rrec.data[0], *pl; 

unsigned short hbtype; 

unsigned int payload; 

unsigned int padding = 16; /* Use minimum padding */ 

/* ... More code ... */ 

/* Read type and payload length first */ 

if (1 + 2 + 16 > s->s3->rrec.length) 

return 0; /* Silently discard */ 

hbtype = *p++; 

n2s(p, payload); 

if (1 + 2 + payload + 16 > s->s3->rrec.length) 

return 0; /* Silently discard per RFC 6520 */ 

pl = p; 

/* ... More code ... */ 

if (hbtype == TLS1_HB_REQUEST) { 

unsigned char *buffer, *bp; 

int r; 

/* 

* Allocate memory for the response; size is 1 byte 

* message type, plus 2 bytes payload length, plus 

* payload, plus padding. 

*/ 

buffer = OPENSSL_malloc(1 + 2 + payload + padding); 

bp = buffer; 

/* Enter response type, length, and copy payload */ 

*bp++ = TLS1_HB_RESPONSE; 

s2n(payload, bp); 

memcpy(bp, pl, payload); 

/* ... More code ... */ 





} 

} 

/* ... More code ... */ 


Depending on the library function called, an attacker may be able to use a heap or stack overflow 

vulnerability to run arbitrary code. 


ARR38-C High Likely Medium P18 L1 


C Secure Coding Standard 

ISO/IEC TS 17961:2013 

ISO/IEC TR 24772:2013 

MITRE CWE 

API00-C. Functions should validate their parameters 

ARR01-C. Do not apply the sizeof operator to 

a pointer when taking the size of an array 

INT30-C. Ensure that unsigned integer operations 

do not wrap 

Forming invalid pointers by library functions 

[libptr] 

Buffer Boundary Violation (Buffer Overflow) 

[HCB] 

Unchecked Array Copying [XYW] 



CWE-121, Stack-based Buffer Overflow 



CWE-805, Buffer Access with Incorrect 

Length Value 


[Cassidy 2014] 

[IETF: RFC 6520] 

[ISO/IEC TS 17961:2013] 

[VU#720951] 

Existential Type Crisis : Diagnosis of the 

OpenSSL Heartbleed Bug 




Array (ARR) - ARR39-C. Do not add or subtract a scaled integer to a pointer 

7.6 ARR39-C. Do not add or subtract a scaled integer to a pointer 

Pointer arithmetic is appropriate only when the pointer argument refers to an array, including an 

array of bytes. (See ARR37-C. Do not add or subtract an integer to a pointer to a non-array object.) 

When performing pointer arithmetic, the size of the value to add to or subtract from a 

pointer is automatically scaled to the size of the type of the referenced array object. Adding or 

subtracting a scaled integer value to or from a pointer is invalid because it may yield a pointer that 

does not point to an element within or one past the end of the array. (See ARR30-C. Do not form 

or use out-of-bounds pointers or array subscripts.) 

Adding a pointer to an array of a type other than character to the result of the sizeof operator or 

offsetof macro, which returns a size and an offset, respectively, violates this rule. However, 

adding an array pointer to the number of array elements, for example, by using the 

arr[sizeof(arr)/sizeof(arr[0])] idiom, is allowed provided that arr refers to an array 

and not a pointer. 


In this noncompliant code example, sizeof(buf) is added to the array buf. This example is 

noncompliant because sizeof(buf) is scaled by int and then scaled again when added to buf. 

enum { INTBUFSIZE = 80 }; 

extern int getdata(void); 

int buf[INTBUFSIZE]; 


int *buf_ptr = buf; 

} 

while (buf_ptr < (buf + sizeof(buf))) { 

*buf_ptr++ = getdata(); 

} 


This compliant solution uses an unscaled integer to obtain a pointer to the end of the array: 

enum { INTBUFSIZE = 80 }; 

extern int getdata(void); 

int buf[INTBUFSIZE]; 


int *buf_ptr = buf; 

while (buf_ptr < (buf + INTBUFSIZE)) { 

*buf_ptr++ = getdata(); 





} 

} 


In this noncompliant code example, skip is added to the pointer s. However, skip represents the 

byte offset of ull_b in struct big. When added to s, skip is scaled by the size of struct 

big. 

#include 

#include 

#include 

struct big { 

unsigned long long ull_a; 

unsigned long long ull_b; 

unsigned long long ull_c; 

int si_e; 

int si_f; 

}; 


size_t skip = offsetof(struct big, ull_b); 

struct big *s = (struct big *)malloc(sizeof(struct big)); 


/* Handle malloc() error */ 

} 

} 

memset(s + skip, 0, sizeof(struct big) - skip); 

/* ... */ 

free(s); 

s = NULL; 


This compliant solution uses an unsigned char * to calculate the offset instead of using a 

struct big *, which would result in scaled arithmetic: 

#include 

#include 

#include 

struct big { 

unsigned long long ull_a; 

unsigned long long ull_b; 

unsigned long long ull_c; 

int si_d; 

int si_e; 





}; 


size_t skip = offsetof(struct big, ull_b); 

unsigned char *ptr = (unsigned char *)malloc( 

sizeof(struct big) 

); 

if (ptr == NULL) { 


} 

} 

memset(ptr + skip, 0, sizeof(struct big) - skip); 

/* ... */ 

free(ptr); 

ptr = NULL; 


In this noncompliant code example, wcslen(error_msg) * sizeof(wchar_t) bytes are 

scaled by the size of wchar_t when added to error_msg: 

#include 

#include 

enum { WCHAR_BUF = 128 }; 


wchar_t error_msg[WCHAR_BUF]; 

} 

wcscpy(error_msg, L"Error: "); 

fgetws(error_msg + wcslen(error_msg) * sizeof(wchar_t), 

WCHAR_BUF - 7, stdin); 

/* ... */ 


This compliant solution does not scale the length of the string; wcslen() returns the number of 

characters and the addition to error_msg is scaled: 

#include 

#include 

enum { WCHAR_BUF = 128 }; 

const wchar_t ERROR_PREFIX[7] = L"Error: "; 


const size_t prefix_len = wcslen(ERROR_PREFIX); 





wchar_t error_msg[WCHAR_BUF]; 

} 

wcscpy(error_msg, ERROR_PREFIX); 

fgetws(error_msg + prefix_len, 

WCHAR_BUF - prefix_len, stdin); 

/* ... */ 


Failure to understand and properly use pointer arithmetic can allow an attacker to execute arbitrary 

code. 


ARR39-C High Probable High P6 L2 



ISO/IEC TR 24772:2013 

MISRA C:2012 

MITRE CWE 






[HFC] 

Pointer Arithmetic [RVG] 





CWE 468, Incorrect Pointer Scaling 


[Dowd 2006] 

[Murenin 07] 





Characters and Strings (STR) - STR30-C. Do not attempt to modify string literals 

8 Characters and Strings (STR) 

8.1 STR30-C. Do not attempt to modify string literals 

According to the C Standard, 6.4.5, paragraph 3 [ISO/IEC 9899:2011]: 

A character string literal is a sequence of zero or more multibyte characters enclosed in 

double-quotes, as in “xyz.” A UTF−8 string literal is the same, except prefixed by u8. A 

wide string literal is the same, except prefixed by the letter L, u, or U. 

At compile time, string literals are used to create an array of static storage duration of sufficient 

length to contain the character sequence and a terminating null character. String literals are usually 

referred to by a pointer to (or array of) characters. Ideally, they should be assigned only to 

pointers to (or arrays of) const char or const wchar_t. It is unspecified whether these arrays 

of string literals are distinct from each other. The behavior is undefined if a program attempts to 

modify any portion of a string literal. Modifying a string literal frequently results in an access violation 

because string literals are typically stored in read-only memory. (See undefined behavior 

33.) 

Avoid assigning a string literal to a pointer to non-const or casting a string literal to a pointer to 

non-const. For the purposes of this rule, a pointer to (or array of) const characters must be 

treated as a string literal. Similarly, the returned value of the following library functions must be 

treated as a string literal if the first argument is a string literal: 

• strpbrk(), strchr(), strrchr(), strstr() 

• wcspbrk(), wcschr(), wcsrchr(), wcsstr() 

• memchr(), wmemchr() 

This rule is a specific instance of EXP40-C. Do not modify constant objects. 


In this noncompliant code example, the char pointer p is initialized to the address of a string literal. 

Attempting to modify the string literal is undefined behavior: 

char *p = "string literal"; 

p[0] = 'S'; 


As an array initializer, a string literal specifies the initial values of characters in an array as well as 

the size of the array. (See STR11-C. Do not specify the bound of a character array initialized with 





a string literal.) This code creates a copy of the string literal in the space allocated to the character 

array a. The string stored in a can be modified safely. 

char a[] = "string literal"; 

a[0] = 'S'; 


In this noncompliant code example, a string literal is passed to the (pointer to non-const) parameter 

of the POSIX function mkstemp(), which then modifies the characters of the string literal: 

#include 


mkstemp("/tmp/edXXXXXX"); 

} 

The behavior of mkstemp() is described in more detail in FIO21-C. Do not create temporary 

files in shared directories. 


This compliant solution uses a named array instead of passing a string literal: 

#include 


static char fname[] = "/tmp/edXXXXXX"; 

mkstemp(fname); 

} 

8.1.5 Noncompliant Code Example (Result of strrchr()) 

In this noncompliant example, the char * result of the strrchr() function is used to modify 

the object pointed to by pathname. Because the argument to strrchr() points to a string literal, 

the effects of the modification are undefined. 

#include 

#include 

const char *get_dirname(const char *pathname) { 

char *slash; 

slash = strrchr(pathname, '/'); 

if (slash) { 

*slash = '\0'; /* Undefined behavior */ 

} 

return pathname; 





} 


puts(get_dirname(__FILE__)); 

return 0; 

} 

8.1.6 Compliant Solution (Result of strrchr()) 

This compliant solution avoids modifying a const object, even if it is possible to obtain a nonconst 

pointer to such an object by calling a standard C library function, such as strrchr(). To 

reduce the risk to callers of get_dirname(), a buffer and length for the directory name are 

passed into the function. It is insufficient to change pathname to require a char * instead of a 

const char * because conforming compilers are not required to diagnose passing a string literal 

to a function accepting a char *. 

#include 

#include 

#include 

char *get_dirname(const char *pathname, char *dirname, size_t size) 

{ 

const char *slash; 

slash = strrchr(pathname, '/'); 

if (slash) { 

ptrdiff_t slash_idx = slash - pathname; 

if ((size_t)slash_idx < size) { 

memcpy(dirname, pathname, slash_idx); 

dirname[slash_idx] = '\0'; 

return dirname; 

} 

} 

return 0; 

} 


char dirname[260]; 

if (get_dirname(__FILE__, dirname, sizeof(dirname))) { 

puts(dirname); 

} 

return 0; 

} 






Modifying string literals can lead to abnormal program termination and possibly denial-of-service 

attacks. 


STR30-C Low Likely Low P9 L2 



ISO/IEC TS 17961:2013 

EXP05-C. Do not cast away a const qualification 

STR11-C. Do not specify the bound of a character 

array initialized with a string literal 

Modifying string literals [strmod] 


[ISO/IEC 9899:2011] 

6.4.5, “String Literals” 

[Plum 1991] 

Topic 1.26, “Strings—String Literals” 

[Summit 1995] comp.lang.c FAQ List, Question 1.32 




Characters and Strings (STR) - STR31-C. Guarantee that storage for strings has sufficient space for character data and the 

null terminator 

8.2 STR31-C. Guarantee that storage for strings has sufficient space 

for character data and the null terminator 

Copying data to a buffer that is not large enough to hold that data results in a buffer overflow. 

Buffer overflows occur frequently when manipulating strings [Seacord 2013b]. To prevent such 

errors, either limit copies through truncation or, preferably, ensure that the destination is of sufficient 

size to hold the character data to be copied and the null-termination character. (See STR03- 

C. Do not inadvertently truncate a string.) 

When strings live on the heap, this rule is a specific instance of MEM35-C. Allocate sufficient 

memory for an object. Because strings are represented as arrays of characters, this rule is related 

to both ARR30-C. Do not form or use out-of-bounds pointers or array subscripts and ARR38-C. 

Guarantee that library functions do not form invalid pointers. 

8.2.1 Noncompliant Code Example (Off-by-One Error) 

This noncompliant code example demonstrates an off-by-one error [Dowd 2006]. The loop copies 

data from src to dest. However, because the loop does not account for the null-termination character, 

it may be incorrectly written 1 byte past the end of dest. 

#include 

void copy(size_t n, char src[n], char dest[n]) { 

size_t i; 

} 

for (i = 0; src[i] && (i < n); ++i) { 

dest[i] = src[i]; 

} 

dest[i] = '\0'; 

8.2.2 Compliant Solution (Off-by-One Error) 

In this compliant solution, the loop termination condition is modified to account for the null-termination 

character that is appended to dest: 

#include 

void copy(size_t n, char src[n], char dest[n]) { 

size_t i; 

} 

for (i = 0; src[i] && (i < n - 1); ++i) { 

dest[i] = src[i]; 

} 

dest[i] = '\0'; 






8.2.3 Noncompliant Code Example (gets()) 

The gets() function, which was deprecated in the C99 Technical Corrigendum 3 and removed 

from C11, is inherently unsafe and should never be used because it provides no way to control 

how much data is read into a buffer from stdin. This noncompliant code example assumes that 

gets() will not read more than BUFFER_SIZE - 1 characters from stdin. This is an invalid 

assumption, and the resulting operation can result in a buffer overflow. 

The gets() function reads characters from the stdin into a destination array until end-of-file is 

encountered or a newline character is read. Any newline character is discarded, and a null character 

is written immediately after the last character read into the array. 

#include 

#define BUFFER_SIZE 1024 


char buf[BUFFER_SIZE]; 

if (gets(buf) == NULL) { 


} 

} 

See also MSC24-C. Do not use deprecated or obsolescent functions. 

8.2.4 Compliant Solution (fgets()) 

The fgets() function reads, at most, one less than the specified number of characters from a 

stream into an array. This solution is compliant because the number of characters copied from 

stdin to buf cannot exceed the allocated memory: 

#include 

#include 



char buf[BUFFERSIZE]; 

int ch; 

if (fgets(buf, sizeof(buf), stdin)) { 

/* fgets() succeeded; scan for newline character */ 

char *p = strchr(buf, '\n'); 

if (p) { 

*p = '\0'; 

} else { 

/* Newline not found; flush stdin to end of line */ 

while ((ch = getchar()) != '\n' && ch != EOF) 

; 






} 

if (ch == EOF && !feof(stdin) && !ferror(stdin)) { 

/* Character resembles EOF; handle error */ 

} 

} 

} else { 

/* fgets() failed; handle error */ 

} 

The fgets() function is not a strict replacement for the gets() function because fgets() retains 

the newline character (if read) and may also return a partial line. It is possible to use 

fgets() to safely process input lines too long to store in the destination array, but this is not recommended 

for performance reasons. Consider using one of the following compliant solutions 

when replacing gets(). 

8.2.5 Compliant Solution (gets_s()) 

The gets_s() function reads, at most, one less than the number of characters specified from the 

stream pointed to by stdin into an array. 

The C Standard, Annex K [ISO/IEC 9899:2011], states 

No additional characters are read after a new-line character (which is discarded) or after 

end-of-file. The discarded new-line character does not count towards number of characters 

read. A null character is written immediately after the last character read into the array. 

If end-of-file is encountered and no characters have been read into the destination array, or if a 

read error occurs during the operation, then the first character in the destination array is set to the 

null character and the other elements of the array take unspecified values: 

#define __STDC_WANT_LIB_EXT1__ 1 

#include 




} 

if (gets_s(buf, sizeof(buf)) == NULL) { 


} 






8.2.6 Compliant Solution (getline(), POSIX) 

The getline() function is similar to the fgets() function but can dynamically allocate 

memory for the input buffer. If passed a null pointer, getline() dynamically allocates a buffer 

of sufficient size to hold the input. If passed a pointer to dynamically allocated storage that is too 

small to hold the contents of the string, the getline() function resizes the buffer, using realloc(), 

rather than truncating the input. If successful, the getline() function returns the number 

of characters read, which can be used to determine if the input has any null characters before the 

newline. The getline() function works only with dynamically allocated buffers. Allocated 

memory must be explicitly deallocated by the caller to avoid memory leaks. (See MEM31-C. Free 

dynamically allocated memory when no longer needed.) 

#include 

#include 

#include 


int ch; 

size_t buffer_size = 32; 

char *buffer = malloc(buffer_size); 

if (!buffer) { 


return; 

} 

} 

if ((ssize_t size = getline(&buffer, &buffer_size, stdin)) 

== -1) { 


} else { 

char *p = strchr(buffer, '\n'); 

if (p) { 

*p = '\0'; 

} else { 

/* Newline not found; flush stdin to end of line */ 

while ((ch = getchar()) != '\n' && ch != EOF) 

; 

if (ch == EOF && !feof(stdin) && !ferror(stdin)) { 

/* Character resembles EOF; handle error */ 

} 

} 

} 

free (buffer); 

The getline() function uses an in-band error indicator, in violation of ERR02-C. Avoid in-band 

error indicators. 






8.2.7 Noncompliant Code Example (getchar()) 

Reading one character at a time provides more flexibility in controlling behavior, though with additional 

performance overhead. This noncompliant code example uses the getchar() function to 

read one character at a time from stdin instead of reading the entire line at once. The stdin 

stream is read until end-of-file is encountered or a newline character is read. Any newline character 

is discarded, and a null character is written immediately after the last character read into the 

array. Similar to the noncompliant code example that invokes gets(), there are no guarantees 

that this code will not result in a buffer overflow. 

#include 




char *p; 

int ch; 

p = buf; 

while ((ch = getchar()) != '\n' && ch != EOF) { 

*p++ = (char)ch; 

} 

*p++ = 0; 

if (ch == EOF) { 

/* Handle EOF or error */ 

} 

} 

After the loop ends, if ch == EOF, the loop has read through to the end of the stream without encountering 

a newline character, or a read error occurred before the loop encountered a newline 

character. To conform to FIO34-C. Distinguish between characters read from a file and EOF or 

WEOF, the error-handling code must verify that an end-of-file or error has occurred by calling 

feof() or ferror(). 

8.2.8 Compliant Solution (getchar()) 

In this compliant solution, characters are no longer copied to buf once index == BUFFERSIZE 

- 1, leaving room to null-terminate the string. The loop continues to read characters until the end 

of the line, the end of the file, or an error is encountered. When chars_read > index, the input 

string has been truncated. 

#include 




int ch; 






size_t index = 0; 

size_t chars_read = 0; 

} 

while ((ch = getchar()) != '\n' && ch != EOF) { 

if (index < sizeof(buf) - 1) { 

buf[index++] = (char)ch; 

} 

chars_read++; 

} 

buf[index] = '\0'; /* Terminate string */ 

if (ch == EOF) { 

/* Handle EOF or error */ 

} 

if (chars_read > index) { 

/* Handle truncation */ 

} 

8.2.9 Noncompliant Code Example (fscanf()) 

In this noncompliant example, the call to fscanf() can result in a write outside the character array 

buf: 

#include 

enum { BUF_LENGTH = 1024 }; 

void get_data(void) { 

char buf[BUF_LENGTH]; 

if (1 != fscanf(stdin, "%s", buf)) { 


} 

} 

/* Rest of function */ 

8.2.10 Compliant Solution (fscanf()) 

In this compliant solution, the call to fscanf() is constrained not to overflow buf: 

#include 

enum { BUF_LENGTH = 1024 }; 

void get_data(void) { 

char buf[BUF_LENGTH]; 

if (1 != fscanf(stdin, "%1023s", buf)) { 


} 






} 

/* Rest of function */ 

8.2.11 Noncompliant Code Example (argv) 

In a hosted environment, arguments read from the command line are stored in process memory. 

The function main(), called at program startup, is typically declared as follows when the program 

accepts command-line arguments: 

int main(int argc, char *argv[]) { /* ... */ } 

Command-line arguments are passed to main() as pointers to strings in the array members 

argv[0] through argv[argc - 1]. If the value of argc is greater than 0, the string pointed to 

by argv[0] is, by convention, the program name. If the value of argc is greater than 1, the 

strings referenced by argv[1] through argv[argc - 1] are the program arguments. 

Vulnerabilities can occur when inadequate space is allocated to copy a command-line argument or 

other program input. In this noncompliant code example, an attacker can manipulate the contents 

of argv[0] to cause a buffer overflow: 

#include 

int main(int argc, char *argv[]) { 

/* Ensure argv[0] is not null */ 

const char *const name = (argc && argv[0]) ? argv[0] : ""; 

char prog_name[128]; 

strcpy(prog_name, name); 

} 

return 0; 

8.2.12 Compliant Solution (argv) 

The strlen() function can be used to determine the length of the strings referenced by argv[0] 

through argv[argc - 1] so that adequate memory can be dynamically allocated. 

#include 

#include 




char *prog_name = (char *)malloc(strlen(name) + 1); 

if (prog_name != NULL) { 

strcpy(prog_name, name); 

} else { 






} 


} 

free(prog_name); 

return 0; 

Remember to add a byte to the destination string size to accommodate the null-termination character. 


The strcpy_s() function provides additional safeguards, including accepting the size of the destination 

buffer as an additional argument. (See STR07-C. Use the bounds-checking interfaces for 

string manipulation.) 


#include 

#include 




char *prog_name; 

size_t prog_size; 

prog_size = strlen(name) + 1; 

prog_name = (char *)malloc(prog_size); 

} 

if (prog_name != NULL) { 

if (strcpy_s(prog_name, prog_size, name)) { 


} 

} else { 


} 

/* ... */ 

free(prog_name); 

return 0; 

The strcpy_s() function can be used to copy data to or from dynamically allocated memory or 

a statically allocated array. If insufficient space is available, strcpy_s() returns an error. 







If an argument will not be modified or concatenated, there is no reason to make a copy of the 

string. Not copying a string is the best way to prevent a buffer overflow and is also the most efficient 

solution. Care must be taken to avoid assuming that argv[0] is non-null. 



const char * const prog_name = (argc && argv[0]) ? argv[0] : ""; 

/* ... */ 

return 0; 

} 

8.2.15 Noncompliant Code Example (getenv()) 

According to the C Standard, 7.22.4.6 [ISO/IEC 9899:2011]: 

The getenv function searches an environment list, provided by the host environment, 

for a string that matches the string pointed to by name. The set of environment names 

and the method for altering the environment list are implementation defined. 

Environment variables can be arbitrarily large, and copying them into fixed-length arrays without 

first determining the size and allocating adequate storage can result in a buffer overflow. 

#include 

#include 


char buff[256]; 

char *editor = getenv("EDITOR"); 

if (editor == NULL) { 

/* EDITOR environment variable not set */ 

} else { 

strcpy(buff, editor); 

} 

} 

8.2.16 Compliant Solution (getenv()) 

Environmental variables are loaded into process memory when the program is loaded. As a result, 

the length of these strings can be determined by calling the strlen() function, and the resulting 

length can be used to allocate adequate dynamic memory: 

#include 

#include 


char *buff; 






} 

char *editor = getenv("EDITOR"); 

if (editor == NULL) { 

/* EDITOR environment variable not set */ 

} else { 

size_t len = strlen(editor) + 1; 

buff = (char *)malloc(len); 

if (buff == NULL) { 


} 

memcpy(buff, editor, len); 

free(buff); 

} 

8.2.17 Noncompliant Code Example (sprintf()) 

In this noncompliant code example, name refers to an external string; it could have originated 

from user input, the file system, or the network. The program constructs a file name from the 

string in preparation for opening the file. 

#include 

void func(const char *name) { 

char filename[128]; 

sprintf(filename, "%s.txt", name); 

} 

Because the sprintf() function makes no guarantees regarding the length of the generated 

string, a sufficiently long string in name could generate a buffer overflow. 

8.2.18 Compliant Solution (sprintf()) 

The buffer overflow in the preceding noncompliant example can be prevented by adding a precision 

to the %s conversion specification. If the precision is specified, no more than that many bytes 

are written. The precision 123 in this compliant solution ensures that filename can contain the 

first 123 characters of name, the .txt extension, and the null terminator. 

#include 



sprintf(filename, "%.123s.txt", name); 

} 






8.2.19 Compliant Solution (snprintf()) 

A more general solution is to use the snprintf() function: 

#include 



snprintf(filename, sizeof(filename), "%s.txt", name); 

} 


Copying string data to a buffer that is too small to hold that data results in a buffer overflow. Attackers 

can exploit this condition to execute arbitrary code with the permissions of the vulnerable 

process. 


STR31-C High Likely Medium P18 L1 


CVE-2009-1252 results from a violation of this rule. The Network Time Protocol daemon 

(NTPd), before versions 4.2.4p7 and 4.2.5p74, contained calls to sprintf that allow an attacker 

to execute arbitrary code by overflowing a character array [xorl 2009]. 

CVE-2009-0587 results from a violation of this rule. Before version 2.24.5, Evolution Data Server 

performed unchecked arithmetic operations on the length of a user-input string and used the value 

to allocate space for a new buffer. An attacker could thereby execute arbitrary code by inputting a 

long string, resulting in incorrect allocation and buffer overflow [xorl 2009]. 



STR03-C. Do not inadvertently truncate a 

string 

STR07-C. Use the bounds-checking interfaces 

for remediation of existing string manipulation 

code 

MSC24-C. Do not use deprecated or obsolescent 

functions 

MEM00-C. Allocate and free memory in the 

same module, at the same level of abstraction 

FIO34-C. Distinguish between characters read 

from a file and EOF or WEOF 






ISO/IEC TR 24772:2013 

ISO/IEC TS 17961:2013 

MITRE CWE 

String Termination [CJM] 


[HCB] 

Unchecked Array Copying [XYW] 

Using a tainted value to write to an object using 

a formatted input or output function [taintformatio] 

Tainted strings are passed to a string copying 

function [taintstrcpy] 



CWE-120, Buffer Copy without Checking Size 

of Input (“Classic Buffer Overflow”) 



CWE-193, Off-by-one Error 


[Dowd 2006] 

[Drepper 2006] 

[ISO/IEC 9899:2011] 

[Lai 2006] 

[NIST 2006] 


[xorl 2009] 

Chapter 7, “Program Building Blocks” (“Loop 

Constructs,” pp. 327–336) 

Section 2.1.1, “Respecting Memory Bounds” 

K.3.5.4.1, “The gets_s Function” 

SAMATE Reference Dataset Test Case ID 

000-000-088 

Chapter 2, “Strings” 

FreeBSD-SA-09:11: NTPd Remote Stack 

Based Buffer Overflows 




Characters and Strings (STR) - STR32-C. Do not pass a non-null-terminated character sequence to a library function that 

expects a string 

8.3 STR32-C. Do not pass a non-null-terminated character sequence to 

a library function that expects a string 

Many library functions accept a string or wide string argument with the constraint that the string 

they receive is properly null-terminated. Passing a character sequence or wide character sequence 

that is not null-terminated to such a function can result in accessing memory that is outside the 

bounds of the object. Do not pass a character sequence or wide character sequence that is not nullterminated 

to a library function that expects a string or wide string argument. 


This code example is noncompliant because the character sequence c_str will not be null-terminated 

when passed as an argument to printf(). (See STR11-C. Do not specify the bound of a 

character array initialized with a string literal on how to properly initialize character arrays.) 

#include 


char c_str[3] = "abc"; 

printf("%s\n", c_str); 

} 


This compliant solution does not specify the bound of the character array in the array declaration. 

If the array bound is omitted, the compiler allocates sufficient storage to store the entire string literal, 

including the terminating null character. 

#include 


char c_str[] = "abc"; 


} 


This code example is noncompliant because the wide character sequence cur_msg will not be 

null-terminated when passed to wcslen(). This will occur if lessen_memory_usage() is invoked 

while cur_msg_size still has its initial value of 1024. 

#include 

#include 

wchar_t *cur_msg = NULL; 

size_t cur_msg_size = 1024; 

size_t cur_msg_len = 0; 






void lessen_memory_usage(void) { 

wchar_t *temp; 

size_t temp_size; 

/* ... */ 

if (cur_msg != NULL) { 

temp_size = cur_msg_size / 2 + 1; 

temp = realloc(cur_msg, temp_size * sizeof(wchar_t)); 

/* temp &and cur_msg may no longer be null-terminated */ 

if (temp == NULL) { 


} 

} 

} 

cur_msg = temp; 

cur_msg_size = temp_size; 

cur_msg_len = wcslen(cur_msg); 


In this compliant solution, cur_msg will always be null-terminated when passed to wcslen(): 

#include 

#include 

wchar_t *cur_msg = NULL; 

size_t cur_msg_size = 1024; 

size_t cur_msg_len = 0; 

void lessen_memory_usage(void) { 

wchar_t *temp; 

size_t temp_size; 

/* ... */ 

if (cur_msg != NULL) { 

temp_size = cur_msg_size / 2 + 1; 

temp = realloc(cur_msg, temp_size * sizeof(wchar_t)); 

/* temp and cur_msg may no longer be null-terminated */ 

if (temp == NULL) { 


} 

cur_msg = temp; 

/* Properly null-terminate cur_msg */ 

cur_msg[temp_size - 1] = L'\0'; 

cur_msg_size = temp_size; 






} 

} 

cur_msg_len = wcslen(cur_msg); 

8.3.5 Noncompliant Code Example (strncpy()) 

Although the strncpy() function takes a string as input, it does not guarantee that the resulting 

value is still null-terminated. In the following noncompliant code example, if no null character is 

contained in the first n characters of the source array, the result will not be null-terminated. 

Passing a non-null-terminated character sequence to strlen() is undefined behavior. 

#include 

enum { STR_SIZE = 32 }; 

size_t func(const char *source) { 

char c_str[STR_SIZE]; 

size_t ret = 0; 

} 

if (source) { 

c_str[sizeof(c_str) - 1] = '\0'; 

strncpy(c_str, source, sizeof(c_str)); 

ret = strlen(c_str); 

} else { 

/* Handle null pointer */ 

} 

return ret; 

8.3.6 Compliant Solution (Truncation) 

This compliant solution is correct if the programmer’s intent is to truncate the string: 

#include 





if (source) { 

strncpy(c_str, source, sizeof(c_str) - 1); 

c_str[sizeof(c_str) - 1] = '\0'; 


} else { 


} 






} 

return ret; 

8.3.7 Compliant Solution (Truncation, strncpy_s()) 

The C Standard, Annex K strncpy_s() function can also be used to copy with truncation. The 

strncpy_s() function copies up to n characters from the source array to a destination array. If 

no null character was copied from the source array, then the n th position in the destination array is 

set to a null character, guaranteeing that the resulting string is null-terminated. 


#include 



char a[STR_SIZE]; 


} 

if (source) { 

errno_t err = strncpy_s( 

a, sizeof(a), source, strlen(source) 

); 

if (err != 0) { 


} else { 

ret = strnlen_s(a, sizeof(a)); 

} 

} else { 


} 

return ret; 

8.3.8 Compliant Solution (Copy without Truncation) 

If the programmer’s intent is to copy without truncation, this compliant solution copies the data 

and guarantees that the resulting array is null-terminated. If the string cannot be copied, it is handled 

as an error condition. 

#include 





if (source) { 






} 

if (strlen(source) < sizeof(c_str)) { 

strcpy(c_str, source); 


} else { 

/* Handle string-too-large */ 

} 

} else { 


} 

return ret; 


Failure to properly null-terminate a character sequence that is passed to a library function that expects 

a string can result in buffer overflows and the execution of arbitrary code with the permissions 

of the vulnerable process. Null-termination errors can also result in unintended information 

disclosure. 


STR32-C High Probable Medium P12 L1 


ISO/IEC TR 24772:2013 

ISO/IEC TS 17961:2013 

MITRE CWE 

String Termination [CMJ] 

Passing a non-null-terminated character sequence 

to a library function that expects a 

string [strmod] 





CWE-170, Improper Null Termination 


[Seacord 2013] 

[Viega 2005] 


Section 5.2.14, “Miscalculated NULL Termination” 




Characters and Strings (STR) - STR34-C. Cast characters to unsigned char before converting to larger integer sizes 

8.4 STR34-C. Cast characters to unsigned char before converting to 

larger integer sizes 

Signed character data must be converted to unsigned char before being assigned or converted 

to a larger signed type. This rule applies to both signed char and (plain) char characters on 

implementations where char is defined to have the same range, representation, and behaviors as 

signed char. 

However, this rule is applicable only in cases where the character data may contain values that 

can be interpreted as negative numbers. For example, if the char type is represented by a two’s 

complement 8-bit value, any character value greater than +127 is interpreted as a negative value. 

This rule is a generalization of STR37-C. Arguments to character-handling functions must be representable 

as an unsigned char. 


This noncompliant code example is taken from a vulnerability in bash versions 1.14.6 and earlier 

that led to the release of CERT Advisory CA-1996-22. This vulnerability resulted from the sign 

extension of character data referenced by the c_str pointer in the yy_string_get() function 

in the parse.y module of the bash source code: 

static int yy_string_get(void) { 

register char *c_str; 

register int c; 

c_str = bash_input.location.string; 

c = EOF; 

} 

/* If the string doesn't exist or is empty, EOF found */ 

if (c_str && *c_str) { 

c = *c_str++; 

bash_input.location.string = c_str; 

} 

return (c); 

The c_str variable is used to traverse the character string containing the command line to be 

parsed. As characters are retrieved from this pointer, they are stored in a variable of type int. For 

implementations in which the char type is defined to have the same range, representation, and 

behavior as signed char, this value is sign-extended when assigned to the int variable. For 

character code 255 decimal (−1 in two’s complement form), this sign extension results in the 

value −1 being assigned to the integer, which is indistinguishable from EOF. 






This problem can be repaired by explicitly declaring the c_str variable as unsigned char: 


register unsigned char *c_str; 



c = EOF; 

} 



c = *c_str++; 


} 

return (c); 

This example, however, violates STR04-C. Use plain char for characters in the basic character set. 


In this compliant solution, the result of the expression *c_str++ is cast to unsigned char before 

assignment to the int variable c: 


register char *c_str; 



c = EOF; 



/* Cast to unsigned type */ 

c = (unsigned char)*c_str++; 


} 

return (c); 

} 






In this noncompliant code example, the cast of *s to unsigned int can result in a value in excess 

of UCHAR_MAX because of integer promotions, a violation of ARR30-C. Do not form or use 

out-of-bounds pointers or array subscripts: 

#include 

#include 

static const char table[UCHAR_MAX] = { 'a' /* ... */ }; 

ptrdiff_t first_not_in_table(const char *c_str) { 

for (const char *s = c_str; *s; ++s) { 

if (table[(unsigned int)*s] != *s) { 

return s - c_str; 

} 

} 

return -1; 

} 


This compliant solution casts the value of type char to unsigned char before the implicit promotion 

to a larger type: 

#include 

#include 

static const char table[UCHAR_MAX] = { 'a' /* ... */ }; 

ptrdiff_t first_not_in_table(const char *c_str) { 

for (const char *s = c_str; *s; ++s) { 

if (table[(unsigned char)*s] != *s) { 

return s - c_str; 

} 

} 

return -1; 

} 


Conversion of character data resulting in a value in excess of UCHAR_MAX is an often-missed error 

that can result in a disturbingly broad range of potentially severe vulnerabilities. 


STR34-C Medium Probable Medium P8 L2 






CVE-2009-0887 results from a violation of this rule. In Linux PAM (up to version 1.0.3), the 

libpam implementation of strtok() casts a (potentially signed) character to an integer for use 

as an index to an array. An attacker can exploit this vulnerability by inputting a string with non- 

ASCII characters, causing the cast to result in a negative index and accessing memory outside of 

the array [xorl 2009]. 

8.4.6.2 Related Guidelines 


ISO/IEC TS 17961:2013 

MISRA-C 

MITRE CWE 

STR37-C. Arguments to character-handling 

functions must be representable as an unsigned 

char 

STR04-C. Use plain char for characters in the 

basic character set 



Conversion of signed characters to wider integer 

types before a check for EOF [signconv] 

Rule 10.1 through Rule 10.4 (required) 

CWE-704, Incorrect Type Conversion or Cast 


[xorl 2009] 

CVE-2009-0887: Linux-PAM Signedness Issue 




Characters and Strings (STR) - STR37-C. Arguments to character-handling functions must be representable as an unsigned 

char 

8.5 STR37-C. Arguments to character-handling functions must be 

representable as an unsigned char 

According to the C Standard, 7.4 [ISO/IEC 9899:2011], 

The header declares several functions useful for classifying and mapping 

characters. In all cases the argument is an int, the value of which shall be representable 

as an unsigned char or shall equal the value of the macro EOF. If the argument 

has any other value, the behavior is undefined. 


This rule is applicable only to code that runs on platforms where the char data type is defined to 

have the same range, representation, and behavior as signed char. 

Following are the character classification functions that this rule addresses: 

isalnum() isalpha() isascii() XSI isblank() 

iscntrl() isdigit() isgraph() islower() 

isprint() ispunct() isspace() isupper() 

isxdigit() toascii() XSI toupper() tolower() 

XSI denotes an X/Open System Interfaces Extension to ISO/IEC 9945—POSIX. These functions 

are not defined by the C Standard. 

This rule is a specific instance of STR34-C. Cast characters to unsigned char before converting to 

larger integer sizes. 


On implementations where plain char is signed, this code example is noncompliant because the 

parameter to isspace(), *t, is defined as a const char *, and this value might not be representable 

as an unsigned char: 

#include 

#include 

size_t count_preceding_whitespace(const char *s) { 

const char *t = s; 

size_t length = strlen(s) + 1; 

while (isspace(*t) && (t - s < length)) { 

++t; 

} 

return t - s; 

} 




Characters and Strings (STR) - STR37-C. Arguments to character-handling functions must be representable as an unsigned 

char 

The argument to isspace() must be EOF or representable as an unsigned char; otherwise, the 

result is undefined. 


This compliant solution casts the character to unsigned char before passing it as an argument 

to the isspace() function: 

#include 

#include 

size_t count_preceding_whitespace(const char *s) { 

const char *t = s; 

size_t length = strlen(s) + 1; 

while (isspace((unsigned char)*t) && (t - s < length)) { 

++t; 

} 

return t - s; 

} 


Passing values to character handling functions that cannot be represented as an unsigned char 

to character handling functions is undefined behavior. 


STR37-C Low Unlikely Low P3 L3 




MITRE CWE 

STR34-C. Cast characters to unsigned char before 

converting to larger integer sizes 

Passing arguments to character-handling functions 

that are not representable as unsigned 

char [chrsgnext] 

CWE-704, Incorrect Type Conversion or Cast 


Type 


[ISO/IEC 9899:2011] 

[Kettlewell 2002] 

7.4, “Character Handling ” 

Section 1.1, “ and Characters 

Types” 




Characters and Strings (STR) - STR38-C. Do not confuse narrow and wide character strings and functions 

8.6 STR38-C. Do not confuse narrow and wide character strings and 

functions 

Passing narrow string arguments to wide string functions or wide string arguments to narrow 

string functions can lead to unexpected and undefined behavior. Scaling problems are likely because 

of the difference in size between wide and narrow characters. (See ARR39-C. Do not add or 

subtract a scaled integer to a pointer.) Because wide strings are terminated by a null wide character 

and can contain null bytes, determining the length is also problematic. 

Because wchar_t and char are distinct types, many compilers will produce a warning diagnostic 

if an inappropriate function is used. (See MSC00-C. Compile cleanly at high warning levels.) 

8.6.1 Noncompliant Code Example (Wide Strings with Narrow String Functions) 

This noncompliant code example incorrectly uses the strncpy() function in an attempt to copy 

up to 10 wide characters. However, because wide characters can contain null bytes, the copy operation 

may end earlier than anticipated, resulting in the truncation of the wide string. 

#include 

#include 


wchar_t wide_str1[] = L"0123456789"; 


} 

strncpy(wide_str2, wide_str1, 10); 

8.6.2 Noncompliant Code Example (Narrow Strings with Wide String Functions) 

This noncompliant code example incorrectly invokes the wcsncpy() function to copy up to 10 

wide characters from narrow_str1 to narrow_str2. Because narrow_str2 is a narrow 

string, it has insufficient memory to store the result of the copy and the copy will result in a buffer 

overflow. 

#include 


char narrow_str1[] = "01234567890123456789"; 


} 

wcsncpy(narrow_str2, narrow_str1, 10); 






This compliant solution uses the proper-width functions. Using wcsncpy() for wide character 

strings and strncpy() for narrow character strings ensures that data is not truncated and buffer 

overflow does not occur. 

#include 

#include 




/* Use of proper-width function */ 

wcsncpy(wide_str2, wide_str1, 10); 

} 



/* Use of proper-width function */ 

strncpy(narrow_str2, narrow_str1, 10); 

8.6.4 Noncompliant Code Example (strlen()) 

In this noncompliant code example, the strlen() function is used to determine the size of a 

wide character string: 

#include 

#include 



wchar_t *wide_str2 = (wchar_t*)malloc(strlen(wide_str1) + 1); 

if (wide_str2 == NULL) { 


} 

/* ... */ 

free(wide_str2); 

wide_str2 = NULL; 

} 

The strlen() function determines the number of characters that precede the terminating null 

character. However, wide characters can contain null bytes, particularly when expressing characters 

from the ASCII character set, as in this example. As a result, the strlen() function will return 

the number of bytes preceding the first null byte in the wide string. 






This compliant solution correctly calculates the number of bytes required to contain a copy of the 

wide string, including the terminating null wide character: 

#include 

#include 



wchar_t *wide_str2 = (wchar_t *)malloc( 

(wcslen(wide_str1) + 1) * sizeof(wchar_t)); 

if (wide_str2 == NULL) { 


} 

/* ... */ 

} 

free(wide_str2); 

wide_str2 = NULL; 


Confusing narrow and wide character strings can result in buffer overflows, data truncation, and 

other defects. 


STR38-C High Likely Low P27 L1 


[ISO/IEC 9899:2011] 

7.24.2.4, “The strncpy Function” 

7.29.4.2.2, “The wcsncpy Function” 




Memory Management (MEM) - MEM30-C. Do not access freed memory 

9 Memory Management (MEM) 

9.1 MEM30-C. Do not access freed memory 

Evaluating a pointer—including dereferencing the pointer, using it as an operand of an arithmetic 

operation, type casting it, and using it as the right-hand side of an assignment—into memory that 

has been deallocated by a memory management function is undefined behavior. Pointers to 

memory that has been deallocated are called dangling pointers. Accessing a dangling pointer can 

result in exploitable vulnerabilities. 

According to the C Standard, using the value of a pointer that refers to space deallocated by a call 

to the free() or realloc() function is undefined behavior. (See undefined behavior 177.) 

Reading a pointer to deallocated memory is undefined behavior because the pointer value is indeterminate 

and might be a trap representation. Fetching a trap representation might perform a hardware 

trap (but is not required to). 

It is at the memory manager’s discretion when to reallocate or recycle the freed memory. When 

memory is freed, all pointers into it become invalid, and its contents might either be returned to 

the operating system, making the freed space inaccessible, or remain intact and accessible. As a 

result, the data at the freed location can appear to be valid but change unexpectedly. Consequently, 

memory must not be written to or read from once it is freed. 


This example from Brian Kernighan and Dennis Ritchie [Kernighan 1988] shows both the incorrect 

and correct techniques for freeing the memory associated with a linked list. In their (intentionally) 

incorrect example, p is freed before p->next is executed, so that p->next reads 

memory that has already been freed. 

#include 

struct node { 

int value; 

struct node *next; 

}; 

void free_list(struct node *head) { 

for (struct node *p = head; p != NULL; p = p->next) { 

free(p); 

} 

} 






Kernighan and Ritchie correct this error by storing a reference to p->next in q before freeing p: 

#include 

struct node { 

int value; 

struct node *next; 

}; 

void free_list(struct node *head) { 

struct node *q; 

for (struct node *p = head; p != NULL; p = q) { 

q = p->next; 

free(p); 

} 

} 


In this noncompliant code example, buf is written to after it has been freed. Write-after-free vulnerabilities 

can be exploited to run arbitrary code with the permissions of the vulnerable process. 

Typically, allocations and frees are far removed, making it difficult to recognize and diagnose 

these problems. 

#include 

#include 


char *return_val = 0; 

const size_t bufsize = strlen(argv[0]) + 1; 

char *buf = (char *)malloc(bufsize); 

if (!buf) { 

return EXIT_FAILURE; 

} 

/* ... */ 

free(buf); 

/* ... */ 

strcpy(buf, argv[0]); 

/* ... */ 

return EXIT_SUCCESS; 

} 






In this compliant solution, the memory is freed after its final use: 

#include 

#include 


char *return_val = 0; 

const size_t bufsize = strlen(argv[0]) + 1; 

char *buf = (char *)malloc(bufsize); 

if (!buf) { 


} 

/* ... */ 

strcpy(buf, argv[0]); 

/* ... */ 

free(buf); 


} 


In this noncompliant example, realloc() may free c_str1 when it returns a null pointer, resulting 

in c_str1 being freed twice. The C Standards Committee's proposed response to Defect 

Report #400 makes it implementation-defined whether or not the old object is deallocated when 

size is zero and memory for the new object is not allocated. The current implementation of realloc() 

in the GNU C Library and Microsoft Visual Studio's Runtime Library will free c_str1 

and return a null pointer for zero byte allocations. Freeing a pointer twice can result in a potentially 

exploitable vulnerability commonly referred to as a double-free vulnerability [Seacord 

2013b]. 

#include 

void f(char *c_str1, size_t size) { 

char *c_str2 = (char *)realloc(c_str1, size); 

if (c_str2 == NULL) { 

free(c_str1); 

} 

} 


This compliant solution does not pass a size argument of zero to the realloc() function, eliminating 

the possibility of c_str1 being freed twice: 

#include 





void f(char *c_str1, size_t size) { 

if (size != 0) { 

char *c_str2 = (char *)realloc(c_str1, size); 

if (c_str2 == NULL) { 

free(c_str1); 

} 

} 

else { 

free(c_str1); 

} 

} 

If the intent of calling f() is to reduce the size of the object, then doing nothing when the size is 

zero would be unexpected; instead, this compliant solution frees the object. 


In this noncompliant example (CVE-2009-1364) from libwmf version 0.2.8.4, the return value of 

gdRealloc (a simple wrapper around realloc() that reallocates space pointed to by im- 

>clip->list) is set to more. However, the value of im->clip->list is used directly afterwards 

in the code, and the C Standard specifies that if realloc() moves the area pointed to, then 

the original block is freed. An attacker can then execute arbitrary code by forcing a reallocation 

(with a sufficient im->clip->count) and accessing freed memory [xorl 2009]. 

void gdClipSetAdd(gdImagePtr im, gdClipRectanglePtr rect) { 

gdClipRectanglePtr more; 

if (im->clip == 0) { 

/* ... */ 

} 

if (im->clip->count == im->clip->max) { 

more = gdRealloc (im->clip->list,(im->clip->max + 8) * 

sizeof (gdClipRectangle)); 

/* 

* If the realloc fails, then we have not lost the 

* im->clip->list value. 

*/ 

if (more == 0) return; 

im->clip->max += 8; 

} 

im->clip->list[im->clip->count] = *rect; 

im->clip->count++; 

} 






This compliant solution simply reassigns im->clip->list to the value of more after the call to 

realloc(): 

void gdClipSetAdd(gdImagePtr im, gdClipRectanglePtr rect) { 

gdClipRectanglePtr more; 

if (im->clip == 0) { 

/* ... */ 

} 

if (im->clip->count == im->clip->max) { 

more = gdRealloc (im->clip->list,(im->clip->max + 8) * 

sizeof (gdClipRectangle)); 

if (more == 0) return; 

im->clip->max += 8; 

im->clip->list = more; 

} 

im->clip->list[im->clip->count] = *rect; 

im->clip->count++; 

} 


Reading memory that has already been freed can lead to abnormal program termination and denial-of-service 

attacks. Writing memory that has already been freed can additionally lead to the 

execution of arbitrary code with the permissions of the vulnerable process. 

Freeing memory multiple times has similar consequences to accessing memory after it is freed. 

Reading a pointer to deallocated memory is undefined behavior because the pointer value is indeterminate 

and might be a trap representation. When reading from or writing to freed memory does 

not cause a trap, it may corrupt the underlying data structures that manage the heap in a manner 

that can be exploited to execute arbitrary code. Alternatively, writing to memory after it has been 

freed might modify memory that has been reallocated. 

Programmers should be wary when freeing memory in a loop or conditional statement; if coded 

incorrectly, these constructs can lead to double-free vulnerabilities. It is also a common error to 

misuse the realloc() function in a manner that results in double-free vulnerabilities. (See 

MEM04-C. Beware of zero-length allocations.) 


MEM30-C High Likely Medium P18 L1 


VU#623332 describes a double-free vulnerability in the MIT Kerberos 5 function 

krb5_recvauth(). 








ISO/IEC TR 24772:2013 


MISRA C:2012 

MITRE CWE 

MEM01-C. Store a new value in pointers immediately 

after free() 

MEM50-CPP. Do not access freed memory 


Dangling Reference to Heap [XYK] 

Accessing freed memory [accfree] 

Freeing memory multiple times [dblfree] 


CWE-415, Double Free 

CWE-416, Use After Free 


[ISO/IEC 9899:2011] 

[Kernighan 1988] 

[OWASP Freed Memory] 

[MIT 2005] 


[Viega 2005] 

[VU#623332] 

[xorl 2009] 

7.22.3, “Memory Management Functions” 

Section 7.8.5, “Storage Management” 

Chapter 4, “Dynamic Memory Management” 

Section 5.2.19, “Using Freed Memory” 

CVE-2009-1364: LibWMF Pointer Use after 

free() 




Memory Management (MEM) - MEM31-C. Free dynamically allocated memory when no longer needed 

9.2 MEM31-C. Free dynamically allocated memory when no longer 

needed 

Before the lifetime of the last pointer that stores the return value of a call to a standard memory 

allocation function has ended, it must be matched by a call to free() with that pointer value. 


In this noncompliant example, the object allocated by the call to malloc() is not freed before the 

end of the lifetime of the last pointer text_buffer referring to the object: 

#include 


int f(void) { 

char *text_buffer = (char *)malloc(BUFFER_SIZE); 

if (text_buffer == NULL) { 

return -1; 

} 

return 0; 

} 


In this compliant solution, the pointer is deallocated with a call to free(): 

#include 


int f(void) { 

char *text_buffer = (char *)malloc(BUFFER_SIZE); 


return -1; 

} 

} 

free(text_buffer); 

return 0; 




Memory Management (MEM) - MEM31-C. Free dynamically allocated memory when no longer needed 


MEM31-C-EX1: Allocated memory does not need to be freed if it is assigned to a pointer with 

static storage duration whose lifetime is the entire execution of a program. The following code example 

illustrates a pointer that stores the return value from malloc() in a static variable: 

#include 


int f(void) { 

static char *text_buffer = NULL; 


text_buffer = (char *)malloc(BUFFER_SIZE); 


return -1; 

} 

} 

return 0; 

} 


Failing to free memory can result in the exhaustion of system memory resources, which can lead 

to a denial-of-service attack. 


MEM31-C Medium Probable Medium P8 L2 


ISO/IEC TR 24772:2013 


MITRE CWE 

Memory Leak [XYL] 

Failing to close files or free dynamic memory 

when they are no longer needed [fileclose] 

CWE-401, Improper Release of Memory Before 

Removing Last Reference (“Memory 

Leak”) 


[ISO/IEC 9899:2011] 

Subclause 7.22.3, “Memory Management 

Functions” 




Memory Management (MEM) - MEM33-C. Allocate and copy structures containing a flexible array member dynamically 

9.3 MEM33-C. Allocate and copy structures containing a flexible array 

member dynamically 

The C Standard, 6.7.2.1, paragraph 18 [ISO/IEC 9899:2011], says 

As a special case, the last element of a structure with more than one named member 

may have an incomplete array type; this is called a flexible array member. In most situations, 

the flexible array member is ignored. In particular, the size of the structure is as if 

the flexible array member were omitted except that it may have more trailing padding 

than the omission would imply. 

The following is an example of a structure that contains a flexible array member: 

struct flex_array_struct { 

int num; 

int data[]; 

}; 

This definition means that when computing the size of such a structure, only the first member, 

num, is considered. Unless the appropriate size of the flexible array member has been explicitly 

added when allocating storage for an object of the struct, the result of accessing the member 

data of a variable of nonpointer type struct flex_array_struct is undefined. DCL38-C. 

Use the correct syntax when declaring a flexible array member describes the correct way to declare 

a struct with a flexible array member. 

To avoid the potential for undefined behavior, structures that contain a flexible array member 

should always be allocated dynamically. Flexible array structures must 

• Have dynamic storage duration (be allocated via malloc() or another dynamic allocation 

function) 

• Be dynamically copied using memcpy() or a similar function and not by assignment 

• When used as an argument to a function, be passed by pointer and not copied by value 

9.3.1 Noncompliant Code Example (Storage Duration) 

This noncompliant code example uses automatic storage for a structure containing a flexible array 

member: 

#include 


size_t num; 

int data[]; 

}; 






struct flex_array_struct flex_struct; 

size_t array_size = 4; 

/* Initialize structure */ 

flex_struct.num = array_size; 

} 


flex_struct.data[i] = 0; 

} 

Because the memory for flex_struct is reserved on the stack, no space is reserved for the 

data member. Accessing the data member is undefined behavior. 

9.3.2 Compliant Solution (Storage Duration) 

This compliant solution dynamically allocates storage for flex_array_struct: 

#include 


size_t num; 

int data[]; 

}; 


struct flex_array_struct *flex_struct; 


} 

/* Dynamically allocate memory for the struct */ 

flex_struct = (struct flex_array_struct *)malloc( 

sizeof(struct flex_array_struct) 


if (flex_sruct == NULL) { 


} 

/* Initialize structure */ 

flex_struct->num = array_size; 


flex_struct->data[i] = 0; 

} 





9.3.3 Noncompliant Code Example (Copying) 

This noncompliant code example attempts to copy an instance of a structure containing a flexible 

array member (struct flex_array_struct) by assignment: 

#include 


size_t num; 

int data[]; 

}; 

void func(struct flex_array_struct *struct_a, 

struct flex_array_struct *struct_b) { 

*struct_b = *struct_a; 

} 

When the structure is copied, the size of the flexible array member is not considered, and only the 

first member of the structure, num, is copied, leaving the array contents untouched. 

9.3.4 Compliant Solution (Copying) 

This compliant solution uses memcpy() to properly copy the content of struct_a into 

struct_b: 

#include 


size_t num; 

int data[]; 

}; 

void func(struct flex_array_struct *struct_a, 

struct flex_array_struct *struct_b) { 

if (struct_a->num > struct_b->num) { 

/* Insufficient space; handle error */ 

return; 

} 

memcpy(struct_b, struct_a, 

sizeof(struct flex_array_struct) + (sizeof(int) 

* struct_a->num)); 

} 





9.3.5 Noncompliant Code Example (Function Arguments) 

In this noncompliant code example, the flexible array structure is passed by value to a function 

that prints the array elements: 

#include 

#include 


size_t num; 

int data[]; 

}; 

void print_array(struct flex_array_struct struct_p) { 

puts("Array is: "); 

for (size_t i = 0; i < struct_p.num; ++i) { 

printf("%d ", struct_p.data[i]); 

} 

putchar('\n'); 

} 


struct flex_array_struct *struct_p; 



struct_p = (struct flex_array_struct *)malloc( 

sizeof(struct flex_array_struct) 


if (struct_p == NULL) { 


} 

struct_p->num = array_size; 

} 


struct_p->data[i] = i; 

} 

print_array(*struct_p); 

Because the argument is passed by value, the size of the flexible array member is not considered 

when the structure is copied, and only the first member of the structure, num, is copied. 

9.3.6 Compliant Solution (Function Arguments) 

In this compliant solution, the structure is passed by reference and not by value: 

#include 

#include 






size_t num; 

int data[]; 

}; 

void print_array(struct flex_array_struct *struct_p) { 

puts("Array is: "); 

for (size_t i = 0; i < struct_p->num; ++i) { 

printf("%d ", struct_p->data[i]); 

} 

putchar('\n'); 

} 


struct flex_array_struct *struct_p; 


/* Space is allocated for the struct and initialized... */ 

} 

print_array(struct_p); 


Failure to use structures with flexible array members correctly can result in undefined behavior. 


MEM33-C Low Unlikely Low P3 L3 



DCL38-C. Use the correct syntax when declaring 

a flexible array member 


[ISO/IEC 9899:2011] 

[JTC1/SC22/WG14 N791] 

Subclause 6.7.2.1, “Structure and Union Specifiers” 

Solving the Struct Hack Problem 




Memory Management (MEM) - MEM34-C. Only free memory allocated dynamically 

9.4 MEM34-C. Only free memory allocated dynamically 

The C Standard, Annex J [ISO/IEC 9899:2011], states that the behavior of a program is undefined 

when 

The pointer argument to the free or realloc function does not match a pointer earlier 

returned by a memory management function, or the space has been deallocated by a 

call to free or realloc. 


Freeing memory that is not allocated dynamically can result in heap corruption and other serious 

errors. Do not call free() on a pointer other than one returned by a standard memory allocation 

function, such as malloc(), calloc(), realloc(), or aligned_alloc(). 

A similar situation arises when realloc() is supplied a pointer to non-dynamically allocated 

memory. The realloc() function is used to resize a block of dynamic memory. If realloc() 

is supplied a pointer to memory not allocated by a standard memory allocation function, the behavior 

is undefined. One consequence is that the program may terminate abnormally. 

This rule does not apply to null pointers. The C Standard guarantees that if free() is passed a 

null pointer, no action occurs. 


This noncompliant code example sets c_str to reference either dynamically allocated memory or 

a statically allocated string literal depending on the value of argc. In either case, c_str is passed 

as an argument to free(). If anything other than dynamically allocated memory is referenced by 

c_str, the call to free(c_str) is erroneous. 

#include 

#include 

#include 

enum { MAX_ALLOCATION = 1000 }; 

int main(int argc, const char *argv[]) { 

char *c_str = NULL; 

size_t len; 

if (argc == 2) { 

len = strlen(argv[1]) + 1; 

if (len > MAX_ALLOCATION) { 


} 

c_str = (char *)malloc(len); 

if (c_str == NULL) { 






} 

} 

strcpy(c_str, argv[1]); 

} else { 

c_str = "usage: $>a.exe [string]"; 


} 

free(c_str); 

return 0; 


This compliant solution eliminates the possibility of c_str referencing memory that is not allocated 

dynamically when passed to free(): 

#include 

#include 

#include 

enum { MAX_ALLOCATION = 1000 }; 


char *c_str = NULL; 

size_t len; 

} 

if (argc == 2) { 

len = strlen(argv[1]) + 1; 

if (len > MAX_ALLOCATION) { 


} 

c_str = (char *)malloc(len); 

if (c_str == NULL) { 


} 

strcpy(c_str, argv[1]); 

} else { 

printf("%s\n", "usage: $>a.exe [string]"); 


} 

free(c_str); 

return 0; 





9.4.3 Noncompliant Code Example (realloc()) 

In this noncompliant example, the pointer parameter to realloc(), buf, does not refer to dynamically 

allocated memory: 

#include 



char buf[BUFSIZE]; 

char *p = (char *)realloc(buf, 2 * BUFSIZE); 

if (p == NULL) { 


} 

} 


In this compliant solution, buf refers to dynamically allocated memory: 

#include 



char *buf = (char *)malloc(BUFSIZE * sizeof(char)); 

char *p = (char *)realloc(buf, 2 * BUFSIZE); 



} 

} 

Note that realloc() will behave properly even if malloc() failed, because when given a null 

pointer, realloc() behaves like a call to malloc(). 


The consequences of this error depend on the implementation, but they range from nothing to arbitrary 

code execution if that memory is reused by malloc(). 


MEM34-C High Likely Medium P18 L1 






CVE-2015-0240 describes a vulnerability in which an uninitialized pointer is passed to 

TALLOC_FREE(), which is a Samba-specific memory deallocation macro that wraps the talloc_free() 

function. The implementation of talloc_free() would access the uninitialized 

pointer, resulting in a remote exploit. 





MITRE CWE 

MEM31-C. Free dynamically allocated 

memory when no longer needed 

MEM51-CPP. Properly deallocate dynamically 

allocated resources 

Reallocating or freeing memory that was not 

dynamically allocated [xfree] 

CWE-590, Free of Memory Not on the Heap 


[ISO/IEC 9899:2011] 


Subclause J.2, “Undefined Behavior” 





Memory Management (MEM) - MEM35-C. Allocate sufficient memory for an object 

9.5 MEM35-C. Allocate sufficient memory for an object 

The types of integer expressions used as size arguments to malloc(), calloc(), realloc(), 

or aligned_alloc() must have sufficient range to represent the size of the objects to be stored. 

If size arguments are incorrect or can be manipulated by an attacker, then a buffer overflow may 

occur. Incorrect size arguments, inadequate range checking, integer overflow, or truncation can 

result in the allocation of an inadequately sized buffer. 

Typically, the amount of memory to allocate will be the size of the type of object to allocate. 

When allocating space for an array, the size of the object will be multiplied by the bounds of the 

array. When allocating space for a structure containing a flexible array member, the size of the array 

member must be added to the size of the structure. (See MEM33-C. Allocate and copy structures 

containing a flexible array member dynamically.) Use the correct type of the object when 

computing the size of memory to allocate. 

STR31-C. Guarantee that storage for strings has sufficient space for character data and the null 

terminator is a specific instance of this rule. 

9.5.1 Noncompliant Code Example (Integer) 

In this noncompliant code example, an array of long is allocated and assigned to p. The code 

checks for unsigned integer overflow in compliance with INT32-C. Ensure that operations on 

signed integers do not result in overflow and also ensures that len is not equal to zero. (See 

MEM04-C. Beware of zero-length allocations.) However, because sizeof(int) is used to compute 

the size, and not sizeof(long), an insufficient amount of memory can be allocated on implementations 

where sizeof(long) is larger than sizeof(int). 

#include 

#include 

void function(size_t len) { 

long *p; 

if (len == 0 || len > SIZE_MAX / sizeof(long)) { 

/* Handle overflow */ 

} 

p = (long *)malloc(len * sizeof(int)); 



} 

free(p); 

} 





9.5.2 Compliant Solution (Integer) 

This compliant solution uses sizeof(long) to correctly size the memory allocation: 

#include 

#include 


long *p; 

if (len == 0 || len > SIZE_MAX / sizeof(long)) { 


} 

p = (long *)malloc(len * sizeof(long)); 



} 

free(p); 

} 

9.5.3 Compliant Solution (Integer) 

Alternatively, sizeof(*p) can be used to properly size the allocation: 

#include 

#include 


long *p; 

if (len == 0 || len > SIZE_MAX / sizeof(*p)) { 


} 

p = (long *)malloc(len * sizeof(*p)); 



} 

free(p); 

} 

9.5.4 Noncompliant Code Example (Pointer) 

In this noncompliant code example, inadequate space is allocated for a struct tm object because 

the size of the pointer is being used to determine the size of the pointed-to object: 

#include 

#include 

struct tm *make_tm(int year, int mon, int day, int hour, 

int min, int sec) { 

struct tm *tmb; 





} 

tmb = (struct tm *)malloc(sizeof(tmb)); 

if (tmb == NULL) { 

return NULL; 

} 

*tmb = (struct tm) { 

.tm_sec = sec, .tm_min = min, .tm_hour = hour, 

.tm_mday = day, .tm_mon = mon, .tm_year = year 

}; 

return tmb; 

9.5.5 Compliant Solution (Pointer) 

In this compliant solution, the correct amount of memory is allocated for the struct tm object. 

When allocating space for a single object, passing the (dereferenced) pointer type to the sizeof 

operator is a simple way to allocate sufficient memory. Because the sizeof operator does not 

evaluate its operand, dereferencing an uninitialized or null pointer in this context is well-defined 

behavior. 

#include 

#include 

struct tm *make_tm(int year, int mon, int day, int hour, 

int min, int sec) { 

struct tm *tmb; 

tmb = (struct tm *)malloc(sizeof(*tmb)); 

if (tmb == NULL) { 

return NULL; 

} 

*tmb = (struct tm) { 

.tm_sec = sec, .tm_min = min, .tm_hour = hour, 

.tm_mday = day, .tm_mon = mon, .tm_year = year 

}; 

return tmb; 

} 


Providing invalid size arguments to memory allocation functions can lead to buffer overflows and 

the execution of arbitrary code with the permissions of the vulnerable process. 


MEM35-C High Probable High P6 L2 







ISO/IEC TR 24772:2013 

ISO/IEC TS 17961:2013 

MITRE CWE 

ARR01-C. Do not apply the sizeof operator to 

a pointer when taking the size of an array 

INT31-C. Ensure that integer conversions do 

not result in lost or misinterpreted data 



INT18-C. Evaluate integer expressions in a 

larger size before comparing or assigning to 

that size 

MEM04-C. Beware of zero-length allocations 


[HCB] 

Taking the size of a pointer to determine the 

size of the pointed-to type [sizeofptr] 

CWE-131, Incorrect Calculation of Buffer Size 


CWE-467, Use of sizeof() on a Pointer 

Type 





[Viega 2005] 

[xorl 2009] 

Section 2.1.1, “Respecting Memory Bounds” 



Section 5.6.8, “Use of sizeof() on a Pointer 

Type” 

CVE-2009-0587: Evolution Data Server 

Base64 Integer Overflows 




Memory Management (MEM) - MEM36-C. Do not modify the alignment of objects by calling realloc() 

9.6 MEM36-C. Do not modify the alignment of objects by calling 

realloc() 

Do not invoke realloc() to modify the size of allocated objects that have stricter alignment requirements 

than those guaranteed by malloc(). Storage allocated by a call to the standard 

aligned_alloc() function, for example, can have stricter than normal alignment requirements. 

The C standard requires only that a pointer returned by realloc() be suitably aligned so that it 

may be assigned to a pointer to any type of object with a fundamental alignment requirement. 


This noncompliant code example returns a pointer to allocated memory that has been aligned to a 

4096-byte boundary. If the resize argument to the realloc() function is larger than the object 

referenced by ptr, then realloc() will allocate new memory that is suitably aligned so that it 

may be assigned to a pointer to any type of object with a fundamental alignment requirement but 

may not preserve the stricter alignment of the original object. 

#include 


size_t resize = 1024; 

size_t alignment = 1


size_t align = 1


if (NULL == (ptr = (int *)aligned_alloc(alignment, 

sizeof(int)))) { 


} 

if (NULL == (ptr1 = (int *)aligned_alloc(alignment, 

resize))) { 


} 

if (NULL == (memcpy(ptr1, ptr, sizeof(int))) { 


} 

} 

free(ptr); 

9.6.3 Compliant Solution (Windows) 

Windows defines the _aligned_malloc() function to allocate memory on a specified alignment 

boundary. The _aligned_realloc() [MSDN] can be used to change the size of this 

memory. This compliant solution demonstrates one such usage: 

#include 


size_t alignment = 1



Improper alignment can lead to arbitrary memory locations being accessed and written to. 


MEM36-C Low Probable High P2 L3 


[ISO/IEC 9899:2011] 

[MSDN] 

7.22.3.1, “The aligned_alloc Function” 

aligned_malloc() 




Input/Output (FIO) - FIO30-C. Exclude user input from format strings 

10 Input/Output (FIO) 

10.1 FIO30-C. Exclude user input from format strings 

Never call a formatted I/O function with a format string containing a tainted value . An attacker 

who can fully or partially control the contents of a format string can crash a vulnerable process, 

view the contents of the stack, view memory content, or write to an arbitrary memory location. 

Consequently, the attacker can execute arbitrary code with the permissions of the vulnerable process 

[Seacord 2013b]. Formatted output functions are particularly dangerous because many programmers 

are unaware of their capabilities. For example, formatted output functions can be used 

to write an integer value to a specified address using the %n conversion specifier. 


The incorrect_password() function in this noncompliant code example is called during identification 

and authentication to display an error message if the specified user is not found or the 

password is incorrect. The function accepts the name of the user as a string referenced by user. 

This is an exemplar of untrusted data that originates from an unauthenticated user. The function 

constructs an error message that is then output to stderr using the C Standard fprintf() function. 

#include 

#include 

#include 

void incorrect_password(const char *user) { 

int ret; 

/* User names are restricted to 256 or fewer characters */ 

static const char msg_format[] = "%s cannot be authenticated.\n"; 

size_t len = strlen(user) + sizeof(msg_format); 

char *msg = (char *)malloc(len); 

if (msg == NULL) { 


} 

ret = snprintf(msg, len, msg_format, user); 

if (ret < 0) { 


} else if (ret >= len) { 

/* Handle truncated output */ 

} 

fprintf(stderr, msg); 

free(msg); 

} 





The incorrect_password() function calculates the size of the message, allocates dynamic 

storage, and then constructs the message in the allocated memory using the snprintf() function. 

The addition operations are not checked for integer overflow because the string referenced 

by user is known to have a length of 256 or less. Because the %s characters are replaced by the 

string referenced by user in the call to snprintf(), the resulting string needs 1 byte less than is 

allocated. The snprintf() function is commonly used for messages that are displayed in multiple 

locations or messages that are difficult to build. However, the resulting code contains a format-string 

vulnerability because the msg includes untrusted user input and is passed as the formatstring 

argument in the call to fprintf(). 

10.1.2 Compliant Solution (fputs()) 

This compliant solution fixes the problem by replacing the fprintf() call with a call to 

fputs(), which outputs msg directly to stderr without evaluating its contents: 

#include 

#include 

#include 


int ret; 





if (msg == NULL) { 


} 


if (ret < 0) { 




} 

fputs(msg, stderr); 

free(msg); 

} 

10.1.3 Compliant Solution (fprintf()) 

This compliant solution passes the untrusted user input as one of the variadic arguments to 

fprintf() and not as part of the format string, eliminating the possibility of a format-string vulnerability: 

#include 







} 

fprintf(stderr, msg_format, user); 


This noncompliant code example is similar to the first noncompliant code example but uses the 

POSIX function syslog() [IEEE Std 1003.1:2013] instead of the fprintf() function. The 

syslog() function is also susceptible to format-string vulnerabilities. 

#include 

#include 

#include 

#include 


int ret; 





if (msg != NULL) { 


} 


if (ret < 0) { 




} 

syslog(LOG_INFO, msg); 

free(msg); 

} 

The syslog() function first appeared in BSD 4.2 and is supported by Linux and other modern 

UNIX implementations. It is not available on Windows systems. 


This compliant solution passes the untrusted user input as one of the variadic arguments to syslog() 

instead of including it in the format string: 

#include 



syslog(LOG_INFO, msg_format, user); 

} 






Failing to exclude user input from format specifiers may allow an attacker to crash a vulnerable 

process, view the contents of the stack, view memory content, or write to an arbitrary memory location 

and consequently execute arbitrary code with the permissions of the vulnerable process. 


FIO30-C High Likely Medium P18 L1 


Two examples of format-string vulnerabilities resulting from a violation of this rule include Ettercap 

and Samba. 

In Ettercap v.NG-0.7.2, the ncurses user interface suffers from a format-string defect. The 

curses_msg() function in ec_curses.c calls wdg_scroll_print(), which takes a format 

string and its parameters and passes it to vw_printw(). The curses_msg() function uses one 

of its parameters as the format string. This input can include user data, allowing for a formatstring 

vulnerability. 

The Samba AFS ACL mapping VFS plug-in fails to properly sanitize user-controlled file names 

that are used in a format specifier supplied to snprintf(). This security flaw becomes exploitable 

when a user can write to a share that uses Samba’s afsacl.so library for setting Windows 

NT access control lists on files residing on an AFS file system. 


CERT Oracle Secure Coding Standard for Java IDS06-J. Exclude unsanitized user input from 

format strings 

CERT Perl Secure Coding Standard 

IDS30-PL. Exclude user input from format 

strings 

ISO/IEC TR 24772:2013 

Injection [RST] 

ISO/IEC TS 17961:2013 

Including tainted or out-of-domain input in a 

format string [usrfmt] 

MITRE CWE 

CWE-134, Uncontrolled Format String 


[IEEE Std 1003.1:2013] 


[Viega 2005] 

XSH, System Interfaces, syslog 

Chapter 6, “Formatted Output” 

Section 5.2.23, “Format String Problem” 




Input/Output (FIO) - FIO32-C. Do not perform operations on devices that are only appropriate for files 

10.2 FIO32-C. Do not perform operations on devices that are only 

appropriate for files 

File names on many operating systems, including Windows and UNIX, may be used to access 

special files, which are actually devices. Reserved Microsoft Windows device names include AUX, 

CON, PRN, COM1, and LPT1 or paths using the \\.\ device namespace. Device files on UNIX systems 

are used to apply access rights and to direct operations on the files to the appropriate device 

drivers. 

Performing operations on device files that are intended for ordinary character or binary files can 

result in crashes and denial-of-service attacks. For example, when Windows attempts to interpret 

the device name as a file resource, it performs an invalid resource access that usually results in a 

crash [Howard 2002]. 

Device files in UNIX can be a security risk when an attacker can access them in an unauthorized 

way. For example, if attackers can read or write to the /dev/kmem device, they may be able to 

alter the priority, UID, or other attributes of their process or simply crash the system. Similarly, 

access to disk devices, tape devices, network devices, and terminals being used by other processes 

can lead to problems [Garfinkel 1996]. 

On Linux, it is possible to lock certain applications by attempting to open devices rather than 

files. Consider the following example: 

/dev/mouse 

/dev/console 

/dev/tty0 

/dev/zero 

A Web browser that failed to check for these devices would allow an attacker to create a website 

with image tags such as that would lock the user's mouse. 


In this noncompliant code example, the user can specify a locked device or a FIFO (first-in, firstout) 

file name, which can cause the program to hang on the call to fopen(): 

#include 

void func(const char *file_name) { 

FILE *file; 

if ((file = fopen(file_name, "wb")) == NULL) { 


} 

/* Operate on the file */ 





} 

if (fclose(file) == EOF) { 


} 


POSIX defines the O_NONBLOCK flag to open(), which ensures that delayed operations on a file 

do not hang the program [IEEE Std 1003.1:2013]. 

When opening a FIFO with O_RDONLY or O_WRONLY set: 

When opening a block special or character special file that supports nonblocking opens: 

Otherwise, the behavior of O_NONBLOCK is unspecified. 

Once the file is open, programmers can use the POSIX lstat() and fstat() functions to obtain 

information about a file and the S_ISREG() macro to determine if the file is a regular file. 

Because the behavior of O_NONBLOCK on subsequent calls to read() or write() is unspecified, 

it is advisable to disable the flag after it has been determined that the file in question is not a special 

device. 

When available (Linux 2.1.126+, FreeBSD, Solaris 10, POSIX.1-2008), the O_NOFOLLOW flag 

should also be used. (See POS01-C. Check for the existence of links when dealing with files.) 

When O_NOFOLLOW is not available, symbolic link checks should use the method from POS35-C. 

Avoid race conditions while checking for the existence of a symbolic link. 

#include 

#include 

#include 

#include 

#ifdef O_NOFOLLOW 

#define OPEN_FLAGS O_NOFOLLOW | O_NONBLOCK 

#else 

#define OPEN_FLAGS O_NONBLOCK 

#endif 


struct stat orig_st; 

struct stat open_st; 

int fd; 

int flags; 

if ((lstat(file_name, &orig_st) != 0) || 

(!S_ISREG(orig_st.st_mode))) { 






} 

/* Race window */ 

fd = open(file_name, OPEN_FLAGS | O_WRONLY); 

if (fd == -1) { 


} 

if (fstat(fd, &open_st) != 0) { 


} 

if ((orig_st.st_mode != open_st.st_mode) || 

(orig_st.st_ino != open_st.st_ino) || 

(orig_st.st_dev != open_st.st_dev)) { 

/* The file was tampered with */ 

} 

/* 

* Optional: drop the O_NONBLOCK now that we are sure 

* this is a good file. 

*/ 

if ((flags = fcntl(fd, F_GETFL)) == -1) { 


} 

if (fcntl(fd, F_SETFL, flags & ~O_NONBLOCK) == -1) { 


} 


} 

if (close(fd) == -1) { 


} 

This code contains an intractable TOCTOU (time-of-check, time-of-use) race condition under 

which an attacker can alter the file referenced by file_name following the call to lstat() but 

before the call to open(). The switch will be discovered after the file is opened, but opening the 

file cannot be prevented in the case where this action itself causes undesired behavior. (See 

FIO45-C. Avoid TOCTOU race conditions while accessing files for more information about 

TOCTOU race conditions.) 

Essentially, an attacker can switch out a file for one of the file types shown in the following table 

with the specified effect. 





File Types and Effects 

Type 

Another regular file 

FIFO 

Symbolic link 

Special device 

Note on Effect 

The fstat() verification fails. 

Either open() returns -1 and sets errno to 

ENXIO, or open() succeeds and the fstat() 

verification fails. 

open() returns -1 if O_NOFOLLOW is available; 

otherwise, the fstat() verification fails. 

Usually the fstat() verification fails on st_mode. 

This can still be a problem if the device is one for 

which just opening (or closing) it causes a side effect. 

If st_mode compares equal, then the device is one 

that, after opening, appears to be a regular file. It 

would then fail the fstat() verification on 

st_dev and st_ino (unless it happens to be the 

same file, as can happen with /dev/fd/* on Solaris, 

but this would not be a problem). 

To be compliant with this rule and to prevent this TOCTOU race condition, file_name must refer 

to a file in a secure directory. (See FIO15-C. Ensure that file operations are performed in a secure 

directory.) 

10.2.3 Noncompliant Code Example (Windows) 

This noncompliant code example uses the GetFileType() function to attempt to prevent opening 

a special file: 

#include 

void func(const TCHAR *file_name) { 

HANDLE hFile = CreateFile( 

file_name, 

GENERIC_READ | GENERIC_WRITE, 0, 

NULL, OPEN_EXISTING, 

FILE_ATTRIBUTE_NORMAL, NULL 

); 

if (hFile == INVALID_HANDLE_VALUE) { 


} else if (GetFileType(hFile) != FILE_TYPE_DISK) { 


CloseHandle(hFile); 

} else { 


CloseHandle(hFile); 

} 

} 





Although tempting, the Win32 GetFileType() function is dangerous in this case. If the file 

name given identifies a named pipe that is currently blocking on a read request, the call to Get- 

FileType() will block until the read request completes. This provides an effective attack vector 

for a denial-of-service attack on the application. Furthermore, the act of opening a file handle may 

cause side effects, such as line states being set to their default voltage when opening a serial device. 


Microsoft documents a list of reserved identifiers that represent devices and have a device 

namespace to be used specifically by devices [MSDN]. In this compliant solution, the 

isReservedName() function can be used to determine if a specified path refers to a device. Care 

must be taken to avoid a TOCTOU race condition when first testing a path name using the 

isReservedName() function and then later operating on that path name. 

#include 

#include 

#include 

#include 

#include 

static bool isReservedName(const char *path) { 

/* This list of reserved names comes from MSDN */ 

static const char *reserved[] = { 

"nul", "con", "prn", "aux", "com1", "com2", "com3", 

"com4", "com5", "com6", "com7", "com8", "com9", 

"lpt1", "lpt2", "lpt3", "lpt4", "lpt5", "lpt6", 

"lpt7", "lpt8", "lpt9" 

}; 

bool ret = false; 

/* 

* First, check to see if this is a device namespace, which 

* always starts with \\.\, because device namespaces are not 

* valid file paths. 

*/ 

if (!path || 0 == strncmp(path, "\\\\.\\", 4)) { 

return true; 

} 

/* Compare against the list of ancient reserved names */ 

for (size_t i = 0; !ret && 

i < sizeof(reserved) / sizeof(*reserved); ++i) { 

/* 

* Because Windows uses a case-insensitive file system, operate 

on 

* a lowercase version of the given filename. Note: This ignores 

* globalization issues and assumes ASCII characters. 





} 

*/ 

if (0 == _stricmp(path, reserved[i])) { 

ret = true; 

} 

} 

return ret; 


Allowing operations that are appropriate only for regular files to be performed on devices can result 

in denial-of-service attacks or more serious exploits depending on the platform. 


FIO32-C Medium Unlikely Medium P4 L3 



FIO05-C. Identify files using multiple file attributes 

FIO15-C. Ensure that file operations are performed 

in a secure directory 

POS01-C. Check for the existence of links 

when dealing with files 

POS35-C. Avoid race conditions while checking 

for the existence of a symbolic link 

CERT Oracle Secure Coding Standard for Java FIO00-J. Do not operate on files in shared directories 

MITRE CWE 

CWE-67, Improper Handling of Windows Device 

Names 


[Garfinkel 1996] 

[Howard 2002] 

[IEEE Std 1003.1:2013] 

[MSDN] 

Section 5.6, “Device Files” 

Chapter 11, “Canonical Representation Issues” 

XSH, System Interfaces, open 




Input/Output (FIO) - FIO34-C. Distinguish between characters read from a file and EOF or WEOF 

10.3 FIO34-C. Distinguish between characters read from a file and EOF 

or WEOF 

The EOF macro represents a negative value that is used to indicate that the file is exhausted and no 

data remains when reading data from a file. EOF is an example of an in-band error indicator. Inband 

error indicators are problematic to work with, and the creation of new in-band-error indicators 

is discouraged by ERR02-C. Avoid in-band error indicators. 

The byte I/O functions fgetc(), getc(), and getchar() all read a character from a stream and 

return it as an int. (See STR00-C. Represent characters using an appropriate type.) If the stream 

is at the end of the file, the end-of-file indicator for the stream is set and the function returns EOF. 

If a read error occurs, the error indicator for the stream is set and the function returns EOF. If these 

functions succeed, they cast the character returned into an unsigned char. 

Because EOF is negative, it should not match any unsigned character value. However, this is only 

true for implementations where the int type is wider than char. On an implementation where 

int and char have the same width, a character-reading function can read and return a valid character 

that has the same bit-pattern as EOF. This could occur, for example, if an attacker inserted a 

value that looked like EOF into the file or data stream to alter the behavior of the program. 

The C Standard requires only that the int type be able to represent a maximum value of +32767 

and that a char type be no larger than an int. Although uncommon, this situation can result in 

the integer constant expression EOF being indistinguishable from a valid character; that is, 

(int)(unsigned char)65535 == -1. Consequently, failing to use feof() and ferror() to 

detect end-of-file and file errors can result in incorrectly identifying the EOF character on rare implementations 

where sizeof(int) == sizeof(char). 

This problem is much more common when reading wide characters. The fgetwc(), getwc(), 

and getwchar() functions return a value of type wint_t. This value can represent the next wide 

character read, or it can represent WEOF, which indicates end-of-file for wide character streams. 

On most implementations, the wchar_t type has the same width as wint_t, and these functions 

can return a character indistinguishable from WEOF. 

In the UTF-16 character set, 0xFFFF is guaranteed not to be a character, which allows WEOF to be 

represented as the value -1. Similarly, all UTF-32 characters are positive when viewed as a 

signed 32-bit integer. All widely used character sets are designed with at least one value that does 

not represent a character. Consequently, it would require a custom character set designed without 

consideration of the C programming language for this problem to occur with wide characters or 

with ordinary characters that are as wide as int. 

The C Standard feof() and ferror() functions are not subject to the problems associated with 

character and integer sizes and should be used to verify end-of-file and file errors for susceptible 





implementations [Kettlewell 2002]. Calling both functions on each iteration of a loop adds significant 

overhead, so a good strategy is to temporarily trust EOF and WEOF within the loop but verify 

them with feof() and ferror() following the loop. 


This noncompliant code example loops while the character c is not EOF: 

#include 


int c; 

} 

do { 

c = getchar(); 

} while (c != EOF); 

Although EOF is guaranteed to be negative and distinct from the value of any unsigned character, 

it is not guaranteed to be different from any such value when converted to an int. Consequently, 

when int has the same width as char, this loop may terminate prematurely. 

10.3.2 Compliant Solution (Portable) 

This compliant solution uses feof() to test for end-of-file and ferror() to test for errors: 

#include 


int c; 

} 

do { 



if (feof(stdin)) { 

/* Handle end of file */ 

} else if (ferror(stdin)) { 

/* Handle file error */ 

} else { 

/* Received a character that resembles EOF; handle error */ 

} 

10.3.3 Noncompliant Code Example (Nonportable) 

This noncompliant code example uses an assertion to ensure that the code is executed only on architectures 

where int is wider than char and EOF is guaranteed not to be a valid character value. 

However, this code example is noncompliant because the variable c is declared as a char rather 





than an int, making it possible for a valid character value to compare equal to the value of the 

EOF macro when char is signed because of sign extension: 

#include 

#include 

#include 


char c; 

static_assert(UCHAR_MAX < UINT_MAX, "FIO34-C violation"); 

} 

do { 



Assuming that a char is a signed 8-bit type and an int is a 32-bit type, if getchar() returns the 

character value '\xff (decimal 255), it will be interpreted as EOF because this value is sign-extended 

to 0xFFFFFFFF (the value of EOF) to perform the comparison. (See STR34-C. Cast characters 

to unsigned char before converting to larger integer sizes.) 

10.3.4 Compliant Solution (Nonportable) 

This compliant solution declares c to be an int. Consequently, the loop will terminate only when 

the file is exhausted. 

#include 

#include 

#include 


int c; 

static_assert(UCHAR_MAX < UINT_MAX, "FIO34-C violation"); 

} 

do { 



10.3.5 Noncompliant Code Example (Wide Characters) 

In this noncompliant example, the result of the call to the C standard library function getwc() is 

stored into a variable of type wchar_t and is subsequently compared with WEOF: 

#include 

#include 

#include 







wchar_t buf[BUFFER_SIZE]; 

wchar_t wc; 

size_t i = 0; 

} 

while ((wc = getwc(stdin)) != L'\n' && wc != WEOF) { 

if (i < (BUFFER_SIZE - 1)) { 

buf[i++] = wc; 

} 

} 

buf[i] = L'\0'; 

This code suffers from two problems. First, the value returned by getwc() is immediately converted 

to wchar_t before being compared with WEOF. Second, there is no check to ensure that 

wint_t is wider than wchar_t. Both of these problems make it possible for an attacker to terminate 

the loop prematurely by supplying the wide-character value matching WEOF in the file. 

10.3.6 Compliant Solution (Portable) 

This compliant solution declares c to be a wint_t to match the integer type returned by 

getwc(). Furthermore, it does not rely on WEOF to determine end-of-file definitively. 

#include 

#include 

#include 

enum {BUFFER_SIZE = 32 } 


wchar_t buf[BUFFER_SIZE]; 

wint_t wc; 

size_t i = 0; 

while ((wc = getwc(stdin)) != L'\n' && wc != WEOF) { 

if (i < BUFFER_SIZE - 1) { 

buf[i++] = wc; 

} 

} 

if (feof(stdin) || ferror(stdin)) { 

buf[i] = L'\0'; 

} else { 

/* Received a wide character that resembles WEOF; handle error 

*/ 

} 

} 






FIO34-C-EX1: A number of C functions do not return characters but can return EOF as a status 

code. These functions include fclose(), fflush(), fputs(), fscanf(), puts(), scanf(), 

sscanf(), vfscanf(), and vscanf(). These return values can be compared to EOF without 

validating the result. 


Incorrectly assuming characters from a file cannot match EOF or WEOF has resulted in significant 

vulnerabilities, including command injection attacks. (See the *CA-1996-22 advisory.) 


FIO34-C High Probable Medium P12 L1 



STR00-C. Represent characters using an appropriate 

type 

INT31-C. Ensure that integer conversions do 

not result in lost or misinterpreted data 

CERT Oracle Secure Coding Standard for Java FIO08-J. Use an int to capture the return value 

of methods that read a character or byte 

ISO/IEC TS 17961:2013 

Using character values that are indistinguishable 

from EOF [chreof] 



[NIST 2006] 

[Summit 2005] Question 12.2 

Section 1.2, “ and Character 

Types” 

SAMATE Reference Dataset Test Case ID 

000-000-088 




Input/Output (FIO) - FIO37-C. Do not assume that fgets() or fgetws() returns a nonempty string when successful 

10.4 FIO37-C. Do not assume that fgets() or fgetws() returns a nonempty 

string when successful 

Errors can occur when incorrect assumptions are made about the type of data being read. These 

assumptions may be violated, for example, when binary data has been read from a file instead of 

text from a user’s terminal or the output of a process is piped to stdin. (See FIO14-C. Understand 

the difference between text mode and binary mode with file streams.) On some systems, it 

may also be possible to input a null byte (as well as other binary codes) from the keyboard. 

Subclause 7.21.7.2 of the C Standard [ISO/IEC 9899:2011] says, 

The fgets function returns s if successful. If end-of-file is encountered and no characters 

have been read into the array, the contents of the array remain unchanged and a 

null pointer is returned. 

The wide-character function fgetws() has the same behavior. Therefore, if fgets() or 

fgetws() returns a non-null pointer, it is safe to assume that the array contains data. However, it 

is erroneous to assume that the array contains a nonempty string because the data may contain 

null characters. 


This noncompliant code example attempts to remove the trailing newline (\n) from an input line. 

The fgets() function is typically used to read a newline-terminated line of input from a stream. 

It takes a size parameter for the destination buffer and copies, at most, size - 1 characters from 

a stream to a character array. 

#include 

#include 




} 

if (fgets(buf, sizeof(buf), stdin) == NULL) { 


} 

buf[strlen(buf) - 1] = '\0'; 

The strlen() function computes the length of a string by determining the number of characters 

that precede the terminating null character. A problem occurs if the first character read from the 

input by fgets() happens to be a null character. This may occur, for example, if a binary data 

file is read by the fgets() call [Lai 2006]. If the first character in buf is a null character, 





strlen(buf) returns 0, the expression strlen(buf) - 1 wraps around to a large positive 

value, and a write-outside-array-bounds error occurs. 


This compliant solution uses strchr() to replace the newline character in the string if it exists: 

#include 

#include 




char *p; 

} 

if (fgets(buf, sizeof(buf), stdin)) { 

p = strchr(buf, '\n'); 

if (p) { 

*p = '\0'; 

} 

} else { 


} 


Incorrectly assuming that character data has been read can result in an out-of-bounds memory 

write or other flawed logic. 


FIO37-C High Probable Medium P12 L1 



MITRE CWE 

FIO14-C. Understand the difference between 

text mode and binary mode with file streams 

FIO20-C. Avoid unintentional truncation when 

using fgets() or fgetws() 





CWE-241, Improper Handling of Unexpected 

Data Type 






[ISO/IEC 9899:2011] 

[Lai 2006] 


Subclause 7.21.7.2, “The fgets Function” 

Subclause 7.29.3.2, “The fgetws Function” 





Input/Output (FIO) - FIO38-C. Do not copy a FILE object 

10.5 FIO38-C. Do not copy a FILE object 

According to the C Standard, 7.21.3, paragraph 6 [ISO/IEC 9899:2011], 

The address of the FILE object used to control a stream may be significant; a copy of a 

FILE object need not serve in place of the original. 

Consequently, do not copy a FILE object. 


This noncompliant code example can fail because a by-value copy of stdout is being used in the 

call to fputs(): 

#include 


FILE my_stdout = *stdout; 

if (fputs("Hello, World!\n", &my_stdout) == EOF) { 


} 

return 0; 

} 

When compiled under Microsoft Visual Studio 2013 and run on Windows, this noncompliant example 

results in an “access violation” at runtime. 


In this compliant solution, a copy of the stdout pointer to the FILE object is used in the call to 

fputs(): 

#include 


FILE *my_stdout = stdout; 

if (fputs("Hello, World!\n", my_stdout) == EOF) { 


} 

return 0; 

} 




Input/Output (FIO) - FIO38-C. Do not copy a FILE object 


Using a copy of a FILE object in place of the original may result in a crash, which can be used in 

a denial-of-service attack. 


FIO38-C Low Probable Medium P4 L3 


ISO/IEC TS 17961:2013 

Copying a FILE object [filecpy] 


[ISO/IEC 9899:2011] 

7.21.3, “Files” 




Input/Output (FIO) - FIO39-C. Do not alternately input and output from a stream without an intervening flush or positioning 

call 

10.6 FIO39-C. Do not alternately input and output from a stream without 

an intervening flush or positioning call 

The C Standard, 7.21.5.3, paragraph 7 [ISO/IEC 9899:2011], places the following restrictions on 

update streams: 

When a file is opened with update mode . . ., both input and output may be performed on 

the associated stream. However, output shall not be directly followed by input without an 

intervening call to the fflush function or to a file positioning function (fseek, fsetpos, 

or rewind), and input shall not be directly followed by output without an intervening call 

to a file positioning function, unless the input operation encounters end-of-file. Opening 

(or creating) a text file with update mode may instead open (or create) a binary stream in 

some implementations. 

The following scenarios can result in undefined behavior. (See undefined behavior 151.) 

• Receiving input from a stream directly following an output to that stream without an intervening 

call to fflush(), fseek(), fsetpos(), or rewind() if the file is not at end-of-file 

• Outputting to a stream after receiving input from that stream without a call to fseek(), 

fsetpos(), or rewind() if the file is not at end-of-file 

Consequently, a call to fseek(), fflush(), or fsetpos() is necessary between input and output 

to the same stream. See ERR07-C. Prefer functions that support error checking over equivalent 

functions that don’t for more information on why fseek() is preferred over rewind(). 


This noncompliant code example appends data to a file and then reads from the same file: 

#include 


extern void initialize_data(char *data, size_t size); 


char data[BUFFERSIZE]; 

char append_data[BUFFERSIZE]; 

FILE *file; 

file = fopen(file_name, "a+"); 

if (file == NULL) { 


} 

initialize_data(append_data, BUFFERSIZE); 

if (fwrite(append_data, 1, BUFFERSIZE, file) != BUFFERSIZE) { 





call 


} 

if (fread(data, 1, BUFFERSIZE, file) < BUFFERSIZE) { 

/* Handle there not being data */ 

} 

} 



} 

Because there is no intervening flush or positioning call between the calls to fread() and 

fwrite(), the behavior is undefined. 


In this compliant solution, fseek() is called between the output and input, eliminating the undefined 

behavior: 

#include 


extern void initialize_data(char *data, size_t size); 


char data[BUFFERSIZE]; 

char append_data[BUFFERSIZE]; 

FILE *file; 

file = fopen(file_name, "a+"); 



} 

initialize_data(append_data, BUFFERSIZE); 

if (fwrite(append_data, BUFFERSIZE, 1, file) != BUFFERSIZE) { 


} 

if (fseek(file, 0L, SEEK_SET) != 0) { 


} 

if (fread(data, BUFFERSIZE, 1, file) != 0) { 

/* Handle there not being data */ 

} 







call 

} 

} 


Alternately inputting and outputting from a stream without an intervening flush or positioning call 

is undefined behavior. 


FIO39-C Low Likely Medium P6 L2 



ISO/IEC TS 17961:2013 

FIO50-CPP. Do not alternately input and output 

from a file stream without an intervening 

positioning call 

Interleaving stream inputs and outputs without 

a flush or positioning call [ioileave] 


[ISO/IEC 9899:2011] 

7.21.5.3, “The fopen Function” 




Input/Output (FIO) - FIO40-C. Reset strings on fgets() or fgetws() failure 

10.7 FIO40-C. Reset strings on fgets() or fgetws() failure 

If either of the C Standard fgets() or fgetws() functions fail, the contents of the array being 

written is indeterminate. (See undefined behavior 170.) It is necessary to reset the string to a 

known value to avoid errors on subsequent string manipulation functions. 


In this noncompliant code example, an error flag is set if fgets() fails. However, buf is not reset 

and has indeterminate contents: 

#include 


void func(FILE *file) { 


} 

if (fgets(buf, sizeof(buf), file) == NULL) { 

/* Set error flag and continue */ 

} 


In this compliant solution, buf is set to an empty string if fgets() fails. The equivalent solution 

for fgetws() would set buf to an empty wide string. 

#include 


void func(FILE *file) { 


} 

if (fgets(buf, sizeof(buf), file) == NULL) { 

/* Set error flag and continue */ 

*buf = '\0'; 

} 


FIO40-C-EX1: If the string goes out of scope immediately following the call to fgets() or 

fgetws() or is not referenced in the case of a failure, it need not be reset. 




Input/Output (FIO) - FIO40-C. Reset strings on fgets() or fgetws() failure 


Making invalid assumptions about the contents of an array modified by fgets() or fgetws() 

can result in undefined behavior and abnormal program termination. 


FIO40-C Low Probable Medium P4 L3 




Input/Output (FIO) - FIO41-C. Do not call getc(), putc(), getwc(), or putwc() with a stream argument that has side effects 

10.8 FIO41-C. Do not call getc(), putc(), getwc(), or putwc() with a stream 

argument that has side effects 

Do not invoke getc() or putc() or their wide-character analogues getwc() and putwc() with 

a stream argument that has side effects. The stream argument passed to these macros may be evaluated 

more than once if these functions are implemented as unsafe macros. (See PRE31-C. Avoid 

side effects in arguments to unsafe macros for more information.) 

This rule does not apply to the character argument in putc() or the wide-character argument in 

putwc(), which is guaranteed to be evaluated exactly once. 

10.8.1 Noncompliant Code Example (getc()) 

This noncompliant code example calls the getc() function with an expression as the stream argument. 

If getc() is implemented as a macro, the file may be opened multiple times. (See 

FIO24-C. Do not open a file that is already open.) 

#include 


FILE *fptr; 

int c = getc(fptr = fopen(file_name, "r")); 

if (feof(stdin) || ferror(stdin)) { 


} 

} 

if (fclose(fptr) == EOF) { 


} 

This noncompliant code example also violates ERR33-C. Detect and handle standard library errors 

because the value returned by fopen() is not checked for errors. 

10.8.2 Compliant Solution (getc()) 

In this compliant solution, fopen() is called before getc() and its return value is checked for 

errors: 

#include 


int c; 

FILE *fptr; 

fptr = fopen(file_name, "r"); 

if (fptr == NULL) { 





} 


c = getc(fptr); 

if (c == EOF) { 


} 

} 

if (fclose(fptr) == EOF) { 


} 

10.8.3 Noncompliant Code Example (putc()) 

In this noncompliant example, putc() is called with an expression as the stream argument. If 

putc() is implemented as a macro, this expression might be evaluated multiple times. 

#include 


FILE *fptr = NULL; 

int c = 'a'; 

while (c


10.8.4 Compliant Solution (putc()) 

In this compliant solution, the stream argument to putc() no longer has side effects: 

#include 


int c = 'a'; 

FILE *fptr = fopen(file_name, "w"); 

if (fptr == NULL) { 


} 

while (c

Input/Output (FIO) - FIO42-C. Close files when they are no longer needed 

10.9 FIO42-C. Close files when they are no longer needed 

A call to the fopen() or freopen() function must be matched with a call to fclose() before 

the lifetime of the last pointer that stores the return value of the call has ended or before normal 

program termination, whichever occurs first. 

In general, this rule should also be applied to other functions with open and close resources, such 

as the POSIX open() and close() functions, or the Microsoft Windows CreateFile() and 

CloseHandle() functions. 


This code example is noncompliant because the file opened by the call to fopen() is not closed 

before function func() returns: 

#include 

int func(const char *filename) { 

FILE *f = fopen(filename, "r"); 

if (NULL == f) { 

return -1; 

} 

/* ... */ 

return 0; 

} 


In this compliant solution, the file pointed to by f is closed before returning to the caller: 

#include 


FILE *f = fopen(filename, "r"); 


return -1; 

} 

/* ... */ 

if (fclose(f) == EOF) { 

return -1; 

} 

return 0; 

} 





10.9.3 Noncompliant Code Example (exit()) 

This code example is noncompliant because the resource allocated by the call to fopen() is not 

closed before the program terminates. Although exit() closes the file, the program has no way 

of determining if an error occurs while flushing or closing the file. 

#include 

#include 


FILE *f = fopen(filename, "w"); 


exit(EXIT_FAILURE); 

} 

/* ... */ 

exit(EXIT_SUCCESS); 

} 

10.9.4 Compliant Solution (exit()) 

In this compliant solution, the program closes f explicitly before calling exit(), allowing any 

error that occurs when flushing or closing the file to be handled appropriately: 

#include 

#include 


FILE *f = fopen(filename, "w"); 



} 

/* ... */ 



} 

exit(EXIT_SUCCESS); 

} 


This code example is noncompliant because the resource allocated by the call to open() is not 

closed before function func() returns: 

#include 

#include 


int fd = open(filename, O_RDONLY, S_IRUSR); 

if (-1 == fd) { 





} 

return -1 

} 

/* ... */ 

return 0; 


In this compliant solution, fd is closed before returning to the caller: 

#include 

#include 

#include 


int fd = open(filename, O_RDONLY, S_IRUSR); 

if (-1 == fd) { 

return -1 

} 

/* ... */ 

if (-1 == close(fd)) { 

return -1; 

} 

return 0; 

} 


In this noncompliant code example, the file opened by the Microsoft Windows CreateFile() function 

is not closed before func() returns: 

#include 

int func(LPCTSTR filename) { 

HANDLE hFile = CreateFile(filename, GENERIC_READ, 0, NULL, 

OPEN_EXISTING, 

FILE_ATTRIBUTE_NORMAL, NULL); 

if (INVALID_HANDLE_VALUE == hFile) { 

return -1; 

} 

/* ... */ 

return 0; 

} 






In this compliant solution, hFile is closed by invoking the CloseHandle() function before returning 

to the caller: 

#include 

int func(LPCTSTR filename) { 

HANDLE hFile = CreateFile(filename, GENERIC_READ, 0, NULL, 

OPEN_EXISTING, 

FILE_ATTRIBUTE_NORMAL, NULL); 

if (INVALID_HANDLE_VALUE == hFile) { 

return -1; 

} 

/* ... */ 

if (!CloseHandle(hFile)) { 

return -1; 

} 

} 

return 0; 


Failing to properly close files may allow an attacker to exhaust system resources and can increase 

the risk that data written into in-memory file buffers will not be flushed in the event of abnormal 

program termination. 





FIO51-CPP. Close files when they are no 

longer needed 

CERT Oracle Secure Coding Standard for Java FIO04-J. Release resources when they are no 

longer needed 

ISO/IEC TS 17961:2013 

MITRE CWE 

Failing to close files or free dynamic memory 

when they are no longer needed [fileclose] 

CWE-404, Improper Resource Shutdown or 

Release 


[IEEE Std 1003.1:2013] 





Input/Output (FIO) - FIO44-C. Only use values for fsetpos() that are returned from fgetpos() 

10.10 FIO44-C. Only use values for fsetpos() that are returned from 

fgetpos() 

The C Standard, 7.21.9.3 [ISO/IEC 9899:2011], defines the following behavior for fsetpos(): 

The fsetpos function sets the mbstate_t object (if any) and file position indicator for 

the stream pointed to by stream according to the value of the object pointed to by pos, 

which shall be a value obtained from an earlier successful call to the fgetpos function 

on a stream associated with the same file. 

Invoking the fsetpos() function with any other values for pos is undefined behavior. 


This noncompliant code example attempts to read three values from a file and then set the file position 

pointer back to the beginning of the file: 

#include 

#include 

int opener(FILE *file) { 

int rc; 

fpos_t offset; 

memset(&offset, 0, sizeof(offset)); 


return -1; 

} 

/* Read in data from file */ 

rc = fsetpos(file, &offset); 

if (rc != 0 ) { 

return rc; 

} 

} 

return 0; 

Only the return value of an fgetpos() call is a valid argument to fsetpos(); passing a value of 

type fpos_t that was created in any other way is undefined behavior. 




Input/Output (FIO) - FIO44-C. Only use values for fsetpos() that are returned from fgetpos() 


In this compliant solution, the initial file position indicator is stored by first calling fgetpos(), 

which is used to restore the state to the beginning of the file in the later call to fsetpos(): 

#include 

#include 

int opener(FILE *file) { 

int rc; 

fpos_t offset; 


return -1; 

} 

rc = fgetpos(file, &offset); 

if (rc != 0 ) { 

return rc; 

} 

/* Read in data from file */ 

rc = fsetpos(file, &offset); 

if (rc != 0 ) { 

return rc; 

} 

} 

return 0; 


Misuse of the fsetpos() function can position a file position indicator to an unintended location 

in the file. 




ISO/IEC TS 17961:2013 

Using a value for fsetpos other than a value returned 

from fgetpos [xfilepos] 


[ISO/IEC 9899:2011] 

7.21.9.3, “The fsetpos Function” 




Input/Output (FIO) - FIO45-C. Avoid TOCTOU race conditions while accessing files 

10.11 FIO45-C. Avoid TOCTOU race conditions while accessing files 

A TOCTOU (time-of-check, time-of-use) race condition is possible when two or more concurrent 

processes are operating on a shared file system [Seacord 2013b]. Typically, the first access is a 

check to verify some attribute of the file, followed by a call to use the file. An attacker can alter 

the file between the two accesses, or replace the file with a symbolic or hard link to a different 

file. These TOCTOU conditions can be exploited when a program performs two or more file operations 

on the same file name or path name. 

A program that performs two or more file operations on a single file name or path name creates a 

race window between the two file operations. This race window comes from the assumption that 

the file name or path name refers to the same resource both times. If an attacker can modify the 

file, remove it, or replace it with a different file, then this assumption will not hold. 


If an existing file is opened for writing with the w mode argument, the file’s previous contents (if 

any) are destroyed. This noncompliant code example tries to prevent an existing file from being 

overwritten by first opening it for reading before opening it for writing. An attacker can exploit 

the race window between the two calls to fopen() to overwrite an existing file. 

#include 

void open_some_file(const char *file) { 

FILE *f = fopen(file, "r"); 

if (NULL != f) { 

/* File exists, handle error */ 

} else { 



} 

f = fopen(file, "w"); 



} 

} 

} 

/* Write to file */ 



} 






This compliant solution invokes fopen() at a single location and uses the x mode of fopen(), 

which was added in C11. This mode causes fopen() to fail if the file exists. This check and subsequent 

open is performed without creating a race window. The x mode provides exclusive access 

to the file only if the host environment provides this support. 

#include 


FILE *f = fopen(file, "wx") 



} 




} 

} 


This compliant solution uses the O_CREAT and O_EXCL flags of POSIX’s open() function. These 

flags cause open() to fail if the file exists. 

#include 

#include 

#include 


int fd = open(file, O_CREAT | O_EXCL | O_WRONLY); 

if (-1 != fd) { 

FILE *f = fdopen(fd, "w"); 

if (NULL != f) { 


} 

} 



} 

} else { 

if (close(fd) == -1) { 


} 

} 






FIO45-C-EX1: TOCTOU race conditions require that the vulnerable process is more privileged 

than the attacker; otherwise there is nothing to be gained from a successful attack. An unprivileged 

process is not subject to this rule. 

FIO45-C-EX2: Accessing a file name or path name multiple times is permitted if the file referenced 

resides in a secure directory (for more information, see FIO15-C. Ensure that file operations 

are performed in a secure directory). 

FIO45-C-EX3: Accessing a file name or path name multiple times is permitted if the program 

can verify that every operation operates on the same file. 

This POSIX code example verifies that each subsequent file access operates on the same file. In 

POSIX, every file can be uniquely identified by using its device and i-node attributes. This code 

example checks that a file name refers to a regular file (and not a directory, symbolic link, or 

other special file) by invoking lstat(). This call also retrieves its device and i-node. The file is 

subsequently opened. Finally, the program verifies that the file that was opened is the same one 

(matching device and i-nodes) as the file that was confirmed as a regular file. 

#include 

#include 

int open_regular_file(char *filename, int flags) { 

struct stat lstat_info; 

struct stat fstat_info; 

int f; 

if (lstat(filename, &lstat_info) == -1) { 

/* File does not exist, handle error */ 

} 

if (!S_ISREG(lstat_info.st_mode)) { 

/* File is not a regular file, handle error */ 

} 

if ((f = open(filename, flags)) == -1) { 

/* File has disappeared, handle error */ 

} 

if (fstat(f, &fstat_info) == -1) { 


} 

if (lstat_info.st_ino != fstat_info.st_ino || 

lstat_info.st_dev != fstat_info.st_dev) { 

/* Open file is not the expected regular file, handle error */ 

} 





} 

/* f is the expected regular open file */ 

return f; 


TOCTOU race conditions can result in unexpected behavior, including privilege escalation. 


FIO45-C High Probable High P6 L2 



Chapter 7, “Files” 




Input/Output (FIO) - FIO46-C. Do not access a closed file 

10.12 FIO46-C. Do not access a closed file 

Using the value of a pointer to a FILE object after the associated file is closed is undefined behavior. 

(See undefined behavior 148.) Programs that close the standard streams (especially stdout 

but also stderr and stdin) must be careful not to use these streams in subsequent function 

calls, particularly those that implicitly operate on them (such as printf(), perror(), and 

getc()). 

This rule can be generalized to other file representations. 


In this noncompliant code example, the stdout stream is used after it is closed: 

#include 

int close_stdout(void) { 

if (fclose(stdout) == EOF) { 

return -1; 

} 

} 

printf("stdout successfully closed.\n"); 

return 0; 


In this compliant solution, stdout is not used again after it is closed. This must remain true for 

the remainder of the program, or stdout must be assigned the address of an open file object. 

#include 

int close_stdout(void) { 

if (fclose(stdout) == EOF) { 

return -1; 

} 

} 

fputs("stdout successfully closed.", stderr); 

return 0; 




Input/Output (FIO) - FIO46-C. Do not access a closed file 


Using the value of a pointer to a FILE object after the associated file is closed is undefined behavior. 




[IEEE Std 1003.1:2013] 

[ISO/IEC 9899:2011] 


Subclause 7.21.3, “Files” 

Subclause 7.21.5.1, “The fclose Function” 




Input/Output (FIO) - FIO47-C. Use valid format strings 

10.13 FIO47-C. Use valid format strings 

The formatted output functions (fprintf() and related functions) convert, format, and print 

their arguments under control of a format string. The C Standard, 7.21.6.1, paragraph 3 [ISO/IEC 

9899:2011], specifies 

The format shall be a multibyte character sequence, beginning and ending in its initial 

shift state. The format is composed of zero or more directives: ordinary multibyte characters 

(not %), which are copied unchanged to the output stream; and conversion specifications, 

each of which results in fetching zero or more subsequent arguments, converting 

them, if applicable, according to the corresponding conversion specifier, and then 

writing the result to the output stream. 

Each conversion specification is introduced by the % character followed (in order) by 

• Zero or more flags (in any order), which modify the meaning of the conversion specification 

• An optional minimum field width 

• An optional precision that gives the minimum number of digits to appear for certain conversion 

specifiers 

• An optional length modifier that specifies the size of the argument 

• A conversion specifier character that indicates the type of conversion to be applied 

Common mistakes in creating format strings include 

• Providing an incorrect number of arguments for the format string 

• Using invalid conversion specifiers 

• Using a flag character that is incompatible with the conversion specifier 

• Using a length modifier that is incompatible with the conversion specifier 

• Mismatching the argument type and conversion specifier 

• Using an argument of type other than int for width or precision 

The following table summarizes the compliance of various conversion specifications. The first 

column contains one or more conversion specifier characters. The next four columns consider the 

combination of the specifier characters with the various flags (the apostrophe ['], -, +, the space 

character, #, and 0). The next eight columns consider the combination of the specifier characters 

with the various length modifiers (h, hh, l, ll, j, z, t, and L). 

Valid combinations are marked with a type name; arguments matched with the conversion specification 

are interpreted as that type. For example, an argument matched with the specifier %hd is 

interpreted as a short, so short appears in the cell where d and h intersect. The last column denotes 

the expected types of arguments matched with the original specifier characters. 





Valid and meaningful combinations are marked by the symbol (save for the length modifier columns, 

as described previously). Valid combinations that have no effect are labeled N/E. Using a 

combination marked by the symbol, using a specification not represented in the table, or using an 

argument of an unexpected type is undefined behavior. (See undefined behaviors 153, 155, 157, 

158, 161, and 162.) 





Conversion 

Specifier 

Character 

'XSI -, +, 

SPACE 

# 0 h hh l ll j z t L Argument 

Type 

d, i short signed char long long long intmax_t size_t ptrdiff_t Signed integer 

o unsigned 

short 

u unsigned 

short 

x, X unsigned 

short 

unsigned 

char 

unsigned 

char 

unsigned 

char 

unsigned 

long 

unsigned 

long 

unsigned 

long 

unsigned 

long long 

unsigned 

long long 

unsigned 

long long 

uintmax_t size_t ptrdiff_t Unsigned 

integer 


integer 


integer 

f, F N/E N/E long 

double 

e, E N/E N/E long 

double 

g, G N/E N/E long 

double 

a, A N/E N/E long 

double 

double or long 

double 


double 


double 


double 

c wint_t int or wint_t 

s NTWS NTBS or 

NTWS 

p void* 

n short* char* long* long long* intmax_t* size_t* ptrdiff_t* Pointer to 

integer 

CXSI wint_t 

SXSI NTWS 

% None 

SPACE: The space (“ “) character 

N/E: No effect 

NTBS: char* argument pointing to a null-terminated character string 

NTWS: wchar_t* argument pointing to a null-terminated wide character string 

XSI: ISO/IEC 9945-2003 XSI extension 





The formatted input functions (fscanf() and related functions) use similarly specified format 

strings and impose similar restrictions on their format strings and arguments. 

Do not supply an unknown or invalid conversion specification or an invalid combination of flag 

character, precision, length modifier, or conversion specifier to a formatted IO function. Likewise, 

do not provide a number or type of argument that does not match the argument type of the conversion 

specifier used in the format string. 

Format strings are usually string literals specified at the call site, but they need not be. However, 

they should not contain tainted values. (See FIO30-C. Exclude user input from format strings for 

more information.) 


Mismatches between arguments and conversion specifications may result in undefined behavior. 

Compilers may diagnose type mismatches in formatted output function invocations. In this noncompliant 

code example, the error_type argument to printf() is incorrectly matched with 

the s specifier rather than with the d specifier. Likewise, the error_msg argument is incorrectly 

matched with the d specifier instead of the s specifier. These usages result in undefined behavior. 

One possible result of this invocation is that printf() will interpret the error_type argument 

as a pointer and try to read a string from the address that error_type contains, possibly resulting 

in an access violation. 

#include 


const char *error_msg = "Resource not available to user."; 

int error_type = 3; 

/* ... */ 

printf("Error (type %s): %d\n", error_type, error_msg); 

/* ... */ 

} 


This compliant solution ensures that the arguments to the printf() function match their respective 

conversion specifications: 

#include 


const char *error_msg = "Resource not available to user."; 

int error_type = 3; 

/* ... */ 

printf("Error (type %d): %s\n", error_type, error_msg); 





} 

/* ... */ 


Incorrectly specified format strings can result in memory corruption or abnormal program termination. 


FIO47-C High Unlikely Medium P6 L2 



ISO/IEC TS 17961:2013 

MITRE CWE 

FIO00-CPP. Take care when creating format 

strings 

Using invalid format strings [invfmtstr] 


Type 


[ISO/IEC 9899:2011] 

Subclause 7.21.6.1, “The fprintf Function” 




Environment (ENV) - ENV30-C. Do not modify the object referenced by the return value of certain functions 

11 Environment (ENV) 

11.1 ENV30-C. Do not modify the object referenced by the return value 

of certain functions 

Some functions return a pointer to an object that cannot be modified without causing undefined 

behavior. These functions include getenv(), setlocale(), localeconv(), asctime(), and 

strerror(). In such cases, the function call results must be treated as being const-qualified. 

The C Standard, 7.22.4.6, paragraph 4 [ISO/IEC 9899:2011], defines getenv() as follows: 

The getenv function returns a pointer to a string associated with the matched list member. 

The string pointed to shall not be modified by the program, but may be overwritten 

by a subsequent call to the getenv function. If the specified name cannot be found, a 

null pointer is returned. 

If the string returned by getenv() must be altered, a local copy should be created. Altering the 

string returned by getenv() is undefined behavior. (See undefined behavior 184.) 

Similarly, subclause 7.11.1.1, paragraph 8 [ISO/IEC 9899:2011], defines setlocale() as follows: 

The pointer to string returned by the setlocale function is such that a subsequent call 

with that string value and its associated category will restore that part of the program’s 

locale. The string pointed to shall not be modified by the program, but may be overwritten 

by a subsequent call to the setlocale function. 

And subclause 7.11.2.1, paragraph 8 [ISO/IEC 9899:2011], defines localeconv() as follows: 

The localeconv function returns a pointer to the filled-in object. The structure pointed 

to by the return value shall not be modified by the program, but may be overwritten by a 

subsequent call to the localeconv function. In addition, calls to the setlocale function 

with categories LC_ALL, LC_MONETARY, or LC_NUMERIC may overwrite the contents of 

the structure. 

Altering the string returned by setlocale() or the structure returned by localeconv() are undefined 

behaviors. (See undefined behaviors 120 and 121.) Furthermore, the C Standard imposes 

no requirements on the contents of the string by setlocale(). Consequently, no assumptions 

can be made as to the string's internal contents or structure. 





Finally, subclause 7.24.6.2, paragraph 4 [ISO/IEC 9899:2011], states 

The strerror function returns a pointer to the string, the contents of which are localespecific. 

The array pointed to shall not be modified by the program, but may be overwritten 

by a subsequent call to the strerror function. 

Altering the string returned by strerror() is undefined behavior. (See undefined behavior 184.) 

11.1.1 Noncompliant Code Example (getenv()) 

This noncompliant code example modifies the string returned by getenv() by replacing all double 

quotation marks (“) with underscores (_): 

#include 

void trstr(char *c_str, char orig, char rep) { 

while (*c_str != '\0') { 

if (*c_str == orig) { 

*c_str = rep; 

} 

++c_str; 

} 

} 


char *env = getenv("TEST_ENV"); 

if (env == NULL) { 


} 

trstr(env,'"', '_'); 

} 

11.1.2 Compliant Solution (getenv()) (Environment Not Modified) 

If the programmer does not intend to modify the environment, this compliant solution demonstrates 

how to modify a copy of the return value: 

#include 

#include 


while (*c_str != '\0') { 


*c_str = rep; 

} 

++c_str; 

} 

} 






const char *env; 

char *copy_of_env; 

env = getenv("TEST_ENV"); 



} 

copy_of_env = (char *)malloc(strlen(env) + 1); 

if (copy_of_env == NULL) { 


} 

} 

strcpy(copy_of_env, env); 

trstr(copy_of_env,'"', '_'); 

/* ... */ 

free(copy_of_env); 

11.1.3 Compliant Solution (getenv()) (Modifying the Environment in POSIX) 

If the programmer’s intent is to modify the environment, this compliant solution, which saves the 

altered string back into the environment by using the POSIX setenv() and strdup() functions, 

can be used: 

#include 

#include 


while (*c_str != '\0') { 


*c_str = rep; 

} 

++c_str; 

} 

} 


const char *env; 

char *copy_of_env; 

env = getenv("TEST_ENV"); 



} 

copy_of_env = strdup(env); 

if (copy_of_env == NULL) { 






} 

trstr(copy_of_env,'"', '_'); 

} 

if (setenv("TEST_ENV", copy_of_env, 1) != 0) { 


} 

/* ... */ 

free(copy_of_env); 

11.1.4 Noncompliant Code Example (localeconv()) 

In this noncompliant example, the object returned by localeconv() is directly modified: 

#include 


struct lconv *conv = localeconv(); 

} 

if ('\0' == conv->decimal_point[0]) { 

conv->decimal_point = "."; 

} 

11.1.5 Compliant Solution (localeconv()) (Copy) 

This compliant solution modifies a copy of the object returned by localeconv(): 

#include 

#include 

#include 


const struct lconv *conv = localeconv(); 

if (conv == NULL) { 


} 

struct lconv *copy_of_conv = (struct lconv *)malloc( 

sizeof(struct lconv)); 

if (copy_of_conv == NULL) { 


} 

memcpy(copy_of_conv, conv, sizeof(struct lconv)); 

if ('\0' == copy_of_conv->decimal_point[0]) { 

copy_of_conv->decimal_point = "."; 





} 

} 

/* ... */ 

free(copy_of_conv); 


Modifying the object pointed to by the return value of getenv(), setlocale(), localeconv(), 

asctime(), or strerror() is undefined behavior. Even if the modification succeeds, 

the modified object can be overwritten by a subsequent call to the same function. 


ENV30-C Low Probable Medium P4 L3 


ISO/IEC TS 17961:2013 

Modifying the string returned by getenv, localeconv, 

setlocale, and strerror 

[libmod] 


[IEEE Std 1003.1:2013] 

[ISO/IEC 9899:2011] 

XSH, System Interfaces, getenv 

XSH, System Interfaces, setlocale 

XSH, System Interfaces, localeconv 

7.11.1.1, “The setlocale Function” 

7.11.2.1, “The localeconv Function” 

7.22.4.6, “The getenv Function” 

7.24.6.2, “The strerror Function” 




Environment (ENV) - ENV31-C. Do not rely on an environment pointer following an operation that may invalidate it 

11.2 ENV31-C. Do not rely on an environment pointer following an 

operation that may invalidate it 

Some implementations provide a nonportable environment pointer that is valid when main() is 

called but may be invalidated by operations that modify the environment. 

The C Standard, J.5.1 [ISO/IEC 9899:2011], states 

In a hosted environment, the main function receives a third argument, char *envp[], 

that points to a null-terminated array of pointers to char, each of which points to a string 

that provides information about the environment for this execution of the program. 

Consequently, under a hosted environment supporting this common extension, it is possible to access 

the environment through a modified form of main(): 

main(int argc, char *argv[], char *envp[]){ /* ... */ } 

However, modifying the environment by any means may cause the environment memory to be reallocated, 

with the result that envp now references an incorrect location. For example, when compiled 

with GCC 4.8.1 and run on a 32-bit Intel GNU/Linux machine, the following code, 

#include 

#include 

extern char **environ; 

int main(int argc, const char *argv[], const char *envp[]) { 

printf("environ: %p\n", environ); 

printf("envp: %p\n", envp); 

setenv("MY_NEW_VAR", "new_value", 1); 

puts("--Added MY_NEW_VAR--"); 

printf("environ: %p\n", environ); 

printf("envp: %p\n", envp); 

return 0; 

} 

yields 

% ./envp-environ 

environ: 0xbf8656ec 

envp: 0xbf8656ec 

--Added MY_NEW_VAR-- 

environ: 0x804a008 

envp: 0xbf8656ec 





It is evident from these results that the environment has been relocated as a result of the call to 

setenv(). The external variable environ is updated to refer to the current environment; the 

envp parameter is not. 

An environment pointer may also become invalidated by subsequent calls to getenv(). (See 

ENV34-C. Do not store pointers returned by certain functions for more information.) 


After a call to the POSIX setenv() function or to another function that modifies the environment, 

the envp pointer may no longer reference the current environment. The Portable Operating 

System Interface (POSIX®), Base Specifications, Issue 7 [IEEE Std 1003.1:2013], states 

Unanticipated results may occur if setenv() changes the external variable environ. In 

particular, if the optional envp argument to main() is present, it is not changed, and 

thus may point to an obsolete copy of the environment (as may any other copy of environ). 

This noncompliant code example accesses the envp pointer after calling setenv(): 

#include 

#include 


if (setenv("MY_NEW_VAR", "new_value", 1) != 0) { 


} 

if (envp != NULL) { 

for (size_t i = 0; envp[i] != NULL; ++i) { 

puts(envp[i]); 

} 

} 

return 0; 

} 

Because envp may no longer point to the current environment, this program has undefined behavior. 


Use environ in place of envp when defined: 

#include 

#include 







if (setenv("MY_NEW_VAR", "new_value", 1) != 0) { 


} 

if (environ != NULL) { 

for (size_t i = 0; environ[i] != NULL; ++i) { 

puts(environ[i]); 

} 

} 

return 0; 

} 


After a call to the Windows _putenv_s() function or to another function that modifies the environment, 

the envp pointer may no longer reference the environment. 

According to the Visual C++ reference [MSDN] 

The environment block passed to main and wmain is a “frozen” copy of the current environment. 

If you subsequently change the environment via a call to _putenv or 

_wputenv, the current environment (as returned by getenv / _wgetenv and the _environ 

/ _wenviron variable) will change, but the block pointed to by envp will not change. 

This noncompliant code example accesses the envp pointer after calling _putenv_s(): 

#include 

#include 


if (_putenv_s("MY_NEW_VAR", "new_value") != 0) { 


} 

if (envp != NULL) { 

for (size_t i = 0; envp[i] != NULL; ++i) { 

puts(envp[i]); 

} 

} 

return 0; 

} 

Because envp no longer points to the current environment, this program has undefined behavior. 






This compliant solution uses the _environ variable in place of envp: 

#include 

#include 

_CRTIMP extern char **_environ; 


if (_putenv_s("MY_NEW_VAR", "new_value") != 0) { 


} 

if (_environ != NULL) { 

for (size_t i = 0; _environ[i] != NULL; ++i) { 

puts(_environ[i]); 

} 

} 

return 0; 

} 


This compliant solution can reduce remediation time when a large amount of noncompliant envp 

code exists. It replaces 

int main(int argc, char *argv[], char *envp[]) { 

/* ... */ 

} 

with 

#if defined (_POSIX_) || defined (__USE_POSIX) 


#define envp environ 

#elif defined(_WIN32) 

_CRTIMP extern char **_environ; 

#define envp _environ 

#endif 


/* ... */ 

} 

This compliant solution may need to be extended to support other implementations that support 

forms of the external variable environ. 






Using the envp environment pointer after the environment has been modified can result in undefined 

behavior. 





VOID ENV31-CPP. Do not rely on an environment 

pointer following an operation that may 

invalidate it 


[IEEE Std 1003.1:2013] 

[ISO/IEC 9899:2011] 

[MSDN] 

XSH, System Interfaces, setenv 

J.5.1, “Environment Arguments” 

_environ, _wenviron, 

getenv, _wgetenv, 

_putenv_s, _wputenv_s 




Environment (ENV) - ENV32-C. All exit handlers must return normally 

11.3 ENV32-C. All exit handlers must return normally 

The C Standard provides three functions that cause an application to terminate normally: 

_Exit(), exit(), and quick_exit(). These are collectively called exit functions. When the 

exit() function is called, or control transfers out of the main() entry point function, functions 

registered with atexit() are called (but not at_quick_exit()). When the quick_exit() 

function is called, functions registered with at_quick_exit() (but not atexit()) are called. 

These functions are collectively called exit handlers. When the _Exit() function is called, no 

exit handlers or signal handlers are called. 

Exit handlers must terminate by returning. It is important and potentially safety-critical for all exit 

handlers to be allowed to perform their cleanup actions. This is particularly true because the application 

programmer does not always know about handlers that may have been installed by support 

libraries. Two specific issues include nested calls to an exit function and terminating a call to an 

exit handler by invoking longjmp. 

A nested call to an exit function is undefined behavior. (See undefined behavior 182.) This behavior 

can occur only when an exit function is invoked from an exit handler or when an exit function 

is called from within a signal handler. (See SIG30-C. Call only asynchronous-safe functions 

within signal handlers.) 

If a call to the longjmp() function is made that would terminate the call to a function registered 

with atexit(), the behavior is undefined. 


In this noncompliant code example, the exit1() and exit2() functions are registered by 

atexit() to perform required cleanup upon program termination. However, if some_condition 

evaluates to true, exit() is called a second time, resulting in undefined behavior. 

#include 

void exit1(void) { 

/* ... Cleanup code ... */ 

return; 

} 


extern int some_condition; 

if (some_condition) { 

/* ... More cleanup code ... */ 

exit(0); 

} 

return; 

} 






} 

if (atexit(exit1) != 0) { 


} 



} 

/* ... Program code ... */ 

return 0; 

Functions registered by the atexit() function are called in the reverse order from which they 

were registered. Consequently, if exit2() exits in any way other than by returning, exit1() 

will not be executed. The same may also be true for atexit() handlers installed by support libraries. 


A function that is registered as an exit handler by atexit() must exit by returning, as in this 

compliant solution: 

#include 


/* ... Cleanup code ... */ 

return; 

} 


extern int some_condition; 

if (some_condition) { 

/* ... More cleanup code ... */ 

} 

return; 

} 




} 



} 

/* ... Program code ... */ 

return 0; 

} 






In this noncompliant code example, exit1() is registered by atexit() so that upon program 

termination, exit1() is called. The exit1() function jumps back to main() to return, with undefined 

results. 

#include 

#include 

jmp_buf env; 

int val; 


longjmp(env, 1); 

} 




} 

if (setjmp(env) == 0) { 

exit(0); 

} else { 

return 0; 

} 

} 


This compliant solution does not call longjmp()but instead returns from the exit handler normally: 

#include 


return; 

} 




} 

return 0; 

} 






Terminating a call to an exit handler in any way other than by returning is undefined behavior and 

may result in abnormal program termination or other unpredictable behavior. It may also prevent 

other registered handlers from being invoked. 


ENV32-C Medium Likely Medium P12 L1 



ISO/IEC TR 24772:2013 

MITRE CWE 

SIG30-C. Call only asynchronous-safe functions 

within signal handlers 

Structured Programming [EWD] 

Termination Strategy [REU] 

CWE-705, Incorrect Control Flow Scoping 




Environment (ENV) - ENV33-C. Do not call system() 

11.4 ENV33-C. Do not call system() 

The C Standard system() function executes a specified command by invoking an implementation-defined 

command processor, such as a UNIX shell or CMD.EXE in Microsoft Windows. The 

POSIX popen() and Windows _popen() functions also invoke a command processor but create a 

pipe between the calling program and the executed command, returning a pointer to a stream that 

can be used to either read from or write to the pipe [IEEE Std 1003.1:2013]. 

Use of the system() function can result in exploitable vulnerabilities, in the worst case allowing 

execution of arbitrary system commands. Situations in which calls to system() have high risk 

include the following: 

• When passing an unsanitized or improperly sanitized command string originating from a 

tainted source 

• If a command is specified without a path name and the command processor path name resolution 

mechanism is accessible to an attacker 

• If a relative path to an executable is specified and control over the current working directory 

is accessible to an attacker 

• If the specified executable program can be spoofed by an attacker 

Do not invoke a command processor via system() or equivalent functions to execute a command. 


In this noncompliant code example, the system() function is used to execute any_cmd in the 

host environment. Invocation of a command processor is not required. 

#include 

#include 

#include 


void func(const char *input) { 

char cmdbuf[BUFFERSIZE]; 

int len_wanted = snprintf(cmdbuf, BUFFERSIZE, 

"any_cmd '%s'", input); 

if (len_wanted >= BUFFERSIZE) { 


} else if (len_wanted < 0) { 


} else if (system(cmdbuf) == -1) { 


} 

} 





If this code is compiled and run with elevated privileges on a Linux system, for example, an attacker 

can create an account by entering the following string: 

happy'; useradd 'attacker 

The shell would interpret this string as two separate commands: 

any_cmd 'happy'; 

useradd 'attacker' 

and create a new user account that the attacker can use to access the compromised system. 

This noncompliant code example also violates STR02-C. Sanitize data passed to complex subsystems. 


In this compliant solution, the call to system() is replaced with a call to execve(). The exec 

family of functions does not use a full shell interpreter, so it is not vulnerable to command-injection 

attacks, such as the one illustrated in the noncompliant code example. 

The execlp(), execvp(), and (nonstandard) execvP() functions duplicate the actions of the 

shell in searching for an executable file if the specified file name does not contain a forward slash 

character (/). As a result, they should be used without a forward slash character (/) only if the 

PATH environment variable is set to a safe value, as described in ENV03-C. Sanitize the environment 

when invoking external programs. 

The execl(), execle(), execv(), and execve() functions do not perform path name substitution. 

Additionally, precautions should be taken to ensure the external executable cannot be modified by 

an untrusted user, for example, by ensuring the executable is not writable by the user. 

#include 

#include 

#include 

#include 

#include 

void func(char *input) { 

pid_t pid; 

int status; 

pid_t ret; 

char *const args[3] = {"any_exe", input, NULL}; 

char **env; 






/* ... Sanitize arguments ... */ 

} 

pid = fork(); 

if (pid == -1) { 


} else if (pid != 0) { 

while ((ret = waitpid(pid, &status, 0)) == -1) { 

if (errno != EINTR) { 


break; 

} 

} 

if ((ret != -1) && 

(!WIFEXITED(status) || !WEXITSTATUS(status)) ) { 

/* Report unexpected child status */ 

} 

} else { 

/* ... Initialize env as a sanitized copy of environ ... */ 

if (execve("/usr/bin/any_cmd", args, env) == -1) { 


_Exit(127); 

} 

} 

This compliant solution is significantly different from the preceding noncompliant code example. 

First, input is incorporated into the args array and passed as an argument to execve(), eliminating 

concerns about buffer overflow or string truncation while forming the command string. 

Second, this compliant solution forks a new process before executing “/usr/bin/any_cmd” in 

the child process. Although this method is more complicated than calling system(), the added 

security is worth the additional effort. 

The exit status of 127 is the value set by the shell when a command is not found, and POSIX recommends 

that applications should do the same. XCU, Section 2.8.2, of Standard for Information 

Technology—Portable Operating System Interface (POSIX®), Base Specifications, Issue 7 [IEEE 

Std 1003.1:2013], says 

If a command is not found, the exit status shall be 127. If the command name is found, 

but it is not an executable utility, the exit status shall be 126. Applications that invoke 

utilities without using the shell should use these exit status values to report similar errors. 






This compliant solution uses the Microsoft Windows CreateProcess() API: 

#include 

void func(TCHAR *input) { 

STARTUPINFO si = { 0 }; 

PROCESS_INFORMATION pi; 

si.cb = sizeof(si); 

if (!CreateProcess(TEXT("any_cmd.exe"), input, NULL, NULL, FALSE, 

0, 0, 0, &si, &pi)) { 


} 

CloseHandle(pi.hThread); 

CloseHandle(pi.hProcess); 

} 

This compliant solution relies on the input parameter being non-const. If it were const, the 

solution would need to create a copy of the parameter because the CreateProcess() function 

can modify the command-line arguments to be passed into the newly created process. 

This solution creates the process such that the child process does not inherit any handles from the 

parent process, in compliance with WIN03-C. Understand HANDLE inheritance. 


This noncompliant code invokes the C system() function to remove the .config file in the 

user’s home directory. 

#include 


system("rm ~/.config"); 

} 

If the vulnerable program has elevated privileges, an attacker can manipulate the value of the 

HOME environment variable such that this program can remove any file named .config anywhere 

on the system. 


An alternative to invoking the system() call to execute an external program to perform a required 

operation is to implement the functionality directly in the program using existing library 





calls. This compliant solution calls the POSIX unlink() function to remove a file without invoking 

the system() function [IEEE Std 1003.1:2013]: 

#include 

#include 

#include 

#include 

#include 


const char *file_format = "%s/.config"; 

size_t len; 

char *pathname; 

struct passwd *pwd; 

/* Get /etc/passwd entry for current user */ 

pwd = getpwuid(getuid()); 

if (pwd == NULL) { 


} 

/* Build full path name home dir from pw entry */ 

len = strlen(pwd->pw_dir) + strlen(file_format) + 1; 

pathname = (char *)malloc(len); 

if (NULL == pathname) { 


} 

int r = snprintf(pathname, len, file_format, pwd->pw_dir); 

if (r < 0 || r >= len) { 


} 

if (unlink(pathname) != 0) { 


} 

} 

free(pathname); 

The unlink() function is not susceptible to a symlink attack where the final component of 

pathname (the file name) is a symbolic link because unlink() will remove the symbolic link 

and not affect any file or directory named by the contents of the symbolic link. (See FIO01-C. Be 

careful using functions that use file names for identification.) While this reduces the susceptibility 

of the unlink() function to symlink attacks, it does not eliminate it. The unlink() function is 

still susceptible if one of the directory names included in the pathname is a symbolic link. This 

could cause the unlink() function to delete a similarly named file in a different directory. 






This compliant solution uses the Microsoft Windows SHGetKnownFolderPath() API to get the 

current user’s My Documents folder, which is then combined with the file name to create the path 

to the file to be deleted. The file is then removed using the DeleteFile() API. 

#include 

#include 

#include 

#if defined(_MSC_VER) 

#pragma comment(lib, "Shlwapi") 

#endif 


HRESULT hr; 

LPWSTR path = 0; 

WCHAR full_path[MAX_PATH]; 

} 

hr = SHGetKnownFolderPath(&FOLDERID_Documents, 0, NULL, &path); 

if (FAILED(hr)) { 


} 

if (!PathCombineW(full_path, path, L".config")) { 


} 

CoTaskMemFree(path); 

if (!DeleteFileW(full_path)) { 


} 


ENV33-C-EX1: It is permissible to call system() with a null pointer argument to determine the 

presence of a command processor for the system. 

11.4.8 Risk Assessments 

If the command string passed to system(), popen(), or other function that invokes a command 

processor is not fully sanitized, the risk of exploitation is high. In the worst case scenario, an attacker 

can execute arbitrary system commands on the compromised machine with the privileges 

of the vulnerable process. 


ENV33-C High Probable Medium P12 L1 








ENV03-C. Sanitize the environment when invoking 

external programs. 

ENV02-CPP. Do not call system() if you do 

not need a command processor 

CERT Oracle Secure Coding Standard for Java IDS07-J. Sanitize untrusted data passed to the 

Runtime.exec() method 

ISO/IEC TR 24772:2013 

ISO/IEC TS 17961:2013 

MITRE CWE 

Unquoted Search Path or Element [XZQ] 

Calling system [syscall] 

CWE-78, Improper Neutralization of Special 

Elements Used in an OS Command (aka “OS 

Command Injection”) 

CWE-88, Argument Injection or Modification 


[IEEE Std 1003.1:2013] 

[Wheeler 2004] 

XSH, System Interfaces, exec 

XSH, System Interfaces, popen 

XSH, System Interfaces, unlink 




Environment (ENV) - ENV34-C. Do not store pointers returned by certain functions 

11.5 ENV34-C. Do not store pointers returned by certain functions 


The getenv function returns a pointer to a string associated with the matched list member. 

The string pointed to shall not be modified by the program but may be overwritten 

by a subsequent call to the getenv function. 

This paragraph gives an implementation the latitude, for example, to return a pointer to a statically 

allocated buffer. Consequently, do not store this pointer because the string data it points to may be 

overwritten by a subsequent call to the getenv() function or invalidated by modifications to the 

environment. This string should be referenced immediately and discarded. If later use is anticipated, 

the string should be copied so the copy can be safely referenced as needed. 

The getenv() function is not thread-safe. Make sure to address any possible race conditions resulting 

from the use of this function. 

The asctime(), localeconv(), setlocale(), and strerror() functions have similar restrictions. 

Do not access the objects returned by any of these functions after a subsequent call. 


This noncompliant code example attempts to compare the value of the TMP and TEMP environment 

variables to determine if they are the same: 

#include 

#include 

#include 


char *tmpvar; 

char *tempvar; 

} 

tmpvar = getenv("TMP"); 

if (!tmpvar) { 


} 

tempvar = getenv("TEMP"); 

if (!tempvar) { 


} 

if (strcmp(tmpvar, tempvar) == 0) { 

printf("TMP and TEMP are the same.\n"); 

} else { 

printf("TMP and TEMP are NOT the same.\n"); 

} 





This code example is noncompliant because the string referenced by tmpvar may be overwritten 

as a result of the second call to the getenv() function. As a result, it is possible that both 

tmpvar and tempvar will compare equal even if the two environment variables have different 

values. 


This compliant solution uses the malloc() and strcpy() functions to copy the string returned 

by getenv() into a dynamically allocated buffer: 

#include 

#include 

#include 


char *tmpvar; 


const char *temp = getenv("TMP"); 

if (temp != NULL) { 

tmpvar = (char *)malloc(strlen(temp)+1); 

if (tmpvar != NULL) { 

strcpy(tmpvar, temp); 

} else { 


} 

} else { 


} 

temp = getenv("TEMP"); 


tempvar = (char *)malloc(strlen(temp)+1); 

if (tempvar != NULL) { 

strcpy(tempvar, temp); 

} else { 


} 

} else { 


} 



} else { 


} 

free(tmpvar); 





} 

free(tempvar); 

11.5.3 Compliant Solution (Annex K) 

The C Standard, Annex K, provides the getenv_s() function for getting a value from the current 

environment. However, getenv_s() can still have data races with other threads of execution that 

modify the environment list. 


#include 

#include 

#include 

#include 


char *tmpvar; 


size_t requiredSize; 

errno_t err; 

err = getenv_s(&requiredSize, NULL, 0, "TMP"); 

if (err) { 


} 

tmpvar = (char *)malloc(requiredSize); 

if (!tmpvar) { 


} 

err = getenv_s(&requiredSize, tmpvar, requiredSize, "TMP" ); 

if (err) { 


} 

err = getenv_s(&requiredSize, NULL, 0, "TEMP"); 

if (err) { 


} 

tempvar = (char *)malloc(requiredSize); 

if (!tempvar) { 


} 

err = getenv_s(&requiredSize, tempvar, requiredSize, "TEMP" ); 

if (err) { 


} 






} 


} else { 


} 

free(tmpvar); 

tmpvar = NULL; 


tempvar = NULL; 


Microsoft Windows provides the _dupenv_s() and wdupenv_s() functions for getting a value 

from the current environment [MSDN]. The _dupenv_s() function searches the list of environment 

variables for a specified name. If the name is found, a buffer is allocated; the variable’s 

value is copied into the buffer, and the buffer’s address and number of elements are returned. The 

_dupenv_s() and _wdupenv_s() functions provide more convenient alternatives to 

getenv_s() and _wgetenv_s() because each function handles buffer allocation directly. 

The caller is responsible for freeing any allocated buffers returned by these functions by calling 

free(). 

#include 

#include 

#include 

#include 


char *tmpvar; 


size_t len; 

errno_t err = _dupenv_s(&tmpvar, &len, "TMP"); 

if (err) { 


} 

err = _dupenv_s(&tempvar, &len, "TEMP"); 

if (err) { 


} 



} else { 


} 

free(tmpvar); 







} 



POSIX provides the strdup() function, which can make a copy of the environment variable 

string [IEEE Std 1003.1:2013]. The strdup() function is also included in Extensions to the C 

Library—Part II [ISO/IEC TR 24731-2:2010]. 

#include 

#include 

#include 


char *tmpvar; 


} 

const char *temp = getenv("TMP"); 


tmpvar = strdup(temp); 

if (tmpvar == NULL) { 


} 

} else { 


} 

temp = getenv("TEMP"); 


tempvar = strdup(temp); 

if (tempvar == NULL) { 


} 

} else { 


} 



} else { 


} 

free(tmpvar); 









Storing the pointer to the string returned by getenv(), localeconv(), setlocale(), or 

strerror() can result in overwritten data. 




C Secure Coding Standard 

ISO/IEC TR 24731-2 

ISO/IEC TS 17961:2013 

ENV00-C. Do not store objects that can be 

overwritten by multiple calls to getenv() and 

similar functions 

5.3.1.1, “The strdup Function” 

Using an object overwritten by getenv, localeconv, 

setlocale, and strerror [libuse] 


[IEEE Std 1003.1:2013] 

[ISO/IEC 9899:2011] 

[MSDN] 

[Viega 2003] 

Chapter 8, “Environment Variables” 

XSH, System Interfaces, strdup 

Subclause 7.22.4, “Communication with the 

Environment” 

Subclause 7.22.4.6, “The getenv Function” 

Subclause K.3.6.2.1, “The getenv_s Function” 

_dupenv_s(), _wdupenv_s() 

Section 3.6, “Using Environment Variables Securely” 




Signals (SIG) - SIG30-C. Call only asynchronous-safe functions within signal handlers 

12 Signals (SIG) 

12.1 SIG30-C. Call only asynchronous-safe functions within signal 

handlers 

Call only asynchronous-safe functions within signal handlers. For strictly conforming programs, 

only the C standard library functions abort(), _Exit(), quick_exit(), and signal() can be 

safely called from within a signal handler. 

The C Standard, 7.14.1.1, paragraph 5 [ISO/IEC 9899:2011], states that if the signal occurs other 

than as the result of calling the abort() or raise() function, the behavior is undefined if 

...the signal handler calls any function in the standard library other than the abort function, 

the _Exit function, the quick_exit function, or the signal function with the first 

argument equal to the signal number corresponding to the signal that caused the invocation 

of the handler. 

Implementations may define a list of additional asynchronous-safe functions. These functions can 

also be called within a signal handler. This restriction applies to library functions as well as application-defined 

functions. 

According to the C Rationale, 7.14.1.1 [C99 Rationale 2003], 

When a signal occurs, the normal flow of control of a program is interrupted. If a signal 

occurs that is being trapped by a signal handler, that handler is invoked. When it is finished, 

execution continues at the point at which the signal occurred. This arrangement 

can cause problems if the signal handler invokes a library function that was being executed 

at the time of the signal. 

In general, it is not safe to invoke I/O functions from within signal handlers. Programmers should 

ensure a function is included in the list of an implementation’s asynchronous-safe functions for all 

implementations the code will run on before using them in signal handlers. 


In this noncompliant example, the C standard library functions fprintf() and free() are 

called from the signal handler via the function log_message(). Neither function is asynchronous-safe. 

#include 

#include 

#include 

enum { MAXLINE = 1024 }; 





char *info = NULL; 

void log_message(void) { 

fputs(info, stderr); 

} 

void handler(int signum) { 

log_message(); 

free(info); 

info = NULL; 

} 


if (signal(SIGINT, handler) == SIG_ERR) { 


} 

info = (char *)malloc(MAXLINE); 

if (info == NULL) { 

/* Handle Error */ 

} 

while (1) { 

/* Main loop program code */ 


} 

/* More program code */ 

} 

return 0; 


Signal handlers should be as concise as possible—ideally by unconditionally setting a flag and returning. 

This compliant solution sets a flag of type volatile sig_atomic_t and returns; the 

log_message() and free() functions are called directly from main(): 

#include 

#include 

#include 


volatile sig_atomic_t eflag = 0; 

char *info = NULL; 

void log_message(void) { 

fputs(info, stderr); 

} 






} 

eflag = 1; 




} 

info = (char *)malloc(MAXLINE); 

if (info == NULL) { 


} 

while (!eflag) { 



} 



free(info); 

info = NULL; 

} 

return 0; 

12.1.3 Noncompliant Code Example (longjmp()) 

Invoking the longjmp() function from within a signal handler can lead to undefined behavior if 

it results in the invocation of any non-asynchronous-safe functions. Consequently, neither 

longjmp() nor the POSIX siglongjmp() functions should ever be called from within a signal 

handler. 

This noncompliant code example is similar to a vulnerability in an old version of Sendmail [VU 

#834865]. The intent is to execute code in a main() loop, which also logs some data. Upon receiving 

a SIGINT, the program transfers out of the loop, logs the error, and terminates. 

However, an attacker can exploit this noncompliant code example by generating a SIGINT just 

before the second if statement in log_message(). The result is that longjmp() transfers control 

back to main(), where log_message() is called again. However, the first if statement 

would not be executed this time (because buf is not set to NULL as a result of the interrupt), and 

the program would write to the invalid memory location referenced by buf0. 

#include 

#include 

#include 






static jmp_buf env; 


longjmp(env, 1); 

} 

void log_message(char *info1, char *info2) { 

static char *buf = NULL; 

static size_t bufsize; 

char buf0[MAXLINE]; 

if (buf == NULL) { 

buf = buf0; 

bufsize = sizeof(buf0); 

} 

/* 

* Try to fit a message into buf, else reallocate 

* it on the heap and then log the message. 

*/ 

/* Program is vulnerable if SIGINT is raised here */ 

} 

if (buf == buf0) { 

buf = NULL; 

} 




} 

char *info1; 

char *info2; 

/* info1 and info2 are set by user input here */ 

if (setjmp(env) == 0) { 

while (1) { 


log_message(info1, info2); 


} 

} else { 


} 

} 

return 0; 






In this compliant solution, the call to longjmp() is removed; the signal handler sets an error flag 

instead: 

#include 

#include 


volatile sig_atomic_t eflag = 0; 


eflag = 1; 

} 

void log_message(char *info1, char *info2) { 

static char *buf = NULL; 

static size_t bufsize; 

char buf0[MAXLINE]; 

if (buf == NULL) { 

buf = buf0; 

bufsize = sizeof(buf0); 

} 

} 

/* 

* Try to fit a message into buf, else reallocate 

* it on the heap and then log the message. 

*/ 

if (buf == buf0) { 

buf = NULL; 

} 




} 

char *info1; 

char *info2; 

/* info1 and info2 are set by user input here */ 

while (!eflag) { 




} 






} 

return 0; 

12.1.5 Noncompliant Code Example (raise()) 

In this noncompliant code example, the int_handler() function is used to carry out tasks specific 

to SIGINT and then raises SIGTERM. However, there is a nested call to the raise() function, 

which is undefined behavior. 

#include 

#include 

void term_handler(int signum) { 

/* SIGTERM handler */ 

} 

void int_handler(int signum) { 

/* SIGINT handler */ 

if (raise(SIGTERM) != 0) { 


} 

} 


if (signal(SIGTERM, term_handler) == SIG_ERR) { 


} 

if (signal(SIGINT, int_handler) == SIG_ERR) { 


} 

/* Program code */ 

if (raise(SIGINT) != 0) { 


} 

/* More code */ 

} 



In this compliant solution, int_handler() invokes term_handler() instead of raising 

SIGTERM: 

#include 

#include 

void term_handler(int signum) { 





} 

/* SIGTERM handler */ 

void int_handler(int signum) { 

/* SIGINT handler */ 

/* Pass control to the SIGTERM handler */ 

term_handler(SIGTERM); 

} 


if (signal(SIGTERM, term_handler) == SIG_ERR) { 


} 

if (signal(SIGINT, int_handler) == SIG_ERR) { 


} 

/* Program code */ 

if (raise(SIGINT) != 0) { 


} 

/* More code */ 

} 


12.1.7 Implementation Details 

12.1.7.1 POSIX 

The following table from the POSIX standard [IEEE Std 1003.1:2013] defines a set of functions 

that are asynchronous-signal-safe. Applications may invoke these functions, without restriction, 

from a signal handler. 

_Exit() fexecve() posix_trace_event sigprocmask() 

() 

_exit() fork() pselect() sigqueue() 

abort() fstat() pthread_kill() sigset() 

accept() fstatat() pthread_self() sigsuspend() 

access() fsync() pthread_sigmask() sleep() 

aio_error() ftruncate() raise() sockatmark() 

aio_return() futimens() read() socket() 

aio_suspend() getegid() readlink() socketpair() 

alarm() geteuid() readlinkat() stat() 

bind() getgid() recv() symlink() 





cfgetispeed() getgroups() recvfrom() symlinkat() 

cfgetospeed() getpeername() recvmsg() tcdrain() 

cfsetispeed() getpgrp() rename() tcflow() 

cfsetospeed() getpid() renameat() tcflush() 

chdir() getppid() rmdir() tcgetattr() 

chmod() getsockname() select() tcgetpgrp() 

chown() getsockopt() sem_post() tcsendbreak() 

clock_gettime() getuid() send() tcsetattr() 

close() kill() sendmsg() tcsetpgrp() 

connect() link() sendto() time() 

creat() linkat() setgid() timer_getoverrun() 

dup() listen() setpgid() timer_gettime() 

dup2() lseek() setsid() timer_settime() 

execl() lstat() setsockopt() times() 

execle() mkdir() setuid() umask() 

execv() mkdirat() shutdown() uname() 

execve() mkfifo() sigaction() unlink() 

faccessat() mkfifoat() sigaddset() unlinkat() 

fchdir() mknod() sigdelset() utime() 

fchmod() mknodat() sigemptyset() utimensat() 

fchmodat() open() sigfillset() utimes() 

fchown() openat() sigismember() wait() 

fchownat() pause() signal() waitpid() 

fcntl() pipe() sigpause() write() 

fdatasync() poll() sigpending() 

All functions not listed in this table are considered to be unsafe with respect to signals. In the 

presence of signals, all POSIX functions behave as defined when called from or interrupted by a 

signal handler, with a single exception: when a signal interrupts an unsafe function and the signal 

handler calls an unsafe function, the behavior is undefined. 


If the signal occurs as the result of calling the abort or raise function, the signal handler 

shall not call the raise function. 

However, in the description of signal(), POSIX [IEEE Std 1003.1:2013] states 





This restriction does not apply to POSIX applications, as POSIX.1-2008 requires 

raise() to be async-signal-safe. 


12.1.7.2 OpenBSD 

The OpenBSD signal() manual page lists a few additional functions that are asynchronous-safe 

in OpenBSD but “probably not on other systems” [OpenBSD], including snprintf(), vsnprintf(), 

and syslog_r() but only when the syslog_data struct is initialized as a local 

variable. 


Invoking functions that are not asynchronous-safe from within a signal handler is undefined behavior. 


SIG30-C High Likely Medium P18 L1 


For an overview of software vulnerabilities resulting from improper signal handling, see Michal 

Zalewski’s paper “Delivering Signals for Fun and Profit” [Zalewski 2001]. 

CERT Vulnerability Note VU #834865, “Sendmail signal I/O race condition,” describes a vulnerability 

resulting from a violation of this rule. Another notable case where using the longjmp() 

function in a signal handler caused a serious vulnerability is wu-ftpd 2.4 [Greenman 1997]. The 

effective user ID is set to 0 in one signal handler. If a second signal interrupts the first, a call is 

made to longjmp(), returning the program to the main thread but without lowering the user’s 

privileges. These escalated privileges can be used for further exploitation. 


ISO/IEC TS 17961:2013 

MITRE CWE 

Calling functions in the C Standard Library 

other than abort, _Exit, and signal from 

within a signal handler [asyncsig] 

CWE-479, Signal Handler Use of a Non-reentrant 

Function 


[C99 Rationale 2003] Subclause 5.2.3, “Signals and Interrupts” 

Subclause 7.14.1.1, “The signal Function” 

[Dowd 2006] 

Chapter 13, “Synchronization and State” 

[Greenman 1997] 





[IEEE Std 1003.1:2013] 

[ISO/IEC 9899:2011] 

[OpenBSD] 

[VU #834865] 

[Zalewski 2001] 

XSH, System Interfaces, longjmp 

XSH, System Interfaces, raise 

7.14.1.1, “The signal Function” 

signal() Man Page 

“Delivering Signals for Fun and Profit” 




Signals (SIG) - SIG31-C. Do not access shared objects in signal handlers 

12.2 SIG31-C. Do not access shared objects in signal handlers 

Accessing or modifying shared objects in signal handlers can result in race conditions that can 

leave data in an inconsistent state. The two exceptions (C Standard, 5.1.2.3, paragraph 5) to this 

rule are the ability to read from and write to lock-free atomic objects and variables of type volatile 

sig_atomic_t. Accessing any other type of object from a signal handler is undefined behavior. 

(See undefined behavior 131.) 

The need for the volatile keyword is described in DCL22-C. Use volatile for data that cannot 

be cached. 

The type sig_atomic_t is the integer type of an object that can be accessed as an atomic entity 

even in the presence of asynchronous interrupts. The type of sig_atomic_t is implementationdefined, 

though it provides some guarantees. Integer values ranging from SIG_ATOMIC_MIN 

through SIG_ATOMIC_MAX, inclusive, may be safely stored to a variable of the type. In addition, 

when sig_atomic_t is a signed integer type, SIG_ATOMIC_MIN must be no greater than −127 

and SIG_ATOMIC_MAX no less than 127. Otherwise, SIG_ATOMIC_MIN must be 0 and 

SIG_ATOMIC_MAX must be no less than 255. The macros SIG_ATOMIC_MIN and 

SIG_ATOMIC_MAX are defined in the header . 

According to the C99 Rationale [C99 Rationale 2003], other than calling a limited, prescribed set 

of library functions, 

the C89 Committee concluded that about the only thing a strictly conforming program 

can do in a signal handler is to assign a value to a volatile static variable which 

can be written uninterruptedly and promptly return. 

However, this issue was discussed at the April 2008 meeting of ISO/IEC WG14, and it was 

agreed that there are no known implementations in which it would be an error to read a value 

from a volatile sig_atomic_t variable, and the original intent of the committee was that 

both reading and writing variables of volatile sig_atomic_t would be strictly conforming. 

The signal handler may also call a handful of functions, including abort(). (See SIG30-C. Call 

only asynchronous-safe functions within signal handlers for more information.) 


In this noncompliant code example, err_msg is updated to indicate that the SIGINT signal was 

delivered. The err_msg variable is a character pointer and not a variable of type volatile 

sig_atomic_t. 

#include 

#include 

#include 

enum { MAX_MSG_SIZE = 24 }; 





char *err_msg; 


strcpy(err_msg, "SIGINT encountered."); 

} 


signal(SIGINT, handler); 

} 

err_msg = (char *)malloc(MAX_MSG_SIZE); 

if (err_msg == NULL) { 


} 

strcpy(err_msg, "No errors yet."); 

/* Main code loop */ 

return 0; 

12.2.2 Compliant Solution (Writing volatile sig_atomic_t) 

For maximum portability, signal handlers should only unconditionally set a variable of type volatile 

sig_atomic_t and return, as in this compliant solution: 

#include 

#include 

#include 


volatile sig_atomic_t e_flag = 0; 


e_flag = 1; 

} 


char *err_msg = (char *)malloc(MAX_MSG_SIZE); 

if (err_msg == NULL) { 


} 

} 

signal(SIGINT, handler); 



if (e_flag) { 

strcpy(err_msg, "SIGINT received."); 

} 

return 0; 





12.2.3 Compliant Solution (Lock-Free Atomic Access) 

Signal handlers can refer to objects with static or thread storage a duration that are lock-free 

atomic objects, as in this compliant solution: 

#include 

#include 

#include 

#include 

#ifdef __STDC_NO_ATOMICS__ 

#error "Atomics are not supported" 

#elif ATOMIC_INT_LOCK_FREE == 0 

#error "int is never lock-free" 

#endif 

atomic_int e_flag = ATOMIC_VAR_INIT(0); 


e_flag = 1; 

} 



char err_msg[MAX_MSG_SIZE]; 

#if ATOMIC_INT_LOCK_FREE == 1 

if (!atomic_is_lock_free(&e_flag)) { 


} 

#endif 



} 



if (e_flag) { 

strcpy(err_msg, "SIGINT received."); 

} 


} 


SIG31-C-EX1: The C Standard, 7.14.1.1 paragraph 5 [ISO/IEC 9899:2011], makes a special exception 

for errno when a valid call to the signal() function results in a SIG_ERR return, allowing 

errno to take an indeterminate value. (See ERR32-C. Do not rely on indeterminate values of 

errno.) 






Accessing or modifying shared objects in signal handlers can result in accessing data in an inconsistent 

state. Michal Zalewski’s paper “Delivering Signals for Fun and Profit” [Zalewski 2001] 

provides some examples of vulnerabilities that can result from violating this and other signal-handling 

rules. 


SIG31-C High Likely High P9 L2 


ISO/IEC TS 17961:2013 

MITRE CWE 

Accessing shared objects in signal handlers [accsig] 

CWE-662, Improper Synchronization 


[C99 Rationale 2003] 5.2.3, “Signals and Interrupts” 

[ISO/IEC 9899:2011] 






Signals (SIG) - SIG34-C. Do not call signal() from within interruptible signal handlers 

12.3 SIG34-C. Do not call signal() from within interruptible signal 

handlers 

A signal handler should not reassert its desire to handle its own signal. This is often done on nonpersistent 

platforms—that is, platforms that, upon receiving a signal, reset the handler for the signal 

to SIG_DFL before calling the bound signal handler. Calling signal() under these conditions 

presents a race condition. (See SIG01-C. Understand implementation-specific details 

regarding signal handler persistence.) 

A signal handler may call signal() only if it does not need to be asynchronous-safe (that is, if 

all relevant signals are masked so that the handler cannot be interrupted). 


On nonpersistent platforms, this noncompliant code example contains a race window, starting 

when the host environment resets the signal and ending when the handler calls signal(). During 

that time, a second signal sent to the program will trigger the default signal behavior, consequently 

defeating the persistent behavior implied by the call to signal() from within the handler 

to reassert the binding. 

If the environment is persistent (that is, it does not reset the handler when the signal is received), 

the signal() call from within the handler() function is redundant. 

#include 


if (signal(signum, handler) == SIG_ERR) { 


} 

/* Handle signal */ 

} 


if (signal(SIGUSR1, handler) == SIG_ERR) { 


} 

} 


Calling the signal() function from within the signal handler to reassert the binding is unnecessary 

for persistent platforms, as in this compliant solution: 

#include 







} 




} 

} 


POSIX defines the sigaction() function, which assigns handlers to signals in a similar manner 

to signal() but allows the caller to explicitly set persistence. Consequently, the sigaction() 

function can be used to eliminate the race window on nonpersistent platforms, as in this compliant 

solution: 

#include 

#include 



} 


struct sigaction act; 

act.sa_handler = handler; 

act.sa_flags = 0; 

if (sigemptyset(&act.sa_mask) != 0) { 


} 

if (sigaction(SIGUSR1, &act, NULL) != 0) { 


} 

} 

Although the handler in this example does not call signal(), it could do so safely because the 

signal is masked and the handler cannot be interrupted. If the same handler is installed for more 

than one signal, the signals must be masked explicitly in act.sa_mask to ensure that the handler 

cannot be interrupted because the system masks only the signal being delivered. 

POSIX recommends that new applications should use sigaction() rather than signal(). The 

sigaction() function is not defined by the C Standard and is not supported on some platforms, 

including Windows. 


There is no safe way to implement persistent signal-handler behavior on Windows platforms, and 

it should not be attempted. If a design depends on this behavior, and the design cannot be altered, 





it may be necessary to claim a deviation from this rule after completing an appropriate risk analysis. 

The reason for this is that Windows is a nonpersistent platform as discussed above. Just before 

calling the current handler function, Windows resets the handler for the next occurrence of the 

same signal to SIG_DFL. If the handler calls signal() to reinstall itself, there is still a race window. 

A signal might occur between the start of the handler and the call to signal(), which 

would invoke the default behavior instead of the desired handler. 


SIG34-C-EX1: For implementations with persistent signal handlers, it is safe for a handler to 

modify the behavior of its own signal. Behavior modifications include ignoring the signal, resetting 

to the default behavior, and having the signal handled by a different handler. A handler reasserting 

its binding is also safe but unnecessary. 

The following code example resets a signal handler to the system’s default behavior: 

#include 


#if !defined(_WIN32) 

if (signal(signum, SIG_DFL) == SIG_ERR) { 


} 

#endif 


} 




} 

} 


Two signals in quick succession can trigger a race condition on nonpersistent platforms, causing 

the signal’s default behavior despite a handler’s attempt to override it. 


SIG34-C Low Unlikely Low P3 L3 







ISO/IEC TS 17961:2013 

MITRE CWE 

SIG01-C. Understand implementation-specific 

details regarding signal handler persistence 

Calling signal from interruptible signal handlers 

[sigcall] 

CWE-479, Signal Handler Use of a Non-reentrant 

Function 




Signals (SIG) - SIG35-C. Do not return from a computational exception signal handler 

12.4 SIG35-C. Do not return from a computational exception signal 

handler 

According to the C Standard, 7.14.1.1 [ISO/IEC 9899:2011], if a signal handler returns when it 

has been entered as a result of a computational exception (that is, with the value of its argument of 

SIGFPE, SIGILL, SIGSEGV, or any other implementation-defined value corresponding to such an 

exception) returns, then the behavior is undefined. (See undefined behavior 130.) 

The Portable Operating System Interface (POSIX®), Base Specifications, Issue 7 [IEEE Std 

1003.1:2013], adds SIGBUS to the list of computational exception signal handlers: 

The behavior of a process is undefined after it returns normally from a signal-catching 

function for a SIGBUS, SIGFPE, SIGILL, or SIGSEGV signal that was not generated by 

kill(), sigqueue(), or raise(). 

Do not return from SIGFPE, SIGILL, SIGSEGV, or any other implementation-defined value corresponding 

to a computational exception, such as SIGBUS on POSIX systems, regardless of how the 

signal was generated. 


In this noncompliant code example, the division operation has undefined behavior if denom 

equals 0 and may result in a SIGFPE signal to the program. (See INT33-C. Ensure that division 

and remainder operations do not result in divide-by-zero errors.) 

#include 

#include 

#include 

#include 

volatile sig_atomic_t denom; 

void sighandle(int s) { 

/* Fix the offending volatile */ 

if (denom == 0) { 

denom = 1; 

} 

} 


if (argc < 2) { 

return 0; 

} 

char *end = NULL; 

long temp = strtol(argv[1], &end, 10); 

if (end == argv[1] || 0 != *end || 





{ 

} 

((LONG_MIN == temp || LONG_MAX == temp) && errno == ERANGE)) 


denom = (sig_atomic_t)temp; 

signal(SIGFPE, sighandle); 

} 

long result = 100 / (long)denom; 

return 0; 

When compiled with some implementations, this noncompliant code example will loop infinitely 

if given the input 0. It illustrates that even when a SIGFPE handler attempts to fix the error condition 

while obeying all other rules of signal handling, the program still does not behave as expected. 


The only portably safe way to leave a SIGFPE, SIGILL, or SIGSEGV handler is to invoke 

abort(), quick_exit(), or _Exit(). In the case of SIGFPE, the default action is abnormal 

termination, so no user-defined handler is required: 

#include 

#include 

#include 

#include 


if (argc < 2) { 

return 0; 

} 

char *end = NULL; 

long denom = strtol(argv[1], &end, 10); 

if (end == argv[1] || 0 != *end || 

((LONG_MIN == denom || LONG_MAX == denom) && errno == 

ERANGE)) { 


} 

} 

long result = 100 / denom; 

return 0; 






Some implementations define useful behavior for programs that return from one or more of these 

signal handlers. For example, Solaris provides the sigfpe() function specifically to set a 

SIGFPE handler that a program may safely return from. Oracle also provides platform-specific 

computational exceptions for the SIGTRAP, SIGBUS, and SIGEMT signals. Finally, GNU libsigsegv 

takes advantage of the ability to return from a SIGSEGV handler to implement page-level 

memory management in user mode. 


Returning from a computational exception signal handler is undefined behavior. 


SIG35-C Low Unlikely High P1 L3 


[IEEE Std 1003.1:2013] 

[ISO/IEC 9899:2011] 

2.4.1, Signal Generation and Delivery 





Error Handling (ERR) - ERR30-C. Set errno to zero before calling a library function known to set errno, and check errno only 

after the function returns a value indicating failure 

13 Error Handling (ERR) 

13.1 ERR30-C. Set errno to zero before calling a library function known 

to set errno, and check errno only after the function returns a value 

indicating failure 

The value of errno is initialized to zero at program startup, but it is never subsequently set to 

zero by any C standard library function. The value of errno may be set to nonzero by a C standard 

library function call whether or not there is an error, provided the use of errno is not documented 

in the description of the function. It is meaningful for a program to inspect the contents of 

errno only after an error might have occurred. More precisely, errno is meaningful only after a 

library function that sets errno on error has returned an error code. 

According to Question 20.4 of C-FAQ [Summit 2005] 

In general, you should detect errors by checking return values, and use errno only to 

distinguish among the various causes of an error, such as “File not found” or “Permission 

denied.” (Typically, you use perror or strerror to print these discriminating error 

messages.) It’s only necessary to detect errors with errno when a function does not 

have a unique, unambiguous, out-of-band error return (that is, because all of its possible 

return values are valid; one example is atoi [sic]). In these cases (and in these 

cases only; check the documentation to be sure whether a function allows this), you can 

detect errors by setting errno to 0, calling the function, and then testing errno. (Setting 

errno to 0 first is important, as no library function ever does that for you.) 

Note that atoi() is not required to set the value of errno. 

Library functions fall into the following categories: 

• Those that set errno and return and out-of-band error indicator 

• Those that set errno and return and in-band error indicator 

• Those that do not promise to set errno 

• Those with differing standards documentation 

13.1.1 Library Functions that Set errno and Return an Out-of-Band Error 

Indicator 

The C Standard specifies that the functions listed in the following table set errno and return an 

out-of-band error indicator. That is, their return value on error can never be returned by a successful 

call. 

A program may set and check errno for these library functions but is not required to do so. The 

program should not check the value of errno without first verifying that the function returned an 






error indicator. For example, errno should not be checked after calling signal() without first 

ensuring that signal() actually returned SIG_ERR. 

Functions That Set errno and Return an Out-of-Band Error Indicator 

Function Name Return Value errno Value 

ftell() -1L Positive 

fgetpos(), fsetpos() Nonzero Positive 

mbrtowc(), mbsrtowcs() (size_t)(-1) EILSEQ 

signal() SIG_ERR Positive 

wcrtomb(), wcsrtombs() (size_t)(-1) EILSEQ 

mbrtoc16(), mbrtoc32() (size_t)(-1) EILSEQ 

c16rtomb(), cr32rtomb() (size_t)(-1) EILSEQ 

13.1.2 Library Functions that Set errno and Return an In-Band Error Indicator 

The C Standard specifies that the functions listed in the following table set errno and return an 

in-band error indicator. That is, the return value when an error occurs is also a valid return value 

for successful calls. For example, the strtoul() function returns ULONG_MAX and sets errno to 

ERANGE if an error occurs. Because ULONG_MAX is a valid return value, errno must be used to 

check whether an error actually occurred. A program that uses errno for error checking must set 

it to 0 before calling one of these library functions and then inspect errno before a subsequent 

library function call. 

The fgetwc() and fputwc() functions return WEOF in multiple cases, only one of which results 

in setting errno. The string conversion functions will return the maximum or minimum representable 

value and set errno to ERANGE if the converted value cannot be represented by the data 

type. However, if the conversion cannot happen because the input is invalid, the function will return 

0, and the output pointer parameter will be assigned the value of the input pointer parameter, 

provided the output parameter is non-null. 

Functions that Set errno and Return an In-Band Error Indicator 


fgetwc(), fputwc() WEOF EILSEQ 

strtol(), wcstol() LONG_MIN or LONG_MAX ERANGE 

strtoll(), wcstoll() LLONG_MIN or LLONG_MAX ERANGE 

strtoul(), wcstoul() ULONG_MAX ERANGE 

strtoull(), wcstoull() ULLONG_MAX ERANGE 

strtoumax(),wcstoumax() UINTMAX_MAX 

ERANGE 

strtod(), wcstod() 0 or ±HUGE_VAL ERANGE 

strtof(), wcstof() 0 or ±HUGE_VALF ERANGE 







strtold(), wcstold() 0 or ±HUGE_VALL ERANGE 

strtoimax(),wcstoimax() INTMAX_MIN, INTMAX_MAX ERANGE 

13.1.3 Library Functions that Do Not Promise to Set errno 

The C Standard fails to document the behavior of errno for some functions. For example, the 

setlocale() function normally returns a null pointer in the event of an error, but no guarantees 

are made about setting errno. 

After calling one of these functions, a program should not rely solely on the value of errno to determine 

if an error occurred. The function might have altered errno, but this does not ensure that 

errno will properly indicate an error condition. 

13.1.4 Library Functions with Differing Standards Documentation 

Some functions behave differently regarding errno in various standards. The fopen() function 

is one such example. When fopen() encounters an error, it returns a null pointer. The C Standard 

makes no mention of errno when describing fopen(). However, POSIX.1 declares that when 

fopen() encounters an error, it returns a null pointer and sets errno to a value indicating the error 

[IEEE Std 1003.1-2013]. The implication is that a program conforming to C but not to POSIX 

(such as a Windows program) should not check errno after calling fopen(), but a POSIX program 

may check errno if fopen() returns a null pointer. 

13.1.5 Library Functions and errno 

The following uses of errno are documented in the C Standard: 

• Functions defined in may set errno but are not required to. 

• For numeric conversion functions in the strtod, strtol, wcstod, and wcstol families, if 

the correct result is outside the range of representable values, an appropriate minimum or 

maximum value is returned and the value ERANGE is stored in errno. For floating-point conversion 

functions in the strtod and wcstod families, if an underflow occurs, whether 

errno acquires the value ERANGE is implementation-defined. If the conversion fails, 0 is returned 

and errno is not set. 

• The numeric conversion function atof() and those in the atoi family “need not affect the 

value of” errno. 

• For mathematical functions in , if the integer expression math_errhandling & 

MATH_ERRNO is nonzero, on a domain error, errno acquires the value EDOM; on an overflow 

with default rounding or if the mathematical result is an exact infinity from finite arguments, 

errno acquires the value ERANGE; and on an underflow, whether errno acquires the value 

ERANGE is implementation-defined. 

• If a request made by calling signal() cannot be honored, a value of SIG_ERR is returned 

and a positive value is stored in errno. 






• The byte I/O functions, wide-character I/O functions, and multibyte conversion functions 

store the value of the macro EILSEQ in errno if and only if an encoding error occurs. 

• On failure, fgetpos() and fsetpos() return nonzero and store an implementation-defined 

positive value in errno. 

• On failure, ftell() returns -1L and stores an implementation-defined positive value in 

errno. 

• The perror() function maps the error number in errno to a message and writes it to 

stderr. 

The POSIX.1 standard defines the use of errno by many more functions (including the C standard 

library function). POSIX also has a small set of functions that are exceptions to the rule. These 

functions have no return value reserved to indicate an error, but they still set errno on error. To 

detect an error, an application must set errno to 0 before calling the function and check whether 

it is nonzero after the call. Affected functions include strcoll(), strxfrm(), strerror(), 

wcscoll(), wcsxfrm(), and fwide().The C Standard allows these functions to set errno to a 

nonzero value on success. Consequently, this type of error checking should be performed only on 

POSIX systems. 

13.1.6 Noncompliant Code Example (strtoul()) 

This noncompliant code example fails to set errno to 0 before invoking strtoul(). If an error 

occurs, strtoul() returns a valid value (ULONG_MAX), so errno is the only means of determining 

if strtoul() ran successfully. 

#include 

#include 

#include 

void func(const char *c_str) { 

unsigned long number; 

char *endptr; 

} 

number = strtoul(c_str, &endptr, 0); 

if (endptr == c_str || (number == ULONG_MAX 

&& errno == ERANGE)) { 


} else { 

/* Computation succeeded */ 

} 

Any error detected in this manner may have occurred earlier in the program or may not represent 

an actual error. 






13.1.7 Compliant Solution (strtoul()) 

This compliant solution sets errno to 0 before the call to strtoul() and inspects errno after 

the call: 

#include 

#include 

#include 

void func(const char *c_str) { 

unsigned long number; 

char *endptr; 

} 

errno = 0; 

number = strtoul(c_str, &endptr, 0); 

if (endptr == c_str || (number == ULONG_MAX 

&& errno == ERANGE)) { 


} else { 

/* Computation succeeded */ 

} 

13.1.8 Noncompliant Code Example (fopen()) 

This noncompliant code example may fail to diagnose errors because fopen() might not set 

errno even if an error occurs: 

#include 

#include 

void func(const char *filename) { 

FILE *fileptr; 

} 

errno = 0; 

fileptr = fopen(filename, "rb"); 

if (errno != 0) { 


} 

13.1.9 Compliant Solution (fopen(), C) 

The C Standard makes no mention of errno when describing fopen(). In this compliant solution, 

the results of the call to fopen() are used to determine failure and errno is not checked: 

#include 







} 

FILE *fileptr = fopen(filename, "rb"); 

if (fileptr == NULL) { 

/* An error occurred in fopen() */ 

} 

13.1.10 Compliant Solution (fopen(), POSIX) 

In this compliant solution, errno is checked only after an error has already been detected by another 

means: 

#include 

#include 


FILE *fileptr; 

} 

errno = 0; 

fileptr = fopen(filename, "rb"); 

if (fileptr == NULL) { 

/* 

* An error occurred in fopen(); now it's valid 

* to examine errno. 

*/ 

perror(filename); 

} 


The improper use of errno may result in failing to detect an error condition or in incorrectly 

identifying an error condition when none exists. 


ERR30-C Medium Probable Medium P8 L2 



ISO/IEC TS 17961:2013 

MITRE CWE 

EXP12-C. Do not ignore values returned by 

functions 

Incorrectly setting and using errno [inverrno] 

CWE-456, Missing Initialization of a Variable 







[Brainbell.com] 

Macros and Miscellaneous Pitfalls 

[Horton 1990] Section 11, p. 168 

Section 14, p. 254 

[IEEE Std 1003.1-2013] 

XSH, System Interfaces, fopen 

[Koenig 1989] Section 5.4, p. 73 

[Summit 2005] 




Error Handling (ERR) - ERR32-C. Do not rely on indeterminate values of errno 

13.2 ERR32-C. Do not rely on indeterminate values of errno 

According to the C Standard [ISO/IEC 9899:2011], the behavior of a program is undefined when 

the value of errno is referred to after a signal occurred other than as the result of calling 

the abort or raise function and the corresponding signal handler obtained a SIG_ERR 

return from a call to the signal function. 


A signal handler is allowed to call signal(); if that fails, signal() returns SIG_ERR and sets 

errno to a positive value. However, if the event that caused a signal was external (not the result 

of the program calling abort() or raise()), the only functions the signal handler may call are 

_Exit() or abort(), or it may call signal() on the signal currently being handled; if signal() 

fails, the value of errno is indeterminate. 

This rule is also a special case of SIG31-C. Do not access shared objects in signal handlers. The 

object designated by errno is of static storage duration and is not a volatile sig_atomic_t. 

As a result, performing any action that would require errno to be set would normally cause undefined 

behavior. The C Standard, 7.14.1.1, paragraph 5, makes a special exception for errno in 

this case, allowing errno to take on an indeterminate value but specifying that there is no other 

undefined behavior. This special exception makes it possible to call signal() from within a signal 

handler without risking undefined behavior, but the handler, and any code executed after the 

handler returns, must not depend on the value of errno being meaningful. 


The handler() function in this noncompliant code example attempts to restore default handling 

for the signal indicated by signum. If the request to set the signal to default can be honored, the 

signal() function returns the value of the signal handler for the most recent successful call to 

the signal() function for the specified signal. Otherwise, a value of SIG_ERR is returned and a 

positive value is stored in errno. Unfortunately, the value of errno is indeterminate because the 

handler() function is called when an external signal is raised, so any attempt to read errno (for 

example, by the perror() function) is undefined behavior: 

#include 

#include 

#include 

typedef void (*pfv)(int); 


pfv old_handler = signal(signum, SIG_DFL); 

if (old_handler == SIG_ERR) { 

perror("SIGINT handler"); /* Undefined behavior */ 


} 





} 


pfv old_handler = signal(SIGINT, handler); 


perror("SIGINT handler"); 


} 


} 


The call to perror() from handler() also violates SIG30-C. Call only asynchronous-safe 

functions within signal handlers. 


This compliant solution does not reference errno and does not return from the signal handler if 

the signal() call fails: 

#include 

#include 

#include 

typedef void (*pfv)(int); 


pfv old_handler = signal(signum, SIG_DFL); 


abort(); 

} 

} 


pfv old_handler = signal(SIGINT, handler); 


perror("SIGINT handler"); 


} 


} 







POSIX is less restrictive than C about what applications can do in signal handlers. It has a long 

list of asynchronous-safe functions that can be called. (See SIG30-C. Call only asynchronous-safe 

functions within signal handlers.) Many of these functions set errno on error, which can lead to a 

signal handler being executed between a call to a failed function and the subsequent inspection of 

errno. Consequently, the value inspected is not the one set by that function but the one set by a 

function call in the signal handler. POSIX applications can avoid this problem by ensuring that 

signal handlers containing code that might alter errno always save the value of errno on entry 

and restore it before returning. 

The signal handler in this noncompliant code example alters the value of errno. As a result, it 

can cause incorrect error handling if executed between a failed function call and the subsequent 

inspection of errno: 

#include 

#include 

#include 

#include 

void reaper(int signum) { 

errno = 0; 

for (;;) { 

int rc = waitpid(-1, NULL, WNOHANG); 

if ((0 == rc) || (-1 == rc && EINTR != errno)) { 

break; 

} 

} 

if (ECHILD != errno) { 


} 

} 



act.sa_handler = reaper; 




} 

if (sigaction(SIGCHLD, &act, NULL) != 0) { 


} 

/* ... */ 

} 







This compliant solution saves and restores the value of errno in the signal handler: 

#include 

#include 

#include 

#include 

void reaper(int signum) { 

errno_t save_errno = errno; 

errno = 0; 

for (;;) { 

int rc = waitpid(-1, NULL, WNOHANG); 

if ((0 == rc) || (-1 == rc && EINTR != errno)) { 

break; 

} 

} 

if (ECHILD != errno) { 


} 

errno = save_errno; 

} 



act.sa_handler = reaper; 




} 

if (sigaction(SIGCHLD, &act, NULL) != 0) { 


} 

/* ... */ 

} 



Referencing indeterminate values of errno is undefined behavior. 


ERR32-C Low Unlikely Low P3 L3 







SIG30-C. Call only asynchronous-safe functions 

within signal handlers 

SIG31-C. Do not access shared objects in signal 

handlers 


[ISO/IEC 9899:2011] 





Error Handling (ERR) - ERR33-C. Detect and handle standard library errors 

13.3 ERR33-C. Detect and handle standard library errors 

The majority of the standard library functions, including I/O functions and memory allocation 

functions, return either a valid value or a value of the correct return type that indicates an error 

(for example, −1 or a null pointer). Assuming that all calls to such functions will succeed and failing 

to check the return value for an indication of an error is a dangerous practice that may lead to 

unexpected or undefined behavior when an error occurs. It is essential that programs detect and 

appropriately handle all errors in accordance with an error-handling policy, as discussed in 

ERR00-C. Adopt and implement a consistent and comprehensive error-handling policy. 

The successful completion or failure of each of the standard library functions listed in the following 

table shall be determined either by comparing the function’s return value with the value listed 

in the column labeled “Error Return” or by calling one of the library functions mentioned in the 

footnotes. 

Standard Library Functions 

Function Successful Return Error Return 

aligned_alloc() Pointer to space NULL 

asctime_s() 0 Nonzero 

at_quick_exit() 0 Nonzero 

atexit() 0 Nonzero 

bsearch() Pointer to matching element NULL 

bsearch_s() Pointer to matching element NULL 

btowc() Converted wide character WEOF 

c16rtomb() Number of bytes (size_t)(-1) 

c32rtomb() Number of bytes (size_t)(-1) 

calloc() Pointer to space NULL 

clock() Processor time (clock_t)(-1) 

cnd_broadcast() thrd_success thrd_error 

cnd_init() thrd_success thrd_nomem or thrd_error 

cnd_signal() thrd_success thrd_error 

cnd_timedwait() thrd_success thrd_timedout or 

thrd_error 

cnd_wait() thrd_success thrd_error 

ctime_s() 0 Nonzero 

fclose() 0 EOF (negative) 

fflush() 0 EOF (negative) 






fgetc() Character read EOF 6 

fgetpos() 0 Nonzero, errno > 0 

fgets() Pointer to string NULL 

fgetwc() Wide character read WEOF 6 

fopen() Pointer to stream NULL 

fopen_s() 0 Nonzero 

fprintf() 

fprintf_s() 

Number of characters 

(nonnegative) 


(nonnegative) 

Negative 

Negative 

fputc() Character written EOF 7 

fputs() Nonnegative EOF (negative) 

fputwc() Wide character written WEOF 

fputws() Nonnegative EOF (negative) 

fread() Elements read Elements read 

freopen() Pointer to stream NULL 

freopen_s() 0 Nonzero 

fscanf() 

fscanf_s() 

Number of conversions 

(nonnegative) 


(nonnegative) 

EOF (negative) 


fseek() 0 Nonzero 

fsetpos() 0 Nonzero, errno > 0 

ftell() File position −1L, errno > 0 

fwprintf() 

Number of wide characters 

(nonnegative) 

Negative 

fwprintf_s() 

Number of wide characters Negative 

(nonnegative) 

fwrite() Elements written Elements written 

fwscanf() 

Number of conversions EOF (negative) 

(nonnegative) 

fwscanf_s() 

Number of conversions EOF (negative) 

(nonnegative) 

getc() Character read EOF 6 

______________________ 

6 

By calling ferror() and feof() 

7 

By calling ferror() 






getchar() Character read EOF 6 

getenv() Pointer to string NULL 

getenv_s() Pointer to string NULL 

gets_s() Pointer to string NULL 

getwc() Wide character read WEOF 

getwchar() Wide character read WEOF 

gmtime() Pointer to broken-down time NULL 

gmtime_s() Pointer to broken-down time NULL 

localtime() Pointer to broken-down time NULL 

localtime_s() Pointer to broken-down time NULL 

malloc() Pointer to space NULL 

mblen(), s != NULL Number of bytes −1 

mbrlen(), s != NULL Number of bytes or status (size_t)(-1) 

mbrtoc16() Number of bytes or status (size_t)(-1), errno == 

EILSEQ 

mbrtoc32() Number of bytes or status (size_t)(-1), errno == 

EILSEQ 

mbrtowc(), s != NULL Number of bytes or status (size_t)(-1), errno == 

EILSEQ 

mbsrtowcs() Number of non-null elements (size_t)(-1), errno == 

EILSEQ 

mbsrtowcs_s() 0 Nonzero 

mbstowcs() Number of non-null elements (size_t)(-1) 

mbstowcs_s() 0 Nonzero 

mbtowc(), s != NULL Number of bytes −1 

memchr() Pointer to located character NULL 

mktime() Calendar time (time_t)(-1) 

mtx_init() thrd_success thrd_error 

mtx_lock() thrd_success thrd_error 

mtx_timedlock() thrd_success thrd_timedout or 

thrd_error 

mtx_trylock() thrd_success thrd_busy or thrd_error 

mtx_unlock() thrd_success thrd_error 

printf_s() 


(nonnegative) 

Negative 

putc() Character written EOF 7 






putwc() Wide character written WEOF 

raise() 0 Nonzero 

realloc() Pointer to space NULL 

remove() 0 Nonzero 

rename() 0 Nonzero 

setlocale() Pointer to string NULL 

setvbuf() 0 Nonzero 

scanf() 

scanf_s() 


(nonnegative) 


(nonnegative) 



signal() Pointer to previous function SIG_ERR, errno > 0 

snprintf() 

snprintf_s() 

sprintf() 

sprintf_s() 

sscanf() 

sscanf_s() 

Number of characters that 

would be written (nonnegative) 



Number of non-null characters 

written 


written 


(nonnegative) 


(nonnegative) 

Negative 

Negative 

Negative 

Negative 

strchr() Pointer to located character NULL 



strerror_s() 0 Nonzero 

strftime() Number of non-null characters 0 

strpbrk() Pointer to located character NULL 

strrchr() Pointer to located character NULL 

strstr() Pointer to located string NULL 

strtod() Converted value 0, errno == ERANGE 

strtof() Converted value 0, errno == ERANGE 

strtoimax() Converted value INTMAX_MAX or INTMAX_MIN, 

strtok() 

Pointer to first character of a 

token 

errno == ERANGE 

NULL 






strtok_s() 

Pointer to first character of a 

token 

NULL 

strtol() Converted value LONG_MAX or LONG_MIN, 


strtold() Converted value 0, errno == ERANGE 

strtoll() Converted value LLONG_MAX or LLONG_MIN, 


strtoumax() Converted value UINTMAX_MAX, errno == 

ERANGE 

strtoul() Converted value ULONG_MAX, errno == 

ERANGE 

strtoull() Converted value ULLONG_MAX, errno == 

ERANGE 

strxfrm() Length of transformed string >= n 

swprintf() 

swprintf_s() 

swscanf() 

swscanf_s() 

Number of non-null wide characters 



(nonnegative) 


(nonnegative) 

Negative 

Negative 



thrd_create() thrd_success thrd_nomem or thrd_error 

thrd_detach() thrd_success thrd_error 

thrd_join() thrd_success thrd_error 

thrd_sleep() 0 Negative 

time() Calendar time (time_t)(-1) 

timespec_get() Base 0 

tmpfile() Pointer to stream NULL 

tmpfile_s() 0 Nonzero 

tmpnam() Non-null pointer NULL 

tmpnam_s() 0 Nonzero 

tss_create() thrd_success thrd_error 

tss_get() 

Value of thread-specific storage 

tss_set() thrd_success thrd_error 

0 






ungetc() Character pushed back EOF 8 

ungetwc() Character pushed back WEOF 

vfprintf() 

vfprintf_s() 

vfscanf() 

vfscanf_s() 

vfwprintf() 

vfwprintf_s() 

vfwscanf() 

vfwscanf_s() 

vprintf_s() 

vscanf() 

vscanf_s() 

vsnprintf() 

vsnprintf_s() 

vsprintf() 

vsprintf_s() 


(nonnegative) 


(nonnegative) 


(nonnegative) 


(nonnegative) 


(nonnegative) 


(nonnegative) 


(nonnegative) 


(nonnegative) 


(nonnegative) 


(nonnegative) 


(nonnegative) 






(nonnegative) 


(nonnegative) 

Negative 

Negative 



Negative 

Negative 



Negative 



Negative 

Negative 

Negative 

Negative 

______________________ 

8 

The ungetc() function does not set the error indicator even when it fails, so it is not possible to check for 

errors reliably unless it is known that the argument is not equal to EOF. The C Standard [ISO/IEC 9899:2011] 

states that “one character of pushback is guaranteed,” so this should not be an issue if, at most, one character 

is ever pushed back before reading again. (See FIO13-C. Never push back anything other than one read character.) 






vsscanf() 

vsscanf_s() 

vswprintf() 

vswprintf_s() 

vswscanf() 

vswscanf_s() 

vwprintf_s() 

vwscanf() 

vwscanf_s() 


(nonnegative) 


(nonnegative) 




(nonnegative) 


(nonnegative) 


(nonnegative) 


(nonnegative) 


(nonnegative) 



Negative 

Negative 



Negative 



wcrtomb() Number of bytes stored (size_t)(-1) 

wcschr() 

wcsftime() 

wcspbrk() 

wcsrchr() 

Pointer to located wide character 




NULL 

0 

NULL 

NULL 

wcsrtombs() Number of non-null bytes (size_t)(-1), errno == 

EILSEQ 

wcsrtombs_s() 0 Nonzero 

wcsstr() Pointer to located wide string NULL 

wcstod() Converted value 0, errno == ERANGE 

wcstof() Converted value 0, errno == ERANGE 

wcstoimax() Converted value INTMAX_MAX or INTMAX_MIN, 

wcstok() 

Pointer to first wide character 

of a token 


NULL 






wcstok_s() 

Pointer to first wide character 

of a token 

NULL 

wcstol() Converted value LONG_MAX or LONG_MIN, 


wcstold() Converted value 0, errno == ERANGE 

wcstoll() Converted value LLONG_MAX or LLONG_MIN, 


wcstombs() Number of non-null bytes (size_t)(-1) 

wcstombs_s() 0 Nonzero 

wcstoumax() Converted value UINTMAX_MAX, errno == 

ERANGE 

wcstoul() Converted value ULONG_MAX, errno == 

ERANGE 

wcstoull() Converted value ULLONG_MAX, errno == 

wcsxfrm() 

Length of transformed wide 

string 

ERANGE 

>= n 

wctob() Converted character EOF 

wctomb(), s != NULL Number of bytes stored −1 

wctomb_s(), s != NULL Number of bytes stored −1 

wctrans() 

Valid argument to 

towctrans 

wctype() Valid argument to iswctype 0 

wmemchr() 

wprintf_s() 

wscanf() 

wscanf_s() 



(nonnegative) 


(nonnegative) 


(nonnegative) 

0 

NULL 

Negative 



Note: According to FIO35-C. Use feof() and ferror() to detect end-of-file and file errors when 

sizeof(int) == sizeof(char), callers should verify end-of-file and file errors for the functions in this 

table as follows: 

1 By calling ferror() and feof() 

2 By calling ferror() 





13.3.1 Noncompliant Code Example (setlocale()) 

In this noncompliant code example, the function utf8_to_wcs() attempts to convert a sequence 

of UTF-8 characters to wide characters. It first invokes setlocale() to set the global locale to 

the implementation-defined “en_US.UTF-8” but does not check for failure. The setlocale() 

function will fail by returning a null pointer, for example, when the locale is not installed. The 

function may fail for other reasons as well, such as the lack of resources. Depending on the sequence 

of characters pointed to by utf8, the subsequent call to mbstowcs() may fail or result in 

the function storing an unexpected sequence of wide characters in the supplied buffer wcs. 

#include 

#include 

int utf8_to_wcs(wchar_t *wcs, size_t n, const char *utf8, 

size_t *size) { 

if (NULL == size) { 

return -1; 

} 

setlocale(LC_CTYPE, "en_US.UTF-8"); 

*size = mbstowcs(wcs, utf8, n); 

return 0; 

} 

13.3.2 Compliant Solution (setlocale()) 

This compliant solution checks the value returned by setlocale() and avoids calling 

mbstowcs() if the function fails. The function also takes care to restore the locale to its initial 

setting before returning control to the caller. 

#include 

#include 

int utf8_to_wcs(wchar_t *wcs, size_t n, const char *utf8, 

size_t *size) { 

if (NULL == size) { 

return -1; 

} 

const char *save = setlocale(LC_CTYPE, "en_US.UTF-8"); 

if (NULL == save) { 

return -1; 

} 

*size = mbstowcs(wcs, utf8, n); 

if (NULL == setlocale(LC_CTYPE, save)) { 

return -1; 

} 





} 

return 0; 

13.3.3 Noncompliant Code Example (calloc()) 

In this noncompliant code example, temp_num, tmp2, and num_of_records are derived from a 

tainted source. Consequently, an attacker can easily cause calloc() to fail by providing a large 

value for num_of_records. 

#include 

#include 

enum { SIG_DESC_SIZE = 32 }; 

typedef struct { 

char sig_desc[SIG_DESC_SIZE]; 

} signal_info; 

void func(size_t num_of_records, size_t temp_num, 

const char *tmp2, size_t tmp2_size_bytes) { 

signal_info *start = (signal_info *)calloc(num_of_records, 

sizeof(signal_info)); 

if (tmp2 == NULL) { 


} else if (temp_num > num_of_records) { 


} else if (tmp2_size_bytes < SIG_DESC_SIZE) { 


} 

} 

signal_info *point = start + temp_num - 1; 

memcpy(point->sig_desc, tmp2, SIG_DESC_SIZE); 

point->sig_desc[SIG_DESC_SIZE - 1] = '\0'; 

/* ... */ 

free(start); 

When calloc() fails, it returns a null pointer that is assigned to start. If start is null, an attacker 

can provide a value for temp_num that, when scaled by sizeof(signal_info), references 

a writable address to which control is eventually transferred. The contents of the string referenced 

by tmp2 can then be used to overwrite the address, resulting in an arbitrary code 

execution vulnerability. 





13.3.4 Compliant Solution (calloc()) 

To correct this error, ensure the pointer returned by calloc() is not null: 

#include 

#include 

enum { SIG_DESC_SIZE = 32 }; 


char sig_desc[SIG_DESC_SIZE]; 

} signal_info; 

void func(size_t num_of_records, size_t temp_num, 

const char *tmp2, size_t tmp2_size_bytes) { 

signal_info *start = (signal_info *)calloc(num_of_records, 

sizeof(signal_info)); 

if (start == NULL) { 

/* Handle allocation error */ 

} else if (tmp2 == NULL) { 


} else if (temp_num > num_of_records) { 


} else if (tmp2_size_bytes < SIG_DESC_SIZE) { 


} 

} 

signal_info *point = start + temp_num - 1; 

memcpy(point->sig_desc, tmp2, SIG_DESC_SIZE); 

point->sig_desc[SIG_DESC_SIZE - 1] = '\0'; 

/* ... */ 

free(start); 

13.3.5 Noncompliant Code Example (realloc()) 

This noncompliant code example calls realloc() to resize the memory referred to by p. However, 

if realloc() fails, it returns a null pointer and the connection between the original block of 

memory and p is lost, resulting in a memory leak. 

#include 

void *p; 

void func(size_t new_size) { 

if (new_size == 0) { 


} 

p = realloc(p, new_size); 







} 

} 

This code example complies with MEM04-C. Do not perform zero-length allocations. 


In this compliant solution, the result of realloc() is assigned to the temporary pointer q and 

validated before it is assigned to the original pointer p: 

#include 

void *p; 

void func(size_t new_size) { 

void *q; 

if (new_size == 0) { 


} 

} 

q = realloc(p, new_size); 

if (q == NULL) { 


} else { 

p = q; 

} 

13.3.7 Noncompliant Code Example (fseek()) 

In this noncompliant code example, the fseek() function is used to set the file position to a location 

offset in the file referred to by file prior to reading a sequence of bytes from the file. 

However, if an I/O error occurs during the seek operation, the subsequent read will fill the buffer 

with the wrong contents. 

#include 

size_t read_at(FILE *file, long offset, 

void *buf, size_t nbytes) { 

fseek(file, offset, SEEK_SET); 

return fread(buf, 1, nbytes, file); 

} 





13.3.8 Compliant Solution (fseek()) 

According to the C Standard, the fseek() function returns a nonzero value to indicate that an error 

occurred. This compliant solution tests for this condition before reading from a file to eliminate 

the chance of operating on the wrong portion of the file if fseek() fails: 

#include 

size_t read_at(FILE *file, long offset, 

void *buf, size_t nbytes) { 

if (fseek(file, offset, SEEK_SET) != 0) { 

/* Indicate error to caller */ 

return 0; 

} 

return fread(buf, 1, nbytes, file); 

} 

13.3.9 Noncompliant Code Example (snprintf()) 

In this noncompliant code example, snprintf() is assumed to succeed. However, if the call 

fails (for example, because of insufficient memory, as described in GNU libc bug 441945), the 

subsequent call to log_message() has undefined behavior because the character buffer is uninitialized 

and need not be null-terminated. 

#include 

extern void log_message(const char *); 

void f(int i, int width, int prec) { 

char buf[40]; 

snprintf(buf, sizeof(buf), "i = %*.*i", width, prec, i); 

log_message(buf); 

/* ... */ 

} 

13.3.10 Compliant Solution (snprintf()) 

This compliant solution does not assume that snprintf() will succeed regardless of its arguments. 

It tests the return value of snprintf() before subsequently using the formatted buffer. 

This compliant solution also treats the case where the static buffer is not large enough for 

snprintf() to append the terminating null character as an error. 

#include 

#include 







} 

char buf[40]; 

int n; 

n = snprintf(buf, sizeof(buf), "i = %*.*i", width, prec, i); 

if (n < 0 || n >= sizeof(buf)) { 

/* Handle snprintf() error */ 

strcpy(buf, "unknown error"); 

} 


13.3.11 Compliant Solution (snprintf(null)) 

If unknown, the length of the formatted string can be discovered by invoking snprintf() with a 

null buffer pointer to determine the size required for the output, then dynamically allocating a 

buffer of sufficient size, and finally calling snprintf() again to format the output into the dynamically 

allocated buffer. Even with this approach, the success of all calls still needs to be tested, 

and any errors must be appropriately handled. A possible optimization is to first attempt to format 

the string into a reasonably small buffer allocated on the stack and, only when the buffer turns out 

to be too small, dynamically allocate one of a sufficient size: 

#include 

#include 

#include 




char *buf = buffer; 

int n = sizeof(buffer); 

const char fmt[] = "i = %*.*i"; 

n = snprintf(buf, n, fmt, width, prec, i); 

if (n < 0) { 


strcpy(buffer, "unknown error"); 

goto write_log; 

} 

if (n < sizeof(buffer)) { 


} 

buf = (char *)malloc(n + 1); 

if (NULL == buf) { 




} 





n = snprintf(buf, n, fmt, width, prec, i); 

if (n < 0) { 



} 

write_log: 


} 

if (buf != buffer) { 

free(buf); 

} 

This solution uses the goto statement, as suggested in MEM12-C. Consider using a goto chain 

when leaving a function on error when using and releasing resources. 


ERR33-C-EX1: It is acceptable to ignore the return value of a function that cannot fail, or a function 

whose return value is inconsequential, or when an error condition need not be diagnosed. The 

function’s results should be explicitly cast to void to signify programmer intent. Return values 

from the functions in the following table do not need to be checked because their historical use 

has overwhelmingly omitted error checking and the consequences are not relevant to security. 

Functions for Which Return Values Need Not Be Checked 


putchar() Character written EOF 

putwchar() Wide character written WEOF 

puts() Nonnegative EOF (negative) 

printf(), vprintf() Number of characters Negative 

(nonnegative) 

wprintf(), vwprintf() Number of wide characters Negative 

(nonnegative) 

kill_dependency() The input parameter NA 

memcpy(), wmemcpy() The destination input parameter 

NA 

memmove(), wmemmove() The destination input parameter 

NA 

strcpy(), wcscpy() The destination input parameter 

NA 






strncpy(), wcsncpy() 

strcat(), wcscat() 

strncat(), wcsncat() 

memset(), wmemset() 

The destination input parameter 




NA 

NA 

NA 

NA 


Failing to detect error conditions can lead to unpredictable results, including abnormal program 

termination and denial-of-service attacks or, in some situations, could even allow an attacker to 

run arbitrary code. 


ERR33-C High Likely Medium P18 L1 


The vulnerability in Adobe Flash [VU#159523] arises because Flash neglects to check the return 

value from calloc(). Even when calloc() returns a null pointer, Flash writes to an offset 

from the return value. Dereferencing a null pointer usually results in a program crash, but dereferencing 

an offset from a null pointer allows an exploit to succeed without crashing the program. 




ISO/IEC TS 17961:2013 

ERR00-C. Adopt and implement a consistent 

and comprehensive error-handling policy 

EXP34-C. Do not dereference null pointers 

FIO13-C. Never push back anything other than 

one read character 

MEM04-C. Do not perform zero-length allocations 

MEM12-C. Consider using a goto chain when 

leaving a function on error when using and releasing 

resources 

ERR10-CPP. Check for error conditions 

FIO04-CPP. Detect and handle input and output 

errors 

Failing to detect and handle standard library errors 

[liberr] 





MITRE CWE 

CWE-252, Unchecked Return Value 

CWE-253, Incorrect Check of Function Return 

Value 

CWE-390, Detection of Error Condition without 

Action 

CWE-391, Unchecked Error Condition 

CWE-476, NULL Pointer Dereference 


[DHS 2006] 

[Henricson 1997] 

[ISO/IEC 9899:2011] 

[VU#159523] 

Handle All Errors Safely 

Recommendation 12.1, “Check for All Errors 

Reported from Functions” 

Subclause 7.21.7.10, “The ungetc Function” 




Concurrency (CON) - CON30-C. Clean up thread-specific storage 

14 Concurrency (CON) 

14.1 CON30-C. Clean up thread-specific storage 

The tss_create() function creates a thread-specific storage pointer identified by a key. 

Threads can allocate thread-specific storage and associate the storage with a key that uniquely 

identifies the storage by calling the tss_set() function. If not properly freed, this memory may 

be leaked. Ensure that thread-specific storage is freed. 


In this noncompliant code example, each thread dynamically allocates storage in the get_data() 

function, which is then associated with the global key by the call to tss_set() in the 

add_data() function. This memory is subsequently leaked when the threads terminate. 

#include 

#include 

/* Global key to the thread-specific storage */ 

tss_t key; 

enum { MAX_THREADS = 3 }; 

int *get_data(void) { 

int *arr = (int *)malloc(2 * sizeof(int)); 

if (arr == NULL) { 

return arr; /* Report error */ 

} 

arr[0] = 10; 

arr[1] = 42; 

return arr; 

} 

int add_data(void) { 

int *data = get_data(); 

if (data == NULL) { 

return -1; /* Report error */ 

} 

} 

if (thrd_success != tss_set(key, (void *)data)) { 


} 

return 0; 

void print_data(void) { 

/* Get this thread's global data from key */ 

int *data = tss_get(key); 





} 

if (data != NULL) { 

/* Print data */ 

} 

int function(void *dummy) { 

if (add_data() != 0) { 


} 

print_data(); 

return 0; 

} 


thrd_t thread_id[MAX_THREADS]; 

/* Create the key before creating the threads */ 

if (thrd_success != tss_create(&key, NULL)) { 


} 

{ 

/* Create threads that would store specific storage */ 

for (size_t i = 0; i < MAX_THREADS; i++) { 

if (thrd_success != thrd_create(&thread_id[i], function, NULL)) 

} 

} 



if (thrd_success != thrd_join(thread_id[i], NULL)) { 


} 

} 

} 

tss_delete(key); 

return 0; 


In this compliant solution, each thread explicitly frees the thread-specific storage returned by the 

tss_get() function before terminating: 

#include 

#include 


tss_t key; 





int function(void *dummy) { 

if (add_data() != 0) { 


} 

print_data(); 

free(tss_get(key)); 

return 0; 

} 

/* ... Other functions are unchanged */ 


This compliant solution invokes a destructor function registered during the call to tss_create() 

to automatically free any thread-specific storage: 

#include 

#include 


tss_t key; 

enum { MAX_THREADS = 3 }; 

/* ... Other functions are unchanged */ 

void destructor(void *data) { 

free(data); 

} 


thrd_t thread_id[MAX_THREADS]; 

/* Create the key before creating the threads */ 

if (thrd_success != tss_create(&key, destructor)) { 


} 

{ 

/* Create threads that would store specific storage */ 


if (thrd_success != thrd_create(&thread_id[i], function, NULL)) 

} 

} 



if (thrd_success != thrd_join(thread_id[i], NULL)) { 


} 





} 

} 


return 0; 


Failing to free thread-specific objects results in memory leaks and could result in a denial-of-service 

attack. 


CON30-C Medium Unlikely Medium P4 L3 




Concurrency (CON) - CON31-C. Do not destroy a mutex while it is locked 

14.2 CON31-C. Do not destroy a mutex while it is locked 

Mutexes are used to protect shared data structures being concurrently accessed. If a mutex is destroyed 

while a thread is blocked waiting for that mutex, critical sections and shared data are no 

longer protected. 


The mtx_destroy function releases any resources used by the mutex pointed to by 

mtx. No threads can be blocked waiting for the mutex pointed to by mtx. 

This statement implies that destroying a mutex while a thread is waiting on it is undefined behavior. 


This noncompliant code example creates several threads that each invoke the do_work() function, 

passing a unique number as an ID. The do_work() function initializes the lock mutex if 

the argument is 0 and destroys the mutex if the argument is max_threads - 1. In all other 

cases, the do_work() function provides normal processing. Each thread, except the final cleanup 

thread, increments the atomic completed variable when it is finished. 

Unfortunately, this code contains several race conditions, allowing the mutex to be destroyed before 

it is unlocked. Additionally, there is no guarantee that lock will be initialized before it is 

passed to mtx_lock(). Each of these behaviors is undefined. 

#include 

#include 

#include 

mtx_t lock; 

/* Atomic so multiple threads can modify safely */ 

atomic_int completed = ATOMIC_VAR_INIT(0); 

enum { max_threads = 5 }; 

int do_work(void *arg) { 

int *i = (int *)arg; 

if (*i == 0) { /* Creation thread */ 

if (thrd_success != mtx_init(&lock, mtx_plain)) { 


} 

atomic_store(&completed, 1); 

} else if (*i < max_threads - 1) { /* Worker thread */ 

if (thrd_success != mtx_lock(&lock)) { 


} 

/* Access data protected by the lock */ 





} 

atomic_fetch_add(&completed, 1); 

if (thrd_success != mtx_unlock(&lock)) { 


} 

} else { /* Destruction thread */ 

mtx_destroy(&lock); 

} 

return 0; 


thrd_t threads[max_threads]; 

} 

for (size_t i = 0; i < max_threads; i++) { 

if (thrd_success != thrd_create(&threads[i], do_work, &i)) { 


} 

} 


if (thrd_success != thrd_join(threads[i], 0)) { 


} 

} 

return 0; 


This compliant solution eliminates the race conditions by initializing the mutex in main() before 

creating the threads and by destroying the mutex in main() after joining the threads: 

#include 

#include 

#include 

mtx_t lock; 

/* Atomic so multiple threads can increment safely */ 

atomic_int completed = ATOMIC_VAR_INIT(0); 

enum { max_threads = 5 }; 

int do_work(void *dummy) { 



} 

/* Access data protected by the lock */ 

atomic_fetch_add(&completed, 1); 



} 





} 

return 0; 


thrd_t threads[max_threads]; 

if (thrd_success != mtx_init(&lock, mtx_plain)) { 


} 


if (thrd_success != thrd_create(&threads[i], do_work, NULL)) { 


} 

} 


if (thrd_success != thrd_join(threads[i], 0)) { 


} 

} 

} 

mtx_destroy(&lock); 

return 0; 


Destroying a mutex while it is locked may result in invalid control flow and data corruption. 


CON31-C Medium Probable High P4 L3 


MITRE CWE 

CWE-667, Improper Locking 


[ISO/IEC 9899:2011] 

7.26.4.1, “The mtx_destroy Function” 




Concurrency (CON) - CON32-C. Prevent data races when accessing bit-fields from multiple threads 

14.3 CON32-C. Prevent data races when accessing bit-fields from 

multiple threads 

When accessing a bit-field, a thread may inadvertently access a separate bit-field in adjacent 

memory. This is because compilers are required to store multiple adjacent bit-fields in one storage 

unit whenever they fit. Consequently, data races may exist not just on a bit-field accessed by multiple 

threads but also on other bit-fields sharing the same byte or word. A similar problem is discussed 

in CON00-C. Avoid race conditions with multiple threads, but the issue described by this 

rule can be harder to diagnose because it may not be obvious that the same memory location is being 

modified by multiple threads. 

One approach for preventing data races in concurrent programming is to use a mutex. When 

properly observed by all threads, a mutex can provide safe and secure access to a shared object. 

However, mutexes provide no guarantees with regard to other objects that might be accessed 

when the mutex is not controlled by the accessing thread. Unfortunately, there is no portable way 

to determine which adjacent bit-fields may be stored along with the desired bit-field. 

Another approach is to insert a non-bit-field member between any two bit-fields to ensure that 

each bit-field is the only one accessed within its storage unit. This technique effectively guarantees 

that no two bit-fields are accessed simultaneously. 

14.3.1 Noncompliant Code Example (Bit-field) 

Adjacent bit-fields may be stored in a single memory location. Consequently, modifying adjacent 

bit-fields in different threads is undefined behavior, as shown in this noncompliant code example: 

struct multi_threaded_flags { 

unsigned int flag1 : 2; 

unsigned int flag2 : 2; 

}; 

struct multi_threaded_flags flags; 

int thread1(void *arg) { 

flags.flag1 = 1; 

return 0; 

} 



return 0; 

} 





The C Standard, 3.14, paragraph 3 [ISO/IEC 9899:2011], states 

NOTE 2 A bit-field and an adjacent non-bit-field member are in separate memory locations. 

The same applies to two bit-fields, if one is declared inside a nested structure declaration 

and the other is not, or if the two are separated by a zero-length bit-field declaration, 

or if they are separated by a non-bit-field member declaration. It is not safe to 

concurrently update two non-atomic bit-fields in the same structure if all members declared 

between them are also (non-zero-length) bit-fields, no matter what the sizes of 

those intervening bit-fields happen to be. 

For example, the following instruction sequence is possible: 

Thread 1: register 0 = flags 

Thread 1: register 0 &= ~mask(flag1) 

Thread 2: register 0 = flags 

Thread 2: register 0 &= ~mask(flag2) 

Thread 1: register 0 |= 1


} 

return 0; 


if (thrd_success != mtx_lock(&flags.mutex)) { 


} 

flags.s.flag2 = 2; 

if (thrd_success != mtx_unlock(&flags.mutex)) { 


} 

return 0; 

} 

14.3.3 Compliant Solution (C11) 

In this compliant solution, two threads simultaneously modify two distinct non-bit-field members 

of a structure. Because the members occupy different bytes in memory, no concurrency protection 

is required. 

struct multi_threaded_flags { 

unsigned char flag1; 

unsigned char flag2; 

}; 

struct multi_threaded_flags flags; 



return 0; 

} 



return 0; 

} 

Unlike C99, C11 explicitly defines a memory location and provides the following note in subclause 

3.14.2 [ISO/IEC 9899:2011]: 

NOTE 1 Two threads of execution can update and access separate memory locations 

without interfering with each other. 

It is almost certain that flag1 and flag2 are stored in the same word. Using a compiler that conforms 

to C99 or earlier, if both assignments occur on a thread-scheduling interleaving that ends 

with both stores occurring after one another, it is possible that only one of the flags will be set as 

intended. The other flag will contain its previous value because both members are represented by 

the same word, which is the smallest unit the processor can work on. Before the changes were 





made to the C Standard for C11, there were no guarantees that these flags could be modified concurrently. 


Although the race window is narrow, an assignment or an expression can evaluate improperly because 

of misinterpreted data resulting in a corrupted running state or unintended information disclosure. 


CON32-C Medium Probable Medium P8 L2 


[ISO/IEC 9899:2011] 

3.14, “Memory Location” 




Concurrency (CON) - CON33-C. Avoid race conditions when using library functions 

14.4 CON33-C. Avoid race conditions when using library functions 

Some C standard library functions are not guaranteed to be reentrant with respect to threads. 

Functions such as strtok() and asctime() return a pointer to the result stored in function-allocated 

memory on a per-process basis. Other functions such as rand() store state information in 

function-allocated memory on a per-process basis. Multiple threads invoking the same function 

can cause concurrency problems, which often result in abnormal behavior and can cause more serious 

vulnerabilities, such as abnormal termination, denial-of-service attack, and data integrity violations. 

According to the C Standard, the library functions listed in the following table may contain data 

races when invoked by multiple threads. 

Functions 

rand(), srand() 

getenv(), getenv_s() 

strtok() 

strerror() 

asctime(), ctime(), 

localtime(), gmtime() 

setlocale() 

ATOMIC_VAR_INIT, atomic_init() 

tmpnam() 

mbrtoc16(), c16rtomb(), 

mbrtoc32(), c32rtomb() 

Remediation 

MSC30-C. Do not use the rand() function for 

generating pseudorandom numbers 

ENV34-C. Do not store pointers returned by 

certain functions 

strtok_s() in C11 Annex K 

strtok_r() in POSIX 

strerror_s() in C11 Annex K 

strerror_r() in POSIX 

asctime_s(), ctime_s(), localtime_s(), 

gmtime_s() in C11 Annex K 

Protect multithreaded access to locale-specific 

functions with a mutex 

Do not attempt to initialize an atomic variable 

from multiple threads 

tmpnam_s() in C11 Annex K 

tmpnam_r() in POSIX 

Do not call with a null mbstate_t * argument 

Section 2.9.1 of the Portable Operating System Interface (POSIX®), Base Specifications, Issue 7 

[IEEE Std 1003.1:2013] extends the list of functions that are not required to be thread-safe. 


In this noncompliant code example, the function f() is called from within a multithreaded application 

but encounters an error while calling a system function. The strerror() function returns 

a human-readable error string given an error number. The C Standard, 7.24.6.2 [ISO/IEC 





9899:2011], specifically states that strerror() is not required to avoid data races. An implementation 

could write the error string into a static array and return a pointer to it, and that array 

might be accessible and modifiable by other threads. 

#include 

#include 

#include 

void f(FILE *fp) { 

fpos_t pos; 

errno = 0; 

} 

if (0 != fgetpos(fp, &pos)) { 

char *errmsg = strerror(errno); 

printf("Could not get the file position: %s\n", errmsg); 

} 

This code first sets errno to 0 to comply with ERR30-C. Set errno to zero before calling a library 

function known to set errno, and check errno only after the function returns a value indicating failure. 

14.4.2 Compliant Solution (Annex K, strerror_s()) 

This compliant solution uses the strerror_s() function from Annex K of the C Standard, 

which has the same functionality as strerror() but guarantees thread-safety: 


#include 

#include 

#include 



fpos_t pos; 

errno = 0; 

} 


char errmsg[BUFFERSIZE]; 

if (strerror_s(errmsg, BUFFERSIZE, errno) != 0) { 


} 


} 

Because Annex K is optional, strerror_s() may not be available in all implementations. 





14.4.3 Compliant Solution (POSIX,strerror_r()) 

This compliant solution uses the POSIX strerror_r() function, which has the same functionality 

as strerror() but guarantees thread safety: 

#include 

#include 

#include 



fpos_t pos; 

errno = 0; 

} 


char errmsg[BUFFERSIZE]; 

if (strerror_r(errno, errmsg, BUFFERSIZE) != 0) { 


} 


} 

Linux provides two versions of strerror_r(), known as the XSI-compliant version and the 

GNU-specific version. This compliant solution assumes the XSI-compliant version, which is the 

default when an application is compiled as required by POSIX (that is, by defining 

_POSIX_C_SOURCE or _XOPEN_SOURCE appropriately). The strerror_r() manual page lists 

versions that are available on a particular system. 


Race conditions caused by multiple threads invoking the same library function can lead to abnormal 

termination of the application, data integrity violations, or a denial-of-service attack. 






ERR30-C. Set errno to zero before calling a library 

function known to set errno, and check 

errno only after the function returns a value indicating 

failure 

CON00-CPP. Avoid assuming functions are 

thread safe unless otherwise specified 






[IEEE Std 1003.1:2013] 

[ISO/IEC 9899:2011] 

Section 2.9.1, “Thread Safety” 

Subclause 7.24.6.2, “The strerror Function” 

[Open Group 1997b] Section 10.12, “Thread-Safe POSIX.1 and C- 

Language Functions” 




Concurrency (CON) - CON34-C. Declare objects shared between threads with appropriate storage durations 

14.5 CON34-C. Declare objects shared between threads with 

appropriate storage durations 

Accessing the automatic or thread-local variables of one thread from another thread is implementation-defined 

behavior and can cause invalid memory accesses because the execution of threads 

can be interwoven within the constraints of the synchronization model. As a result, the referenced 

stack frame or thread-local variable may no longer be valid when another thread tries to access it. 

Shared static variables can be protected by thread synchronization mechanisms. 

However, automatic (local) variables cannot be shared in the same manner because the referenced 

stack frame’s thread would need to stop executing, or some other mechanism must be employed 

to ensure that the referenced stack frame is still valid. Do not access automatic or thread-local objects 

from a thread other than the one with which the object is associated. See DCL30-C. Declare 

objects with appropriate storage durations for information on how to declare objects with appropriate 

storage durations when data is not being shared between threads. 

14.5.1 Noncompliant Code Example (Automatic Storage Duration) 

This noncompliant code example passes the address of a variable to a child thread, which prints it 

out. The variable has automatic storage duration. Depending on the execution order, the child 

thread might reference the variable after the variable’s lifetime in the parent thread. This would 

cause the child thread to access an invalid memory location. 

#include 

#include 

int child_thread(void *val) { 

int *res = (int *)val; 

printf("Result: %d\n", *res); 

return 0; 

} 

void create_thread(thrd_t *tid) { 

int val = 1; 

if (thrd_success != thrd_create(tid, child_thread, &val)) { 


} 

} 


thrd_t tid; 

create_thread(&tid); 

} 

if (thrd_success != thrd_join(tid, NULL)) { 


} 

return 0; 





14.5.2 Noncompliant Code Example (Automatic Storage Duration ) 

One solution is to ensure that all objects with automatic storage duration shared between threads 

are declared such that their lifetime extends past the lifetime of the threads. This can be accomplished 

using a thread synchronization mechanism, such as thrd_join(). In this code example, 

val is declared in main(), where thrd_join() is called. Because the parent thread waits until 

the child thread completes before continuing its execution, the shared objects have a lifetime at 

least as great as the thread. However, this example relies on implementation-defined behavior and 

is nonportable. 

#include 

#include 


int *result = (int *)val; 

printf("Result: %d\n", *result); /* Correctly prints 1 */ 

return 0; 

} 

void create_thread(thrd_t *tid, int *val) { 

if (thrd_success != thrd_create(tid, child_thread, val)) { 


} 

} 


int val = 1; 

thrd_t tid; 

create_thread(&tid, &val); 



} 

return 0; 

} 

14.5.3 Compliant Solution (Static Storage Duration) 

This compliant solution stores the value in an object having static storage duration. The lifetime 

of this object is the entire execution of the program; consequently, it can be safely accessed by 

any thread. 

#include 

#include 

int child_thread(void *v) { 

int *result = (int *)v; 


return 0; 

} 





void create_thread(thrd_t *tid) { 

static int val = 1; 

if (thrd_success != thrd_create(tid, child_thread, &val)) { 


} 

} 


thrd_t tid; 

create_thread(&tid); 



} 

return 0; 

} 

14.5.4 Compliant Solution (Allocated Storage Duration) 

This compliant solution stores the value passed to the child thread in a dynamically allocated object. 

Because this object will persist until explicitly freed, the child thread can safely access its 

value. 

#include 

#include 

#include 


int *result = (int *)val; 


return 0; 

} 

void create_thread(thrd_t *tid, int *value) { 

*value = 1; 

if (thrd_success != thrd_create(tid, child_thread, 

value)) { 


} 

} 


thrd_t tid; 

int *value = (int *)malloc(sizeof(int)); 

if (!value) { 


} 

create_thread(&tid, value); 







} 

} 

free(value); 

return 0; 

14.5.5 Noncompliant Code Example (Thread-Specific Storage) 

In this noncompliant code example, the value is stored in thread-specific storage of the parent 

thread. However, because thread-specific data is available only to the thread that stores it, the 

child_thread() function will set result to a null value. 

#include 

#include 

#include 

static tss_t key; 


void *result = tss_get(*(tss_t *)v); 

printf("Result: %d\n", *(int *)result); 

return 0; 

} 

int create_thread(void *thrd) { 

int *val = (int *)malloc(sizeof(int)); 

if (val == NULL) { 


} 

*val = 1; 

if (thrd_success != tss_set(key, val)) { 


} 

if (thrd_success != thrd_create((thrd_t *)thrd, 

child_thread, &key)) { 


} 

return 0; 

} 


thrd_t parent_tid, child_tid; 

if (thrd_success != tss_create(&key, free)) { 


} 

if (thrd_success != thrd_create(&parent_tid, create_thread, 

&child_tid)) { 


} 

if (thrd_success != thrd_join(parent_tid, NULL)) { 





} 


} 

if (thrd_success != thrd_join(child_tid, NULL)) { 


} 


return 0; 

14.5.6 Compliant Solution (Thread-Specific Storage) 

This compliant solution illustrates how thread-specific storage can be combined with a call to a 

thread synchronization mechanism, such as thrd_join(). Because the parent thread waits until 

the child thread completes before continuing its execution, the child thread is guaranteed to access 

a valid live object. 

#include 

#include 

#include 

static tss_t key; 


int *result = v; 


return 0; 

} 

int create_thread(void *thrd) { 

int *val = (int *)malloc(sizeof(int)); 

if (val == NULL) { 


} 

*val = 1; 

if (thrd_success != tss_set(key, val)) { 


} 

/* ... */ 

void *v = tss_get(key); 

if (thrd_success != thrd_create((thrd_t *)thrd, 

child_thread, v)) { 


} 

return 0; 

} 


thrd_t parent_tid, child_tid; 





if (thrd_success != tss_create(&key, free)) { 


} 

if (thrd_success != thrd_create(&parent_tid, create_thread, 

&child_tid)) { 


} 

if (thrd_success != thrd_join(parent_tid, NULL)) { 


} 

if (thrd_success != thrd_join(child_tid, NULL)) { 


} 


return 0; 

} 

This compliant solution uses pointer-to-integer and integer-to-pointer conversions, which have 

implementation-defined behavior. (See INT36-C. Converting a pointer to integer or integer to 

pointer.) 

14.5.7 Compliant Solution (Thread-Local Storage, Windows, Visual Studio) 

Similar to the preceding compliant solution, this compliant solution uses thread-local storage 

combined with thread synchronization to ensure the child thread is accessing a valid live object. It 

uses the Visual Studio–specific __declspec(thread) language extension to provide the threadlocal 

storage and the WaitForSingleObject() API to provide the synchronization. 

#include 

#include 

DWORD WINAPI child_thread(LPVOID v) { 

int *result = (int *)v; 


return NULL; 

} 

int create_thread(HANDLE *tid) { 

/* Declare val as a thread-local value */ 

__declspec(thread) int val = 1; 

*tid = create_thread(NULL, 0, child_thread, &val, 0, NULL); 

return *tid == NULL; 

} 


HANDLE tid; 

if (create_thread(&tid)) { 






} 

if (WAIT_OBJECT_0 != WaitForSingleObject(tid, INFINITE)) { 


} 

CloseHandle(tid); 

} 

return 0; 

14.5.8 Noncompliant Code Example (OpenMP, parallel) 

It is important to note that local data can be used securely with threads when using other thread 

interfaces, so the programmer need not always copy data into nonlocal memory when sharing data 

with threads. For example, the shared keyword in The OpenMP® API Specification for Parallel 

Programming [OpenMP] can be used in combination with OpenMP’s threading interface to share 

local memory without having to worry about whether local automatic variables remain valid. 

In this noncompliant code example, a variable j is declared outside a parallel#pragma and not 

listed as a private variable. In OpenMP, variables outside a parallel #pragma are shared unless 

designated as private. 

#include 

#include 


int j = 0; 

#pragma omp parallel 

{ 

int t = omp_get_thread_num(); 

printf("Running thread - %d\n", t); 

for (int i = 0; i < 5050; i++) { 

j++; /* j not private; could be a race condition */ 

} 

printf("Just ran thread - %d\n", t); 

printf("loop count %d\n", j); 

} 

return 0; 

} 

14.5.9 Compliant Solution (OpenMP, parallel, private) 

In this compliant solution, the variable j is declared outside of the parallel#pragma but is explicitly 

labeled as private: 

#include 

#include 






int j = 0; 

#pragma omp parallel private(j) 

{ 

int t = omp_get_thread_num(); 

printf("Running thread - %d\n", t); 

for (int i = 0; i < 5050; i++) { 

j++; 

} 

printf("Just ran thread - %d\n", t); 

printf("loop count %d\n", j); 

} 

return 0; 

} 


Threads that reference the stack of other threads can potentially overwrite important information 

on the stack, such as function pointers and return addresses. The compiler may not generate warnings 

if the programmer allows one thread to access another thread’s local variables, so a programmer 

may not catch a potential error at compile time. The remediation cost for this error is high because 

analysis tools have difficulty diagnosing problems with concurrency and race conditions. 





DCL30-C. Declare objects with appropriate 

storage durations 


[ISO/IEC 9899:2011] 

[OpenMP] 


The OpenMP® API Specification for Parallel 

Programming 




Concurrency (CON) - CON35-C. Avoid deadlock by locking in a predefined order 

14.6 CON35-C. Avoid deadlock by locking in a predefined order 

Mutexes are used to prevent multiple threads from causing a data race by accessing shared resources 

at the same time. Sometimes, when locking mutexes, multiple threads hold each other’s 

lock, and the program consequently deadlocks. Four conditions are required for deadlock to occur: 

• Mutual exclusion 

• Hold and wait 

• No preemption 

• Circular wait 

Deadlock needs all four conditions, so preventing deadlock requires preventing any one of the 

four conditions. One simple solution is to lock the mutexes in a predefined order, which prevents 

circular wait. 


The behavior of this noncompliant code example depends on the runtime environment and the 

platform’s scheduler. The program is susceptible to deadlock if thread thr1 attempts to lock 

ba2’s mutex at the same time thread thr2 attempts to lock ba1’s mutex in the deposit() function. 

#include 

#include 


int balance; 

mtx_t balance_mutex; 

} bank_account; 


bank_account *from; 

bank_account *to; 

int amount; 

} transaction; 

void create_bank_account(bank_account **ba, 

int initial_amount) { 

bank_account *nba = (bank_account *)malloc( 

sizeof(bank_account) 

); 

if (nba == NULL) { 


} 

nba->balance = initial_amount; 

if (thrd_success 





} 

!= mtx_init(&nba->balance_mutex, mtx_plain)) { 


} 

*ba = nba; 

int deposit(void *ptr) { 

transaction *args = (transaction *)ptr; 

if (thrd_success != mtx_lock(&args->from->balance_mutex)) { 


} 

/* Not enough balance to transfer */ 

if (args->from->balance < args->amount) { 


!= mtx_unlock(&args->from->balance_mutex)) { 


} 

return -1; /* Indicate error */ 

} 

if (thrd_success != mtx_lock(&args->to->balance_mutex)) { 


} 

args->from->balance -= args->amount; 

args->to->balance += args->amount; 


!= mtx_unlock(&args->from->balance_mutex)) { 


} 


!= mtx_unlock(&args->to->balance_mutex)) { 


} 

} 

free(ptr); 

return 0; 


thrd_t thr1, thr2; 

transaction *arg1; 

transaction *arg2; 

bank_account *ba1; 

bank_account *ba2; 

create_bank_account(&ba1, 1000); 





create_bank_account(&ba2, 1000); 

arg1 = (transaction *)malloc(sizeof(transaction)); 

if (arg1 == NULL) { 


} 

arg2 = (transaction *)malloc(sizeof(transaction)); 

if (arg2 == NULL) { 


} 

arg1->from = ba1; 

arg1->to = ba2; 

arg1->amount = 100; 

arg2->from = ba2; 

arg2->to = ba1; 

arg2->amount = 100; 

} 

/* Perform the deposits */ 


!= thrd_create(&thr1, deposit, (void *)arg1)) { 


} 


!= thrd_create(&thr2, deposit, (void *)arg2)) { 


} 

return 0; 


This compliant solution eliminates the circular wait condition by establishing a predefined order 

for locking in the deposit() function. Each thread will lock on the basis of the bank_account 

ID, which is set when the bank_account struct is initialized. 

#include 

#include 


int balance; 

mtx_t balance_mutex; 

/* Should not change after initialization */ 

unsigned int id; 

} bank_account; 


bank_account *from; 

bank_account *to; 





int amount; 

} transaction; 

unsigned int global_id = 1; 

void create_bank_account(bank_account **ba, 

int initial_amount) { 

bank_account *nba = (bank_account *)malloc( 

sizeof(bank_account) 

); 

if (nba == NULL) { 


} 

nba->balance = initial_amount; 


!= mtx_init(&nba->balance_mutex, mtx_plain)) { 


} 

} 

nba->id = global_id++; 

*ba = nba; 

int deposit(void *ptr) { 

transaction *args = (transaction *)ptr; 

int result = -1; 

mtx_t *first; 

mtx_t *second; 

if (args->from->id == args->to->id) { 

return -1; /* Indicate error */ 

} 

/* Ensure proper ordering for locking */ 

if (args->from->id < args->to->id) { 

first = &args->from->balance_mutex; 

second = &args->to->balance_mutex; 

} else { 

first = &args->to->balance_mutex; 

second = &args->from->balance_mutex; 

} 

if (thrd_success != mtx_lock(first)) { 


} 

if (thrd_success != mtx_lock(second)) { 


} 

/* Not enough balance to transfer */ 

if (args->from->balance >= args->amount) { 





} 

args->from->balance -= args->amount; 

args->to->balance += args->amount; 

result = 0; 

} 

if (thrd_success != mtx_unlock(second)) { 


} 

if (thrd_success != mtx_unlock(first)) { 


} 

free(ptr); 



Deadlock prevents multiple threads from progressing, halting program execution. A denial-of-service 

attack is possible if the attacker can create the conditions for deadlock. 


CON35-C Low Probable Medium P4 L3 


CERT Oracle Secure Coding Standard for Java LCK07-J. Avoid deadlock by requesting and 

releasing locks in the same order 

MITRE CWE 

CWE-764, Multiple Locks of a Critical Resource 




Concurrency (CON) - CON36-C. Wrap functions that can spuriously wake up in a loop 

14.7 CON36-C. Wrap functions that can spuriously wake up in a loop 

The cnd_wait() and cnd_timedwait() functions temporarily cede possession of a mutex so 

that other threads that may be requesting the mutex can proceed. These functions must always be 

called from code that is protected by locking a mutex. The waiting thread resumes execution only 

after it has been notified, generally as the result of the invocation of the cnd_signal() or 

cnd_broadcast() function invoked by another thread. The cnd_wait() function must be invoked 

from a loop that checks whether a condition predicate holds. A condition predicate is an expression 

constructed from the variables of a function that must be true for a thread to be allowed 

to continue execution. The thread pauses execution, via cnd_wait(), cnd_timedwait(), or 

some other mechanism, and is resumed later, presumably when the condition predicate is true and 

the thread is notified. 

#include 

#include 

extern bool until_finish(void); 

extern mtx_t lock; 

extern cnd_t condition; 




} 

while (until_finish()) { /* Predicate does not hold */ 

if (thrd_success != cnd_wait(&condition, &lock)) { 


} 

} 

/* Resume when condition holds */ 

} 



} 

The notification mechanism notifies the waiting thread and allows it to check its condition predicate. 

The invocation of cnd_broadcast() in another thread cannot precisely determine which 

waiting thread will be resumed. Condition predicate statements allow notified threads to determine 

whether they should resume upon receiving the notification. 


This noncompliant code example monitors a linked list and assigns one thread to consume list elements 

when the list is nonempty. 





This thread pauses execution using cnd_wait() and resumes when notified, presumably when 

the list has elements to be consumed. It is possible for the thread to be notified even if the list is 

still empty, perhaps because the notifying thread used cnd_broadcast(), which notifies all 

threads. Notification using cnd_broadcast() is frequently preferred over using cnd_signal(). 

(See CON38-C. Preserve thread safety and liveness when using condition variables for 

more information.) 

A condition predicate is typically the negation of the condition expression in the loop. In this noncompliant 

code example, the condition predicate for removing an element from a linked list is 

(list->next != NULL), whereas the condition expression for the while loop condition is 

(list->next == NULL). 

This noncompliant code example nests the cnd_wait() function inside an if block and consequently 

fails to check the condition predicate after the notification is received. If the notification 

was spurious or malicious, the thread would wake up prematurely. 

#include 

#include 

struct node_t { 

void *node; 

struct node_t *next; 

}; 

struct node_t list; 

static mtx_t lock; 

static cnd_t condition; 

void consume_list_element(void) { 



} 

} 

if (list.next == NULL) { 



} 

} 

/* Proceed when condition holds */ 



} 






This compliant solution calls the cnd_wait() function from within a while loop to check the 

condition both before and after the call to cnd_wait(): 

#include 

#include 

struct node_t { 

void *node; 

struct node_t *next; 

}; 

struct node_t list; 

static mtx_t lock; 

static cnd_t condition; 

void consume_list_element(void) { 



} 

while (list.next == NULL) { 



} 

} 

/* Proceed when condition holds */ 

} 



} 


Failure to enclose calls to the cnd_wait() or cnd_timedwait() functions inside a while loop 

can lead to indefinite blocking and denial of service (DoS). 


CON36-C Low Unlikely Medium P2 L3 


CERT Oracle Secure Coding Standard for Java THI03-J. Always invoke wait() and await() 

methods inside a loop 






[ISO/IEC 9899:2011] 

[Lea 2000] 

7.17.7.4, “The atomic_compare_exchange 

Generic Functions” 

1.3.2, “Liveness” 

3.2.2, “Monitor Mechanics” 




Concurrency (CON) - CON37-C. Do not call signal() in a multithreaded program 

14.8 CON37-C. Do not call signal() in a multithreaded program 

Calling the signal() function in a multithreaded program is undefined behavior. (See undefined 

behavior 135.) 


This noncompliant code example invokes the signal() function from a multithreaded program: 

#include 

#include 

#include 

volatile sig_atomic_t flag = 0; 


flag = 1; 

} 

/* Runs until user sends SIGUSR1 */ 

int func(void *data) { 

while (!flag) { 

/* ... */ 

} 

return 0; 

} 


signal(SIGUSR1, handler); /* Undefined behavior */ 

thrd_t tid; 

} 

if (thrd_success != thrd_create(&tid, func, NULL)) { 


} 

/* ... */ 

return 0; 

NOTE: The SIGUSR1 signal value is not defined in the C Standard; consequently, this is not a C- 

compliant code example. 


This compliant solution uses an object of type atomic_bool to indicate when the child thread 

should terminate its loop: 

#include 

#include 

#include 




Concurrency (CON) - CON37-C. Do not call signal() in a multithreaded program 

#include 

atomic_bool flag = ATOMIC_VAR_INIT(false); 

int func(void *data) { 

while (!flag) { 

/* ... */ 

} 

return 0; 

} 


thrd_t tid; 

if (thrd_success != thrd_create(&tid, func, NULL)) { 


} 

/* ... */ 

/* Set flag when done */ 

flag = true; 

} 

return 0; 


CON37-C-EX1: Implementations such as POSIX that provide defined behavior when multithreaded 

programs use custom signal handlers are exempt from this rule [IEEE Std 1003.1-2013]. 


Mixing signals and threads causes undefined behavior. 


CON37-C Low Probable Low P6 L2 


[IEEE Std 1003.1-2013] 

XSH 2.9.1, “Thread Safety” 




Concurrency (CON) - CON38-C. Preserve thread safety and liveness when using condition variables 

14.9 CON38-C. Preserve thread safety and liveness when using 

condition variables 

Both thread safety and liveness are concerns when using condition variables. The thread-safety 

property requires that all objects maintain consistent states in a multithreaded environment [Lea 

2000]. The liveness property requires that every operation or function invocation execute to completion 

without interruption; for example, there is no deadlock. 

Condition variables must be used inside a while loop. (See CON36-C. Wrap functions that can 

spuriously wake up in a loop for more information.) To guarantee liveness, programs must test the 

while loop condition before invoking the cnd_wait() function. This early test checks whether 

another thread has already satisfied the condition predicate and has sent a notification. Invoking 

the cnd_wait() function after the notification has been sent results in indefinite blocking. 

To guarantee thread safety, programs must test the while loop condition after returning from the 

cnd_wait() function. When a given thread invokes the cnd_wait() function, it will attempt to 

block until its condition variable is signaled by a call to cnd_broadcast() or to cnd_signal(). 

The cnd_signal() function unblocks one of the threads that are blocked on the specified condition 

variable at the time of the call. If multiple threads are waiting on the same condition variable, 

the scheduler can select any of those threads to be awakened (assuming that all threads have the 

same priority level). The cnd_broadcast() function unblocks all of the threads that are blocked 

on the specified condition variable at the time of the call. The order in which threads execute following 

a call to cnd_broadcast() is unspecified. Consequently, an unrelated thread could start 

executing, discover that its condition predicate is satisfied, and resume execution even though it 

was supposed to remain dormant. For these reasons, threads must check the condition predicate 

after the cnd_wait() function returns. A while loop is the best choice for checking the condition 

predicate both before and after invoking cnd_wait(). 

The use of cnd_signal() is safe if each thread uses a unique condition variable. If multiple 

threads share a condition variable, the use of cnd_signal() is safe only if the following conditions 

are met: 

• All threads must perform the same set of operations after waking up, which means that any 

thread can be selected to wake up and resume for a single invocation of cnd_signal(). 

• Only one thread is required to wake upon receiving the signal. 

The cnd_broadcast() function can be used to unblock all of the threads that are blocked on the 

specified condition variable if the use of cnd_signal() is unsafe. 

14.9.1 Noncompliant Code Example (cnd_signal()) 

This noncompliant code example uses five threads that are intended to execute sequentially according 

to the step level assigned to each thread when it is created (serialized processing). The 

current_step variable holds the current step level and is incremented when the respective 





thread completes. Finally, another thread is signaled so that the next step can be executed. Each 

thread waits until its step level is ready, and the cnd_wait() function call is wrapped inside a 

while loop, in compliance with CON36-C. Wrap functions that can spuriously wake up in a loop. 

#include 

#include 

enum { NTHREADS = 5 }; 

mtx_t mutex; 

cnd_t cond; 

int run_step(void *t) { 

static int current_step = 0; 

size_t my_step = *(size_t *)t; 

if (thrd_success != mtx_lock(&mutex)) { 


} 

printf("Thread %zu has the lock\n", my_step); 

while (current_step != my_step) { 

printf("Thread %zu is sleeping...\n", my_step); 

if (thrd_success != cnd_wait(&cond, &mutex)) { 


} 

printf("Thread %zu woke up\n", my_step); 

} 

/* Do processing ... */ 

printf("Thread %zu is processing...\n", my_step); 

current_step++; 

/* Signal awaiting task */ 

if (thrd_success != cnd_signal(&cond)) { 


} 

printf("Thread %zu is exiting...\n", my_step); 

if (thrd_success != mtx_unlock(&mutex)) { 


} 

return 0; 

} 


thrd_t threads[NTHREADS]; 

size_t step[NTHREADS]; 





if (thrd_success != mtx_init(&mutex, mtx_plain)) { 


} 

if (thrd_success != cnd_init(&cond)) { 


} 

/* Create threads */ 

for (size_t i = 0; i < NTHREADS; ++i) { 

step[i] = i; 

} 

if (thrd_success != thrd_create(&threads[i], run_step, 

&step[i])) { 


} 

/* Wait for all threads to complete */ 

for (size_t i = NTHREADS; i != 0; --i) { 

if (thrd_success != thrd_join(threads[i-1], NULL)) { 


} 

} 

} 

mtx_destroy(&mutex); 

cnd_destroy(&cond); 

return 0; 

In this example, all threads share a condition variable. Each thread has its own distinct condition 

predicate because each thread requires current_step to have a different value before proceeding. 

When the condition variable is signaled, any of the waiting threads can wake up. 

The following table illustrates a possible scenario in which the liveness property is violated. If, by 

chance, the notified thread is not the thread with the next step value, that thread will wait again. 

No additional notifications can occur, and eventually the pool of available threads will be exhausted. 

Deadlock: Out-of-Sequence Step Value 

Time Thread # 

(my_step) 

current_step 

Action 

0 3 0 Thread 3 executes first time: predicate is FALSE - 

> wait() 


> wait() 


> wait() 





Time Thread # 

(my_step) 

current_step 

Action 

3 0 0 Thread 0 executes first time: predicate is TRUE -> 

current_step++; cnd_signal() 

4 1 1 Thread 1 executes first time: predicate is TRUE -> 

current_step++; cnd_signal() 

5 3 2 Thread 3 wakes up (scheduler choice): predicate is 

FALSE -> wait() 

6 — — Thread exhaustion! No more threads to run, and a 

conditional variable signal is needed to wake up the 

others 

This noncompliant code example violates the liveness property. 

14.9.2 Compliant Solution (cnd_broadcast()) 

This compliant solution uses the cnd_broadcast() function to signal all waiting threads instead 

of a single random thread. Only the run_step() thread code from the noncompliant code example 

is modified, as follows: 

#include 

#include 

mtx_t mutex; 

cnd_t cond; 


static size_t current_step = 0; 




} 




if (thrd_success != cnd_wait(&cond, &mutex)) { 


} 


} 








/* Signal ALL waiting tasks */ 

if (thrd_success != cnd_broadcast(&cond)) { 


} 


} 



} 

return 0; 

Awakening all threads solves and guarantees the liveness property because each thread will execute 

its condition predicate test, and exactly one will succeed and continue execution. 

14.9.3 Compliant Solution (Using cnd_signal() with a Unique Condition 

Variable per Thread) 

Another compliant solution is to use a unique condition variable for each thread (all associated 

with the same mutex). In this case, cnd_signal() wakes up only the thread that is waiting on it. 

This solution is more efficient than using cnd_broadcast() because only the desired thread is 

awakened. 

The condition predicate of the signaled thread must be true; otherwise, a deadlock will occur. 

#include 

#include 


mtx_t mutex; 

cnd_t cond[NTHREADS]; 






} 




if (thrd_success != cnd_wait(&cond[my_step], &mutex)) { 






} 

} 





/* Signal next step thread */ 

if ((my_step + 1) < NTHREADS) { 

if (thrd_success != cnd_signal(&cond[my_step + 1])) { 


} 

} 


} 



} 

return 0; 


thrd_t threads[NTHREADS]; 

size_t step[NTHREADS]; 

if (thrd_success != mtx_init(&mutex, mtx_plain)) { 


} 

for (size_t i = 0; i< NTHREADS; ++i) { 

if (thrd_success != cnd_init(&cond[i])) { 


} 

} 



step[i] = i; 

if (thrd_success != thrd_create(&threads[i], run_step, 

&step[i])) { 


} 

} 


for (size_t i = NTHREADS; i != 0; --i) { 

if (thrd_success != thrd_join(threads[i-1], NULL)) { 





} 

} 


mtx_destroy(&mutex); 

} 


cnd_destroy(&cond[i]); 

} 

return 0; 

14.9.4 Compliant Solution (Windows, Condition Variables) 

This compliant solution uses a CONDITION_VARIABLE object, available on Microsoft Windows 

(Vista and later): 

#include 

#include 

CRITICAL_SECTION lock; 

CONDITION_VARIABLE cond; 

DWORD WINAPI run_step(LPVOID t) { 


size_t my_step = (size_t)t; 

EnterCriticalSection(&lock); 




if (!SleepConditionVariableCS(&cond, &lock, INFINITE)) { 


} 

} 





LeaveCriticalSection(&lock); 

/* Signal ALL waiting tasks */ 

WakeAllConditionVariable(&cond); 





} 


return 0; 



HANDLE threads[NTHREADS]; 

InitializeCriticalSection(&lock); 

InitializeConditionVariable(&cond); 



threads[i] = CreateThread(NULL, 0, run_step, (LPVOID)i, 0, 

NULL); 

} 


WaitForMultipleObjects(NTHREADS, threads, TRUE, INFINITE); 

DeleteCriticalSection(&lock); 

} 

return 0; 


Failing to preserve the thread safety and liveness of a program when using condition variables can 

lead to indefinite blocking and denial of service (DoS). 




CERT Oracle Secure Coding Standard for Java THI02-J. Notify all waiting threads rather than 

a single thread 


[IEEE Std 1003.1:2013] 

[Lea 2000] 

XSH, System Interfaces, 

pthread_cond_broadcast 

XSH, System Interfaces, 

pthread_cond_signal 




Concurrency (CON) - CON39-C. Do not join or detach a thread that was previously joined or detached 

14.10 CON39-C. Do not join or detach a thread that was previously 

joined or detached 

The C Standard, 7.26.5.6 [ISO/IEC 9899:2011], states that a thread shall not be joined once it was 

previously joined or detached. Similarly, subclause 7.26.5.3 states that a thread shall not be detached 

once it was previously joined or detached. Violating either of these subclauses results in 

undefined behavior. 


This noncompliant code example detaches a thread that is later joined. 

#include 

#include 

int thread_func(void *arg) { 

/* Do work */ 

thrd_detach(thrd_current()); 

return 0; 

} 


thrd_t t; 

if (thrd_success != thrd_create(&t, thread_func, NULL)) { 


return 0; 

} 

} 

if (thrd_success != thrd_join(t, 0)) { 


return 0; 

} 

return 0; 


This compliant solution does not detach the thread. Its resources are released upon successfully 

joining with the main thread: 

#include 

#include 

int thread_func(void *arg) { 

/* Do work */ 

return 0; 

} 




Concurrency (CON) - CON39-C. Do not join or detach a thread that was previously joined or detached 


thrd_t t; 

if (thrd_success != thrd_create(&t, thread_func, NULL)) { 


return 0; 

} 

} 

if (thrd_success != thrd_join(t, 0)) { 


return 0; 

} 

return 0; 


Joining or detaching a previously joined or detached thread is undefined behavior. 


CON39-C Low Likely Medium P6 L2 


[ISO/IEC 9899:2011] 

Subclause 7.26.5.3, “The thrd_detach Function” 

Subclause 7.26.5.6, “The thrd_join Function” 




Concurrency (CON) - CON40-C. Do not refer to an atomic variable twice in an expression 

14.11 CON40-C. Do not refer to an atomic variable twice in an 

expression 

A consistent locking policy guarantees that multiple threads cannot simultaneously access or 

modify shared data. Atomic variables eliminate the need for locks by guaranteeing thread safety 

when certain operations are performed on them. The thread-safe operations on atomic variables 

are specified in the C Standard, subclauses 7.17.7 and 7.17.8 [ISO/IEC 9899:2011]. While atomic 

operations can be combined, combined operations do not provide the thread safety provided by 

individual atomic operations. 

Every time an atomic variable appears on the left side of an assignment operator, including a compound 

assignment operator such as *=, an atomic write is performed on the variable. The use of 

the increment (++) or decrement (--) operators on an atomic variable constitutes an atomic 

read-and-write operation and is consequently thread-safe. Any reference of an atomic variable anywhere 

else in an expression indicates a distinct atomic read on the variable. 

If the same atomic variable appears twice in an expression, then two atomic reads, or an atomic 

read and an atomic write, are required. Such a pair of atomic operations is not thread-safe, as another 

thread can modify the atomic variable between the two operations. Consequently, an atomic 

variable must not be referenced twice in the same expression. 

14.11.1 Noncompliant Code Example (atomic_bool) 

This noncompliant code example declares a shared atomic_boolflag variable and provides a 

toggle_flag() method that negates the current value of flag: 

#include 

#include 

static atomic_bool flag = ATOMIC_VAR_INIT(false); 

void init_flag(void) { 

atomic_init(&flag, false); 

} 

void toggle_flag(void) { 

bool temp_flag = atomic_load(&flag); 

temp_flag = !temp_flag; 

atomic_store(&flag, temp_flag); 

} 

bool get_flag(void) { 

return atomic_load(&flag); 

} 

Execution of this code may result in a data race because the value of flag is read, negated, and 

written back. This occurs even though the read and write are both atomic. 





Consider, for example, two threads that call toggle_flag(). The expected effect of toggling 

flag twice is that it is restored to its original value. However, the scenario in the following table 

leaves flag in the incorrect state. 

toggle_flag() without Compare-and-Exchange 

Time flag Thread Action 

1 true t1 Reads the current value 

of flag, true, into a 

cache 

2 true t2 Reads the current value 

of flag, (still) true, 

into a different cache 

3 true t1 Toggles the temporary 

variable in the cache to 

false 

4 true t2 Toggles the temporary 

variable in the different 

cache to false 

5 false t1 Writes the cache variable’s 

value to flag 

6 false t2 Writes the different cache 

variable’s value to flag 

As a result, the effect of the call by t2 is not reflected in flag; the program behaves as if toggle_flag() 

was called only once, not twice. 

14.11.2 Compliant Solution (atomic_compare_exchange_weak()) 

This compliant solution uses a compare-and-exchange to guarantee that the correct value is stored 

in flag. All updates are visible to other threads. The call to atomic_compare_exchange_weak() 

is in a loop in conformance with CON41-C. Wrap functions that can fail spuriously 

in a loop. 

#include 

#include 


void init_flag(void) { 

atomic_init(&flag, false); 

} 


bool old_flag = atomic_load(&flag); 

bool new_flag; 

do { 

new_flag = !old_flag; 

} while (!atomic_compare_exchange_weak(&flag, &old_flag, 





new_flag)); 

} 


return atomic_load(&flag); 

} 

An alternative solution is to use the atomic_flag data type for managing Boolean values atomically. 

However, atomic_flag does not support a toggle operation. 

14.11.3 Compliant Solution (Compound Assignment) 

This compliant solution uses the ^= assignment operation to toggle flag. This operation is guaranteed 

to be atomic, according to the C Standard, 6.5.16.2, paragraph 3. This operation performs a 

bitwise-exclusive-or between its arguments, but for Boolean arguments, this is equivalent to negation. 

#include 

#include 



flag ^= 1; 

} 


return flag; 

} 

An alternative solution is to use a mutex to protect the atomic operation, but this solution loses the 

performance benefits of atomic variables. 


This noncompliant code example takes an atomic global variable n and computes n + (n - 1) 

+ (n - 2) + ... + 1, using the formula n * (n + 1) / 2: 

#include 

atomic_int n = ATOMIC_VAR_INIT(0); 

int compute_sum(void) { 

return n * (n + 1) / 2; 

} 

The value of n may change between the two atomic reads of n in the expression, yielding an incorrect 

result. 






This compliant solution passes the atomic variable as a function parameter, forcing the variable to 

be copied and guaranteeing a correct result: 

#include 

int compute_sum(atomic_int n) { 

return n * (n + 1) / 2; 

} 


When operations on atomic variables are assumed to be atomic, but are not atomic, surprising data 

races can occur, leading to corrupted data and invalid control flow. 


CON40-C Medium Probable Medium P8 L2 


MITRE CWE 

CWE-366, Race Condition within a Thread 

CWE-413, Improper Resource Locking 

CWE-567, Unsynchronized Access to Shared 

Data in a Multithreaded Context 

CWE-667, Improper Locking 


[ISO/IEC 9899:2011] 

6.5.16.2, “Compound Assignment” 

7.17, “Atomics” 




Concurrency (CON) - CON41-C. Wrap functions that can fail spuriously in a loop 

14.12 CON41-C. Wrap functions that can fail spuriously in a loop 

Functions that can fail spuriously should be wrapped in a loop. The atomic_compare_exchange_weak() 

and atomic_compare_exchange_weak_explicit() functions both attempt 

to set an atomic variable to a new value but only if it currently possesses a known old value. Unlike 

the related functions atomic_compare_exchange_strong() and atomic_compare_exchange_strong_explicit(), 

these functions are permitted to fail spuriously. This makes 

these functions faster on some platforms—for example, on architectures that implement compareand-exchange 

using load-linked/store-conditional instructions, such as Alpha, ARM, MIPS, and 

PowerPC. The C Standard, 7.17.7.4, paragraph 4 [ISO/IEC 9899:2011], describes this behavior: 

A weak compare-and-exchange operation may fail spuriously. That is, even when the 

contents of memory referred to by expected and object are equal, it may return zero 

and store back to expected the same memory contents that were originally there. 


In this noncompliant code example, reorganize_data_structure() is to be used as an argument 

to thrd_create(). After reorganizing, the function attempts to replace the head pointer 

so that it points to the new version. If no other thread has changed the head pointer since it was 

originally loaded, reorganize_data_structure() is intended to exit the thread with a result 

of true, indicating success. Otherwise, the new reorganization attempt is discarded and the 

thread is exited with a result of false. However, atomic_compare_exchange_weak() may 

fail even when the head pointer has not changed. Therefore, reorganize_data_structure() 

may perform the work and then discard it unnecessarily. 

#include 

#include 

struct data { 

struct data *next; 

/* ... */ 

}; 

extern void cleanup_data_structure(struct data *head); 

int reorganize_data_structure(void *thread_arg) { 

struct data *_Atomic *ptr_to_head = thread_arg; 

struct data *old_head = atomic_load(ptr_to_head); 

struct data *new_head; 

bool success; 

/* ... Reorganize the data structure ... */ 

success = atomic_compare_exchange_weak(ptr_to_head, 

&old_head, new_head); 

if (!success) { 

cleanup_data_structure(new_head); 





} 

} 

return success; /* Exit the thread */ 

14.12.2 Compliant Solution (atomic_compare_exchange_weak()) 

To recover from spurious failures, a loop must be used. However, atomic_compare_exchange_weak() 

might fail because the head pointer changed, or the failure may be spurious. In 

either case, the thread must perform the work repeatedly until the compare-and-exchange succeeds, 

as shown in this compliant solution: 

#include 

#include 

#include 

struct data { 


/* ... */ 

}; 





struct data *new_head = NULL; 

struct data *saved_old_head; 

bool success; 

do { 

if (new_head != NULL) { 


} 

saved_old_head = old_head; 


} 

} while (!(success = atomic_compare_exchange_weak( 

ptr_to_head, &old_head, new_head 

)) && old_head == saved_old_head); 


This loop could also be part of a larger control flow; for example, the thread from the noncompliant 

code example could be retried if it returns false. 





14.12.3 Compliant Solution (atomic_compare_exchange_strong()) 

When a weak compare-and-exchange would require a loop and a strong one would not, the strong 

one is preferable, as in this compliant solution: 

#include 

#include 

struct data { 


/* ... */ 

}; 





struct data *new_head; 

bool success; 


} 

success = atomic_compare_exchange_strong( 

ptr_to_head, &old_head, new_head 

); 

if (!success) { 


} 



Failing to wrap the atomic_compare_exchange_weak() and atomic_compare_exchange_weak_explicit() 

functions in a loop can result in incorrect values and control flow. 




CERT Oracle Secure Coding Standard for Java THI03-J. Always invoke wait() and await() 

methods inside a loop 


[ISO/IEC 9899:2011] 

7.17.7.4, “The atomic_compare_exchange 

Generic Functions” 





[Lea 2000] 

1.3.2, “Liveness” 

3.2.2, “Monitor Mechanics” 




Miscellaneous (MSC) - MSC30-C. Do not use the rand() function for generating pseudorandom numbers 

15 Miscellaneous (MSC) 

15.1 MSC30-C. Do not use the rand() function for generating 

pseudorandom numbers 

Pseudorandom number generators use mathematical algorithms to produce a sequence of numbers 

with good statistical properties, but the numbers produced are not genuinely random. 

The C Standard rand() function makes no guarantees as to the quality of the random sequence 

produced. The numbers generated by some implementations of rand() have a comparatively 

short cycle and the numbers can be predictable. Applications that have strong pseudorandom 

number requirements must use a generator that is known to be sufficient for their needs. 


The following noncompliant code generates an ID with a numeric part produced by calling the 

rand() function. The IDs produced are predictable and have limited randomness. 

#include 

#include 

enum { len = 12 }; 


/* 

* id will hold the ID, starting with the characters 

* "ID" followed by a random integer. 

*/ 

char id[len]; 

int r; 

int num; 

/* ... */ 

r = rand(); /* Generate a random integer */ 

num = snprintf(id, len, "ID%-d", r); /* Generate the ID */ 

/* ... */ 

} 


This compliant solution replaces the rand() function with the POSIX random() function: 

#include 

#include 

#include 

enum { len = 12 }; 






/* 

* id will hold the ID, starting with the characters 

* "ID" followed by a random integer. 

*/ 

char id[len]; 

int r; 

int num; 

/* ... */ 

struct timespec ts; 

if (timespec_get(&ts, TIME_UTC) == 0) { 


} 

srandom(ts.tv_nsec ^ ts.tv_sec); /* Seed the PRNG */ 

/* ... */ 

r = random(); /* Generate a random integer */ 

num = snprintf(id, len, "ID%-d", r); /* Generate the ID */ 

/* ... */ 

} 

The POSIX random() function is a better pseudorandom number generator. Although on some 

platforms the low dozen bits generated by rand() go through a cyclic pattern, all the bits generated 

by random() are usable. The rand48 family of functions provides another alternative for 

pseudorandom numbers. 

Although not specified by POSIX, arc4random() is another possibility for systems that support 

it. The arc4random(3) manual page [OpenBSD] states 

... provides higher quality of data than those described in rand(3), random(3), and 

drand48(3). 

To achieve the best random numbers possible, an implementation-specific function must be used. 

When unpredictability is crucial and speed is not an issue, as in the creation of strong cryptographic 

keys, use a true entropy source, such as /dev/random, or a hardware device capable of 

generating random numbers. The /dev/random device can block for a long time if there are not 

enough events going on to generate sufficient entropy. 


On Windows platforms, the CryptGenRandom() function can be used to generate cryptographically 

strong random numbers. The exact details of the implementation are unknown, including, 

for example, what source of entropy CryptGenRandom() uses. The Microsoft Developer Network 

CryptGenRandom() reference [MSDN] states 





If an application has access to a good random source, it can fill the pbBuffer buffer 

with some random data before calling CryptGenRandom(). The CSP [cryptographic service 

provider] then uses this data to further randomize its internal seed. It is acceptable 

to omit the step of initializing the pbBuffer buffer before calling CryptGenRandom(). 

#include 

#include 

#include 


HCRYPTPROV prov; 

if (CryptAcquireContext(&prov, NULL, NULL, 

PROV_RSA_FULL, 0)) { 

long int li = 0; 

if (CryptGenRandom(prov, sizeof(li), (BYTE *)&li)) { 

printf("Random number: %ld\n", li); 

} else { 


} 

if (!CryptReleaseContext(prov, 0)) { 


} 

} else { 


} 

} 


The use of the rand() function can result in predictable random numbers. 


MSC30-C Medium Unlikely Low P6 L2 



MSC50-CPP. Do not use std::rand() for generating 

pseudorandom numbers 

CERT Oracle Secure Coding Standard for Java MSC02-J. Generate strong random numbers 

MITRE CWE 

CWE-327, Use of a Broken or Risky Cryptographic 

Algorithm 

CWE-330, Use of Insufficiently Random Values 

CWE-331, Insufficient Entropy 





CWE-338, Use of Cryptographically Weak 

Pseudo-Random Number Generator (PRNG) 


[MSDN] 

[OpenBSD] 

“CryptGenRandom Function“ 

arc4random() 




Miscellaneous (MSC) - MSC32-C. Properly seed pseudorandom number generators 

15.2 MSC32-C. Properly seed pseudorandom number generators 

A pseudorandom number generator (PRNG) is a deterministic algorithm capable of generating sequences 

of numbers that approximate the properties of random numbers. Each sequence is completely 

determined by the initial state of the PRNG and the algorithm for changing the state. Most 

PRNGs make it possible to set the initial state, also called the seed state. Setting the initial state is 

called seeding the PRNG. 

Calling a PRNG in the same initial state, either without seeding it explicitly or by seeding it with 

the same value, results in generating the same sequence of random numbers in different runs of 

the program. Consider a PRNG function that is seeded with some initial seed value and is consecutively 

called to produce a sequence of random numbers, S. If the PRNG is subsequently seeded 

with the same initial seed value, then it will generate the same sequence S. 

As a result, after the first run of an improperly seeded PRNG, an attacker can predict the sequence 

of random numbers that will be generated in the future runs. Improperly seeding or failing to seed 

the PRNG can lead to vulnerabilities, especially in security protocols. 

The solution is to ensure that the PRNG is always properly seeded. A properly seeded PRNG will 

generate a different sequence of random numbers each time it is run. 

Not all random number generators can be seeded. True random number generators that rely on 

hardware to produce completely unpredictable results do not need to be and cannot be seeded. 

Some high-quality PRNGs, such as the /dev/random device on some UNIX systems, also cannot 

be seeded. This rule applies only to algorithmic pseudorandom number generators that can be 

seeded. 


This noncompliant code example generates a sequence of 10 pseudorandom numbers using the 

random() function. When random() is not seeded, it behaves like rand(), producing the same 

sequence of random numbers each time any program that uses it is run. 

#include 

#include 


for (unsigned int i = 0; i < 10; ++i) { 

/* Always generates the same sequence */ 

printf("%ld, ", random()); 

} 

} 





The output is as follows: 

1st run: 1804289383, 846930886, 1681692777, 1714636915, 

1957747793, 424238335, 719885386, 1649760492, 

596516649, 1189641421, 

2nd run: 1804289383, 846930886, 1681692777, 1714636915, 

1957747793, 424238335, 719885386, 1649760492, 

596516649, 1189641421, 

... 

nth run: 1804289383, 846930886, 1681692777, 1714636915, 

1957747793, 424238335, 719885386, 1649760492, 

596516649, 1189641421, 


Call srandom() before invoking random() to seed the random sequence generated by random(). 

This compliant solution produces different random number sequences each time the function 

is called, depending on the resolution of the system clock: 

#include 

#include 

#include 


struct timespec ts; 

if (timespec_get(&ts, TIME_UTC) == 0) { 


} else { 

srandom(ts.tv_nsec ^ ts.tv_sec); 


/* Generates different sequences at different runs */ 

printf("%ld, ", random()); 

} 

} 

} 






1st run: 198682410, 2076262355, 910374899, 428635843, 

2084827500, 1558698420, 4459146, 733695321, 

2044378618, 1649046624, 

2nd run: 1127071427, 252907983, 1358798372, 2101446505, 

1514711759, 229790273, 954268511, 1116446419, 

368192457, 

1297948050, 

3rd run: 2052868434, 1645663878, 731874735, 1624006793, 

938447420, 1046134947, 1901136083, 418123888, 

836428296, 

2017467418, 

This output may not be sufficiently random for concurrent execution, which may lead to correlated 

generated series in different threads. Depending on the application and the desired level of 

security, a programmer may choose alternative ways to seed PRNGs. In general, hardware is more 

capable than software of generating real random numbers (for example, by sampling the thermal 

noise of a diode). 


The CryptGenRandom() function does not run the risk of not being properly seeded because its 

arguments serve as seeders: 

#include 

#include 

#include 


HCRYPTPROV hCryptProv; 

long rand_buf; 

/* Example of instantiating the CSP */ 

if (CryptAcquireContext(&hCryptProv, NULL, NULL, 

PROV_RSA_FULL, 0)) { 

printf("CryptAcquireContext succeeded.\n"); 

} else { 

printf("Error during CryptAcquireContext!\n"); 

} 


if (!CryptGenRandom(hCryptProv, sizeof(rand_buf), 

(BYTE *)&rand_buf)) { 

printf("Error\n"); 

} else { 

printf("%ld, ", rand_buf); 

} 





} 

} 


1st run: -1597837311, 906130682, -1308031886, 1048837407, - 

931041900, -658114613, -1709220953, -1019697289, 

1802206541,406505841, 

2nd run: 885904119, -687379556, -1782296854, 1443701916, - 

624291047, 2049692692, -990451563, -142307804, 

1257079211,897185104, 

3rd run: 190598304, -1537409464, 1594174739, -424401916, - 

1975153474, 826912927, 1705549595, -1515331215, 

474951399, 1982500583, 



MSC32-C Medium Likely Low P18 L1 




MITRE CWE 

MSC30-C. Do not use the rand() function for 

generating pseudorandom numbers 

MSC51-CPP. Ensure your random number 

generator is properly seeded 

CWE-327, Use of a Broken or Risky Cryptographic 

Algorithm 

CWE-330, Use of Insufficiently Random Values 

CWE-331, Insufficient Entropy 

CWE-338, Use of Cryptographically Weak 

Pseudo-Random Number Generator (PRNG) 


[MSDN] 

“CryptGenRandom Function“ 




Miscellaneous (MSC) - MSC33-C. Do not pass invalid data to the asctime() function 

15.3 MSC33-C. Do not pass invalid data to the asctime() function 

The C Standard, 7.27.3.1 [ISO/IEC 9899:2011], provides the following sample implementation of 

the asctime() function: 

char *asctime(const struct tm *timeptr) { 

static const char wday_name[7][3] = { 

"Sun", "Mon", "Tue", "Wed", "Thu", "Fri", "Sat" 

}; 

static const char mon_name[12][3] = { 

"Jan", "Feb", "Mar", "Apr", "May", "Jun", 

"Jul", "Aug", "Sep", "Oct", "Nov", "Dec" 

}; 

static char result[26]; 

sprintf( 

result, 

"%.3s %.3s%3d %.2d:%.2d:%.2d %d\n", 

wday_name[timeptr->tm_wday], 

mon_name[timeptr->tm_mon], 

timeptr->tm_mday, timeptr->tm_hour, 

timeptr->tm_min, timeptr->tm_sec, 

1900 + timeptr->tm_year 

); 


} 

This function is supposed to output a character string of 26 characters at most, including the terminating 

null character. If we count the length indicated by the format directives, we arrive at 25. 

Taking into account the terminating null character, the array size of the string appears sufficient. 

However, this implementation assumes that the values of the struct tm data are within normal 

ranges and does nothing to enforce the range limit. If any of the values print more characters than 

expected, the sprintf() function may overflow the result array. For example, if tm_year has 

the value 12345, then 27 characters (including the terminating null character) are printed, resulting 

in a buffer overflow. 

The POSIX® Base Specifications [IEEE Std 1003.1:2013] says the following about the 

asctime() and asctime_r() functions: 

These functions are included only for compatibility with older implementations. They 

have undefined behavior if the resulting string would be too long, so the use of these 

functions should be discouraged. On implementations that do not detect output string 

length overflow, it is possible to overflow the output buffers in such a way as to cause 

applications to fail, or possible system security violations. Also, these functions do not 

support localized date and time formats. To avoid these problems, applications should 

use strftime() to generate strings from broken-down times. 





The C Standard, Annex K, also defines asctime_s(), which can be used as a secure substitute 

for asctime(). 

The asctime() function appears in the list of obsolescent functions in MSC24-C. Do not use 

deprecated or obsolescent functions. 


This noncompliant code example invokes the asctime() function with potentially unsanitized 

data: 

#include 

void func(struct tm *time_tm) { 

char *time = asctime(time_tm); 

/* ... */ 

} 

15.3.2 Compliant Solution (strftime()) 

The strftime() function allows the programmer to specify a more rigorous format and also to 

specify the maximum size of the resulting time string: 

#include 

enum { maxsize = 26 }; 

void func(struct tm *time) { 

char s[maxsize]; 

/* Current time representation for locale */ 

const char *format = "%c"; 

} 

size_t size = strftime(s, maxsize, format, time); 

This call has the same effects as asctime() but also ensures that no more than maxsize characters 

are printed, preventing buffer overflow. 

15.3.3 Compliant Solution (asctime_s()) 

The C Standard, Annex K, defines the asctime_s() function, which serves as a close replacement 

for the asctime() function but requires an additional argument that specifies the maximum 

size of the resulting time string: 


#include 

enum { maxsize = 26 }; 





void func(struct tm *time_tm) { 

char buffer[maxsize]; 

} 

if (asctime_s(buffer, maxsize, &time_tm)) { 


} 


On implementations that do not detect output-string-length overflow, it is possible to overflow the 

output buffers. 


MSC33-C High Likely Low P27 L1 



MSC24-C. Do not use deprecated or obsolescent 

functions 


[IEEE Std 1003.1:2013] 

[ISO/IEC 9899:2011] 

XSH, System Interfaces, asctime 

7.27.3.1, “The asctime Function” 




Miscellaneous (MSC) - MSC37-C. Ensure that control never reaches the end of a non-void function 

15.4 MSC37-C. Ensure that control never reaches the end of a non-void 

function 

If control reaches the closing curly brace (}) of a non-void function without evaluating a return 

statement, using the return value of the function call is undefined behavior. (See undefined behavior 

88.) 


In this noncompliant code example, control reaches the end of the checkpass() function when 

the two strings passed to strcmp() are not equal, resulting in undefined behavior. Many compilers 

will generate code for the checkpass() function, returning various values along the execution 

path where no return statement is defined. 

#include 

#include 

int checkpass(const char *password) { 

if (strcmp(password, "pass") == 0) { 

return 1; 

} 

} 

void func(const char *userinput) { 

if (checkpass(userinput)) { 

printf("Success\n"); 

} 

} 

This error is frequently diagnosed by compilers. (See MSC00-C. Compile cleanly at high warning 

levels.) 


This compliant solution ensures that the checkpass() function always returns a value: 

#include 

#include 

int checkpass(const char *password) { 

if (strcmp(password, "pass") == 0) { 

return 1; 

} 

return 0; 

} 

void func(const char *userinput) { 

if (checkpass(userinput)) { 





} 

} 

printf("Success!\n"); 


In this noncompliant code example, control reaches the end of the getlen() function when input 

does not contain the integer delim. Because the potentially undefined return value of 

getlen() is later used as an index into an array, a buffer overflow may occur. 

#include 

size_t getlen(const int *input, size_t maxlen, int delim) { 

for (size_t i = 0; i < maxlen; ++i) { 

if (input[i] == delim) { 

return i; 

} 

} 

} 

void func(int userdata) { 

size_t i; 

int data[] = { 1, 1, 1 }; 

i = getlen(data, sizeof(data), 0); 

data[i] = userdata; 

} 

15.4.3.1 Implementation Details (GCC) 

Violating this rule can have unexpected consequences, as in the following example: 

#include 

size_t getlen(const int *input, size_t maxlen, int delim) { 



return i; 

} 

} 

} 

int main(int argc, char **argv) { 

size_t i; 

int data[] = { 1, 1, 1 }; 

i = getlen(data, sizeof(data), 0); 

printf("Returned: %zu\n", i); 

data[i] = 0; 





} 

return 0; 

When this program is compiled with -Wall on most versions of the GCC compiler, the following 

warning is generated: 

example.c: In function 'getlen': 

example.c:12: warning: control reaches end of non-void function 

None of the inputs to the function equal the delimiter, so when run with GCC 5.3 on Linux, control 

reaches the end of the getlen() function, which is undefined behavior and in this test returns 

3, causing an out-of-bounds write to the data array. 


This compliant solution changes the interface of getlen() to store the result in a user-provided 

pointer and return an error code to indicate any error conditions. The best method for handling 

this type of error is specific to the application and the type of error. (See ERR00-C. Adopt and implement 

a consistent and comprehensive error-handling policy for more on error handling.) 

#include 

int getlen(const int *input, size_t maxlen, int delim, 

size_t *result) { 



if (result != NULL) { 

*result = i; 

} 

return 0; 

} 

} 

return -1; 

} 

void func(int userdata) { 

size_t i; 

int data[] = {1, 1, 1}; 

if (getlen(data, sizeof(data), 0, &i) != 0) { 


} else { 

data[i] = userdata; 

} 

} 






MSC37-C-EX1: According to the C Standard, 5.1.2.2.3, paragraph 1 [ISO/IEC 9899:2011], 

“Reaching the } that terminates the main function returns a value of 0.” As a result, it is permissible 

for control to reach the end of the main() function without executing a return statement. 


Using the return value from a non-void function where control reaches the end of the function 

without evaluating a return statement can lead to buffer overflow vulnerabilities as well as other 

unexpected program behaviors. 


MSC37-C High Unlikely Low P9 L2 



MSC01-C. Strive for logical completeness 


[ISO/IEC 9899:2011] 

5.1.2.2.3, “Program Termination” 




Miscellaneous (MSC) - MSC38-C. Do not treat a predefined identifier as an object if it might only be implemented as a macro 

15.5 MSC38-C. Do not treat a predefined identifier as an object if it 

might only be implemented as a macro 

The C Standard, 7.1.4 paragraph 1, [ISO/IEC 9899:2011] states 

Any function declared in a header may be additionally implemented as a function-like 

macro defined in the header, so if a library function is declared explicitly when its header 

is included, one of the techniques shown below can be used to ensure the declaration is 

not affected by such a macro. Any macro definition of a function can be suppressed locally 

by enclosing the name of the function in parentheses, because the name is then 

not followed by the left parenthesis that indicates expansion of a macro function name. 

For the same syntactic reason, it is permitted to take the address of a library function 

even if it is also defined as a macro. 185 

185 

This means that an implementation shall provide an actual function for each library 

function, even if it also provides a macro for that function. 

However, the C Standard enumerates specific exceptions in which the behavior of accessing an 

object or function expanded to be a standard library macro definition is undefined. The macros are 

assert, errno, math_errhandling, setjmp, va_arg, va_copy, va_end, and va_start. 

These cases are described by undefined behaviors110, 114, 122, 124, and 138. Programmers must 

not suppress these macros to access the underlying object or function. 

15.5.1 Noncompliant Code Example (assert) 

In this noncompliant code example, the standard assert() macro is suppressed in an attempt to 

pass it as a function pointer to the execute_handler() function. Attempting to suppress the 

assert() macro is undefined behavior. 

#include 

typedef void (*handler_type)(int); 

void execute_handler(handler_type handler, int value) { 

handler(value); 

} 

void func(int e) { 

execute_handler(&(assert), e < 0); 

} 





15.5.2 Compliant Solution (assert) 

In this compliant solution, the assert() macro is wrapped in a helper function, removing the undefined 

behavior: 

#include 

typedef void (*handler_type)(int); 

void execute_handler(handler_type handler, int value) { 

handler(value); 

} 

static void assert_handler(int value) { 

assert(value); 

} 

void func(int e) { 

execute_handler(&assert_handler, e < 0); 

} 

15.5.3 Noncompliant Code Example (Redefining errno) 

Legacy code is apt to include an incorrect declaration, such as the following in this noncompliant 

code example: 

extern int errno; 

15.5.4 Compliant Solution (Declaring errno) 

This compliant solution demonstrates the correct way to declare errno by including the header 

: 

#include 

C-conforming implementations are required to declare errno in , although some historic 

implementations failed to do so. 


Accessing objects or functions underlying the specific macros enumerated in this rule is undefined 

behavior. 


MSC38-C Low Unlikely Medium P2 L3 







DCL37-C. Do not declare or define a reserved 

identifier 


ISO/IEC 9899:2011 

7.1.4, “Use of Library Functions” 




Miscellaneous (MSC) - MSC39-C. Do not call va_arg() on a va_list that has an indeterminate value 

15.6 MSC39-C. Do not call va_arg() on a va_list that has an 

indeterminate value 

Variadic functions access their variable arguments by using va_start() to initialize an object of 

type va_list, iteratively invoking the va_arg() macro, and finally calling va_end(). The 

va_list may be passed as an argument to another function, but calling va_arg() within that 

function causes the va_list to have an indeterminate value in the calling function. As a result, 

attempting to read variable arguments without reinitializing the va_list can have unexpected 

behavior. According to the C Standard, 7.16, paragraph 3 [ISO/IEC 9899:2011], 

If access to the varying arguments is desired, the called function shall declare an object 

(generally referred to as ap in this subclause) having type va_list. The object ap may 

be passed as an argument to another function; if that function invokes the va_arg 

macro with parameter ap, the value of ap in the calling function is indeterminate and 

shall be passed to the va_end macro prior to any further reference to ap. 253 

253 

It is permitted to create a pointer to a va_list and pass that pointer to another function, 

in which case the original function may take further use of the original list after 

the other function returns. 


This noncompliant code example attempts to check that none of its variable arguments are zero by 

passing a va_list to helper function contains_zero(). After the call to contains_zero(), 

the value of ap is indeterminate. 

#include 

#include 

int contains_zero(size_t count, va_list ap) { 

for (size_t i = 1; i < count; ++i) { 

if (va_arg(ap, double) == 0.0) { 

return 1; 

} 

} 

return 0; 

} 

int print_reciprocals(size_t count, ...) { 

va_list ap; 

va_start(ap, count); 

if (contains_zero(count, ap)) { 

va_end(ap); 

return 1; 

} 






} 

printf("%f ", 1.0 / va_arg(ap, double)); 

} 

va_end(ap); 

return 0; 


The compliant solution modifies contains_zero() to take a pointer to a va_list. It then uses 

the va_copy macro to make a copy of the list, traverses the copy, and cleans it up. Consequently, 

the print_reciprocals() function is free to traverse the original va_list. 

#include 

#include 

int contains_zero(size_t count, va_list *ap) { 

va_list ap1; 

va_copy(ap1, *ap); 


if (va_arg(ap1, double) == 0.0) { 

return 1; 

} 

} 

va_end(ap1); 

return 0; 

} 

int print_reciprocals(size_t count, ...) { 

int status; 

va_list ap; 

va_start(ap, count); 

} 

if (contains_zero(count, &ap)) { 

printf("0 in arguments!\n"); 

status = 1; 

} else { 

for (size_t i = 0; i < count; i++) { 

printf("%f ", 1.0 / va_arg(ap, double)); 

} 

printf("\n"); 

status = 0; 

} 

va_end(ap); 

return status; 






Reading variable arguments using a va_list that has an indeterminate value can have unexpected 

results. 


MSC39-C Low Unlikely Low P3 L3 


[ISO/IEC 9899:2011] 

Subclause 7.16, “Variable Arguments 

” 




Miscellaneous (MSC) - MSC40-C. Do not violate constraints 

15.7 MSC40-C. Do not violate constraints 

According to the C Standard, 3.8 [ISO/IEC 9899:2011], a constraint is a “restriction, either syntactic 

or semantic, by which the exposition of language elements is to be interpreted.” Despite the 

similarity of the terms, a runtime constraint is not a kind of constraint. 

Violating any shall statement within a constraint clause in the C Standard requires an implementation 

to issue a diagnostic message, the C Standard, 5.1.1.3 [ISO/IEC 9899:2011] states 

A conforming implementation shall produce at least one diagnostic message (identified 

in an implementation-defined manner) if a preprocessing translation unit or translation 

unit contains a violation of any syntax rule or constraint, even if the behavior is also explicitly 

specified as undefined or implementation-defined. Diagnostic messages need not 

be produced in other circumstances. 

The C Standard further explains in a footnote 

The intent is that an implementation should identify the nature of, and where possible localize, 

each violation. Of course, an implementation is free to produce any number of diagnostics 

as long as a valid program is still correctly translated. It may also successfully 

translate an invalid program. 

Any constraint violation is a violation of this rule because it can result in an invalid program. 

15.7.1 Noncompliant Code Example (Inline, Internal Linkage) 


An inline definition of a function with external linkage shall not contain a definition of a 

modifiable object with static or thread storage duration, and shall not contain a reference 

to an identifier with internal linkage. 

The motivation behind this constraint lies in the semantics of inline definitions. Paragraph 7 of 

subclause 6.7.4 reads, in part: 

An inline definition provides an alternative to an external definition, which a translator 

may use to implement any call to the function in the same translation unit. It is unspecified 

whether a call to the function uses the inline definition or the external definition. 

That is, if a function has an external and inline definition, implementations are free to choose 

which definition to invoke (two distinct invocations of the function may call different definitions, 

one the external definition, the other the inline definition). Therefore, issues can arise when these 

definitions reference internally linked objects or mutable objects with static or thread storage duration. 





This noncompliant code example refers to a static variable with file scope and internal linkage 

from within an external inline function: 

static int I = 12; 

extern inline void func(int a) { 

int b = a * I; 

/* ... */ 

} 

15.7.2 Compliant Solution (Inline, Internal Linkage) 

This compliant solution omits the static qualifier; consequently, the variable I has external 

linkage by default: 

int I = 12; 

extern inline void func(int a) { 

int b = a * I; 

/* ... */ 

} 

15.7.3 Noncompliant Code Example (inline, Modifiable Static) 

This noncompliant code example defines a modifiable static variable within an extern inline 

function. 

extern inline void func(void) { 

static int I = 12; 

/* Perform calculations which may modify I */ 

} 

15.7.4 Compliant Solution (Inline, Modifiable Static) 

This compliant solution removes the static keyword from the local variable definition. If the 

modifications to I must be retained between invocations of func(), it must be declared at file 

scope so that it will be defined with external linkage. 

extern inline void func(void) { 

int I = 12; 

/* Perform calculations which may modify I */ 

} 





15.7.5 Noncompliant Code Example (Inline, Modifiable static) 

This noncompliant code example includes two translation units: file1.c and file2.c. The first 

file, file1.c, defines a pseudorandom number generation function: 

/* file1.c */ 

/* Externally linked definition of the function get_random() */ 

extern unsigned int get_random(void) { 

/* Initialize the seeds */ 

static unsigned int m_z = 0xdeadbeef; 

static unsigned int m_w = 0xbaddecaf; 

} 

/* Compute the next pseudorandom value and update the seeds */ 

m_z = 36969 * (m_z & 65535) + (m_z >> 16); 

m_w = 18000 * (m_w & 65535) + (m_w >> 16); 

return (m_z > 16); 

m_w = 18000 * (m_w & 65535) + (m_w >> 16); 

return (m_z


the 

} 

/* 

* Get a pseudorandom number. Implementation defined whether 

* inline definition in this file or the external definition 

* in file2.c is called. 

*/ 

rand_no = get_random(); 

/* Use rand_no... */ 

/* ... */ 

} 

/* 

* Get another pseudorandom number. Behavior is 

* implementation defined. 

*/ 

rand_no = get_random(); 

/* Use rand_no... */ 

return 0; 

15.7.6 Compliant Solution (Inline, Modifiable static) 

This compliant solution adds the static modifier to the inline function definition in file2.c, 

giving it internal linkage. All references to get_random() in file.2.c will now reference the 

internally linked definition. The first file, which was not changed, is not shown here. 

/* file2.c */ 

/* Static inline definition of get_random function */ 

static inline unsigned int get_random(void) { 

/* 

* Initialize the seeds. 

* No more constraint violation; the inline function is now 

* internally linked. 

*/ 

static unsigned int m_z = 0xdeadbeef; 

static unsigned int m_w = 0xbaddecaf; 

} 

/* Compute the next pseudorandom value and update the seeds */ 

m_z = 36969 * (m_z & 65535) + (m_z >> 16); 

m_w = 18000 * (m_w & 65535) + (m_w >> 16); 

return (m_z



Constraint violations are a broad category of error that can result in unexpected control flow and 

corrupted data. 


MSC40-C Low Unlikely Medium P2 L3 


[ISO/IEC 9899:2011] 

4, “Conformance” 

5.1.1.3, “Diagnostics” 

6.7.4, “Function Specifiers” 




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Appendix B: Definitions 

abnormal end 

Termination of a process prior to completion [ISO/IEC/IEEE 24765:2010]. 

abnormal program termination 

See abnormal end. 

analyzer 

Mechanism that diagnoses coding flaws in software programs [ISO/IEC 9899:2011]. 

NOTE: Analyzers may include static analysis tools, tools within a compiler suite, or tools in other 

contexts. 

async-signal-safe function 

A function that may be invoked, without restriction, from signal-catching functions. No function 

(defined in ISO/IEC 9945) is async-signal-safe unless explicitly described as such. See asynchronous-safe 

[ISO/IEC 9945:2008]. 

asynchronous-safe function 

A function is asynchronous-safe, or asynchronous-signal safe, if it can be called safely and without 

side effects from within a signal handler context. That is, it must be able to be interrupted at 

any point to run linearly out of sequence without causing an inconsistent state. It must also function 

properly when global data might itself be in an inconsistent state. Some asynchronous-safe 

operations are listed here: 

Call the signal() function to reinstall a signal handler: 

• Unconditionally modify a volatile sig_atomic_t variable (as modification to this type 

is atomic). 

• Call the _Exit() function to immediately terminate program execution. 

• Invoke an asynchronous-safe function, as specified by the implementation. 

Few functions are portably asynchronous-safe [GNU Pth]. 

availability 

The degree to which a system or component is operational and accessible when required for use. 

Often expressed as a probability [IEEE Std 610.12 1990]. 

condition predicate 

An expression constructed from the variables of a function that must be true for a thread to be allowed 

to continue execution. 




conforming 

Conforming programs may depend on nonportable features of a conforming implementation 

[ISO/IEC 9899:2011]. 

critical sections 

Code that accesses shared data and would otherwise be protected from data races. 

dangling pointer 

A pointer to deallocated memory. 

data flow analysis 

Tracking of value constraints along nonexcluded paths through the code. 

NOTE: Tracking can be performed intraprocedurally, with various assumptions made about what 

happens at function call boundaries, or interprocedurally, where values are tracked flowing into 

function calls (directly or indirectly) as arguments and flowing back out either as return values or 

indirectly through arguments. 

Data flow analysis may or may not track values flowing into or out of the heap or take into account 

global variables. When this specification refers to values flowing, the key point is contrast 

with variables or expressions because a given variable or expression may hold different values 

along different paths and a given value may be held by multiple variables or expressions along a 

path. 

data race 

The execution of a program contains a data race if it contains two conflicting actions in different 

threads, at least one of which is not atomic, and neither happens before the other. Any such data 

race results in undefined behavior [ISO/IEC 9899:2011]. 

denial-of-service attack 

Also DoS attack. An attempt to make a computer resource unavailable to its intended users. 

diagnostic message 

A message belonging to an implementation-defined subset of the implementation’s message output. 

A diagnostic message may indicate a constraint violation or a valid but questionable language 

construct. Messages typically include the file name and line number pointing to the offending 

code construct. In addition, implementations often indicate the severity of the problem. Although 

the C Standard does not specify any such requirement, the most severe problems often cause implementations 

to fail to fully translate a translation unit. Diagnostics output in such cases are 

termed errors. Other problems may cause implementations simply to issue a warning message and 

continue translating the rest of the program. See error message and warning message [ISO/IEC 

9899:2011]. 

double-free vulnerability 

An exploitable error resulting from the same allocated object being freed more than once. 




error message 

A diagnostic message generated when source code is encountered that prevents an implementation 

from translating a translation unit. See diagnostic message and warning message. 

error tolerance 

The ability of a system or component to continue normal operation despite the presence of erroneous 

inputs [IEEE Std 610.12 1990]. 

exploit 

Technique that takes advantage of a security vulnerability to violate an explicit or implicit security 

policy [ISO/IEC TS 17961:2013]. 

fail safe 

Pertaining to a system or component that automatically places itself in a safe operating mode in 

the event of a failure—for example, a traffic light that reverts to blinking red in all directions 

when normal operation fails [IEEE Std 610.12 1990]. 

fail soft 

Pertaining to a system or component that continues to provide partial operational capability in the 

event of certain failures—for example, a traffic light that continues to alternate between red and 

green if the yellow light fails [IEEE Std 610.12 1990]. 

fatal diagnostic 

A diagnostic message that causes an implementation not to perform the translation. 

fault tolerance 

The ability of a system or component to continue normal operation despite the presence of hardware 

or software faults [IEEE Std 610.12 1990]. 

freestanding environment 

An environment in which C program execution may take place without any benefit of an operating 

system. Program startup might occur at some function other than main(), complex types 

might not be implemented, and only certain minimal library facilities are guaranteed to be available 

[ISO/IEC 9899:2011]. 

function-like macro 

A #define preprocessing directive that defines an identifier immediately followed by zero or 

more parameters, the ellipsis (...), or a combination of the two, enclosed in parentheses, similar 

syntactically to a function call. Subsequent instances of the macro name followed by a parenthesized 

list of arguments in a translation unit are replaced by the replacement list of preprocessing 

tokens that constitute the remainder of the directive. See object-like macro and unsafe functionlike 

macro [ISO/IEC 9899:2011]. 




hosted environment 

An environment that is not freestanding. Program startup occurs at main(), complex types are 

implemented, and all C standard library facilities are available [ISO/IEC 9899:2011]. 

implementation 

Particular set of software, running in a particular translation environment under particular control 

options, that performs translation of programs for, and supports execution of functions in, a particular 

execution environment [ISO/IEC 9899:2011]. 

implementation-defined behavior 

Unspecified behavior whereby each implementation documents how the choice is made [ISO/IEC 

9899:2011]. 

in-band error indicator 

A library function return value on error that can never be returned by a successful call to that library 

function [ISO/IEC 9899:2011]. 

incomplete type 

A type that describes an identifier but lacks information needed to determine the size of the identifier 

[ISO/IEC 9899:2011]. 

indeterminate value 

Either an unspecified value or a trap representation [ISO/IEC 9899:2011]. 

invalid pointer 

A pointer that is not a valid pointer. 

liveness 

Every operation or method invocation executes to completion without interruptions, even if it 

goes against safety. 

locale-specific behavior 

Behavior that depends on local conventions of nationality, culture, and language that each implementation 

documents [ISO/IEC 9899:2011]. 

lvalue 

An expression with an object type or an incomplete type other than void. The name lvalue comes 

originally from the assignment expression E1 = E2, in which the left operand E1 is required to 

be a (modifiable) lvalue. It is perhaps better considered as representing an object “locator value” 

[ISO/IEC 9899:2011]. 

mitigation 

Methods, techniques, processes, tools, or runtime libraries that can prevent or limit exploits 

against vulnerabilities [Seacord 2005a]. 




mutilated value 

Result of an operation performed on an untainted value that yields either an undefined result (such 

as the result of signed integer overflow), the result of right-shifting a negative number, implicit 

conversion to an integral type where the value cannot be represented in the destination type, or 

unsigned integer wrapping. 

EXAMPLE 

int j = INT_MAX + 1; // j is mutilated 

char c = 1234; // c is mutilated if char is eight bits 

unsigned int u = 0U - 1; // u is mutilated 

NOTE: A mutilated value can be just as dangerous as a tainted value because it can differ either in 

sign or magnitude from what the programmer expects. 

nonpersistent signal handler 

Signal handler running on an implementation that requires the program to again register the signal 

handler after occurrences of the signal to catch subsequent occurrences of that signal. 

normal program termination 

Normal termination occurs by a return from main(), when requested with the exit(), _exit(), 

or _Exit() functions, or when the last thread in the process terminates by returning from its start 

function, by calling the pthread_exit() function or through cancellation. See abnormal termination 

[IEEE Std 1003.1-2013]. 

object-like macro 

A #define preprocessing directive that defines an identifier with no parentheses. Subsequent instances 

of the macro name in a translation unit are replaced by the replacement list of preprocessing 

tokens that constitute the remainder of the directive. See function-like macro [ISO/IEC 

9899:2011]. 

out-of-band error indicator 

A library function return value used to indicate nothing but the error status [ISO/IEC TS 

17961:2013]. 

out-of-domain value 

One of a set of values that is not in the domain of a particular operator or function [ISO/IEC TS 

17961:2013]. 

reentrant 

Pertaining to a software module that can be entered as part of one process while also in execution 

as part of another process and still achieve the desired results [ISO/IEC/IEEE 24765:2010]. 

reliability 

The ability of a system or component to perform its required functions under stated conditions for 

a specified period of time [IEEE Std 610.12 1990]. 




estricted sink 

Operands and arguments whose domain is a subset of the domain described by their types 

[ISO/IEC 9899:2011]. 

robustness 

The degree to which a system or component can function correctly in the presence of invalid inputs 

or stressful environmental conditions [IEEE Std 610.12 1990]. 

rvalue 

Value of an expression [ISO/IEC 9899:2011]. 

sanitize 

Assure by testing or replacement that a tainted or other value conforms to the constraints imposed 

by one or more restricted sinks into which it may flow [ISO/IEC TS 17961:2013]. 

NOTE: If the value does not conform, either the path is diverted to avoid using the value or a different, 

known-conforming value is substituted—for example, adding a null character to the end of 

a buffer before passing it as an argument to the strlen function. 

security flaw 

Defect that poses a potential security risk [ISO/IEC TS 17961:2013]. 

security policy 

Set of rules and practices that specify or regulate how a system or organization provides security 

services to protect sensitive and critical system resources [Internet Society 2000]. 

sequence point 

Evaluation of an expression may produce side effects. At specific points in the execution sequence 

called sequence points, all side effects of previous evaluations have completed, and no 

side effects of subsequent evaluations have yet taken place [ISO/IEC 9899:2011]. 

side effect 

Changes in the state of the execution environment achieved by accessing a volatile object, modifying 

an object, modifying a file, or calling a function that does any of those operations [ISO/IEC 

9899:2011]. 

NOTE: The IEC 60559 standard for binary floating-point arithmetic requires certain user-accessible 

status flags and control modes. Floating-point operations implicitly set the status flags; modes 

affect result values of floating-point operations. Implementations that support such a floatingpoint 

state are required to regard changes to it as side effects. These are detailed in Annex F of the 

C Standard. 

static analysis 

Any process for assessing code without executing it [ISO/IEC TS 17961:2013]. 




strictly conforming 

A strictly conforming program is one that uses only those features of the language and library 

specified in the international standard. Strictly conforming programs are intended to be maximally 

portable among conforming implementations and cannot, for example, depend on implementation-defined 

behavior [ISO/IEC 9899:2011]. 

string 

A contiguous sequence of characters terminated by and including the first null character [ISO/IEC 

9899:2011]. 

tainted source 

External source of untrusted data. 

NOTE: Tainted sources include 

• parameters to the main() function, the returned values from localeconv(), fgetc(), 

getc, getchar(), fgetwc(), getwc(), and getwchar(), and the strings produced by 

getenv(), fscanf(), vfscanf(), vscanf(), fgets(), fread(), fwscanf(), vfwscanf(), 

vwscanf(), wscanf(), and fgetws() 

• parameters to the main() function, 

• the returned values from localeconv(), fgetc(), getc, getchar(), fgetwc(), 

getwc(), and getwchar() 

• the strings produced by getenv(), fscanf(), vfscanf(), vscanf(), fgets(), 

fread(), fwscanf(), vfwscanf(), vwscanf(), wscanf(), and fgetws()[ISO/IEC TS 

17961:2013] 

tainted value 

Value derived from a tainted source that has not been sanitized [ISO/IEC TS 17961:2013]. 

target implementation 

Implementation of the C programming language whose environmental limits and implementationdefined 

behavior are assumed by the analyzer during the analysis of a program [ISO/IEC TS 

17961:2013]. 

TOCTOU, TOCTTOU 

Time-of-check, time-of-use (TOCTOU), also referred to as time-of-check-to-time-of-use 

(TOCTTOU), represents a vulnerability in which access control checks are nonatomic with the 

operations they protect, allowing an attacker to violate access control rules. 

trap representation 

Object representation that does not represent a value of the object type. Attempting to read the 

value of an object that has a trap representation other than by an expression that has a character 

type is undefined. Producing such a representation by a side effect that modifies all or any part of 

the object other than by an expression that has a character type is undefined [ISO/IEC 

9899:2011]. 




undefined behavior (UB) 

Behavior, upon use of a nonportable or erroneous program construct or of erroneous data, for 

which the C Standard imposes no requirements. An example of undefined behavior is the behavior 

on integer overflow [ISO/IEC 9899:2011]. 

unexpected behavior 

Well-defined behavior that may be unexpected or unanticipated by the programmer; incorrect programming 

assumptions. 

unsafe function-like macro 

A function-like macro whose expansion causes one or more of its arguments not to be evaluated 

exactly once. 

unsigned integer wrapping 

Computation involving unsigned operands whose result is reduced modulo the number that is one 

greater than the largest value that can be represented by the resulting type. 

unspecified behavior 

Behavior for which the standard provides two or more possibilities and imposes no further requirements 

on which is chosen in any instance [ISO/IEC 9899:2011]. 

unspecified value 

A valid value of the relevant type where the C Standard imposes no requirements on which value 

is chosen in any instance. An unspecified value cannot be a trap representation [ISO/IEC 

9899:2011]. 

untrusted data 

Data originating from outside of a trust boundary [ISO/IEC 11889-1:2009]. 

valid pointer 

Pointer that refers to an element within an array or one past the last element of an array. See invalid 

pointer [ISO/IEC TS 17961:2013]. 

NOTE: For the purposes of this definition, a pointer to an object that is not an element of an array 

behaves the same as a pointer to the first element of an array of length one with the type of the object 

as its element type. (See C Standard, 6.5.8, paragraph 4.) 

For the purposes of this definition, an object can be considered to be an array of a certain number 

of bytes; that number is the size of the object, as produced by the sizeof operator. (See C Standard, 

6.3.2.3, paragraph 7.) 

validation 

Confirmation by examination and provision of objective evidence that the particular requirements 

for a specific intended use are fulfilled [IEC 61508-4]. 




verification 

Confirmation by examination and provision of objective evidence that the requirements have been 

fulfilled [IEC 61508-4]. 

vulnerability 

Set of conditions that allows an attacker to violate an explicit or implicit security policy [ISO/IEC 

TS 17961:2013]. 

warning message 

A diagnostic message generated when source code is encountered that does not prevent an implementation 

from translating a translation unit. See diagnostic message and error message. 




Appendix C: Undefined Behavior 

According to the C Standard, Annex J, J.2 [ISO/IEC 9899:2011], the behavior of a program is undefined 

in the circumstances outlined in the following table. The “Guideline” column in the table 

identifies the coding practices that address the specific case of undefined behavior (UB). The descriptions 

of undefined behaviors in the “Description” column are direct quotes from the standard. 

The parenthesized numbers refer to the subclause of the C Standard (C11) that identifies the undefined 

behavior. 

UB Description Guideline 

1 A “shall” or “shall not” requirement that appears outside of a constraint 

MSC15-C 

is violated (clause 4). 

2 A nonempty source file does not end in a new-line character which is 

not immediately preceded by a backslash character or ends in a partial 

preprocessing token or comment (5.1.1.2). 

3 Token concatenation produces a character sequence matching the PRE30-C 

syntax of a universal character name (5.1.1.2). 

4 A program in a hosted environment does not define a function named 

main using one of the specified forms (5.1.2.2.1). 

5 The execution of a program contains a data race (5.1.2.4). 

6 A character not in the basic source character set is encountered in a 

source file, except in an identifier, a character constant, a string literal, 

a header name, a comment, or a preprocessing token that is 

never converted to a token (5.2.1). 

7 An identifier, comment, string literal, character constant, or header 

name contains an invalid multibyte character or does not begin and 

end in the initial shift state (5.2.1.2). 

8 The same identifier has both internal and external linkage in the same DCL36-C 

translation unit (6.2.2). 

9 An object is referred to outside of its lifetime (6.2.4). DCL21-C, 

DCL30-C 

10 The value of a pointer to an object whose lifetime has ended is used 

(6.2.4). 

11 The value of an object with automatic storage duration is used while 

it is indeterminate (6.2.4, 6.7.9, 6.8). 

12 A trap representation is read by an lvalue expression that does not 

have character type (6.2.6.1). 

DCL30-C, 

EXP33-C 

EXP33-C, 

MSC22-C 

EXP33-C 





13 A trap representation is produced by a side effect that modifies any 

part of the object using an lvalue expression that does not have character 

type (6.2.6.1). 

14 The operands to certain operators are such that they could produce a 

negative zero result, but the implementation does not support negative 

zeros (6.2.6.2). 

15 Two declarations of the same object or function specify types that are 

not compatible (6.2.7). 

16 A program requires the formation of a composite type from a variable 

length array type whose size is specified by an expression that is not 

evaluated (6.2.7). 

17 Conversion to or from an integer type produces a value outside the 

range that can be represented (6.3.1.4). 

18 Demotion of one real floating type to another produces a value outside 

the range that can be represented (6.3.1.5). 

19 An lvalue does not designate an object when evaluated (6.3.2.1). 

20 A non-array lvalue with an incomplete type is used in a context that 

requires the value of the designated object (6.3.2.1). 

21 An lvalue designation an object of automatic storage duration that 

could have been declared with the register storage class is used in 

a context that requires the value of the designated object, but the object 

is uninitialized (6.3.2.1). 

22 An lvalue having array type is converted to a pointer to the initial element 

of the array, and the array object has register storage class 

(6.3.2.1). 

23 An attempt is made to use the value of a void expression, or an implicit 

or explicit conversion (except to void) is applied to a void expression 

(6.3.2.2). 

24 Conversion of a pointer to an integer type produces a value outside 

the range that can be represented (6.3.2.3). 

25 Conversion between two pointer types produces a result that is incorrectly 

aligned (6.3.2.3). 

26 A pointer is used to call a function whose type is not compatible with 

the pointed-to type (6.3.2.3). 

27 An unmatched ' or " character is encountered on a logical source line 

during tokenization (6.4). 

28 A reserved keyword token is used in translation phase 7 or 8 for some 

purpose other than as a keyword (6.4.1). 

DCL23-C, 

DCL40-C 

FLP34-C 

FLP34-C 

INT36-C 

EXP36-C 

EXP37-C 





29 A universal character name in an identifier does not designate a character 

whose encoding falls into one of the specified ranges (6.4.2.1). 

30 The initial character of an identifier is a universal character name designating 

a digit (6.4.2.1). 

31 Two identifiers differ only in nonsignificant characters (6.4.2.1). DCL23-C, 

DCL31-C 

32 The identifier func{} is explicitly declared (6.4.2.2). 

33 The program attempts to modify a string literal (6.4.5). STR30-C 

34 The characters ', back-slash, ", /, or /* occur in the sequence between 

the < and > delimiters, or the characters ', back-slash, //, or 

/* occur in the sequence between the " delimiters, in a header name 

preprocessing token (6.4.7). 

35 A side effect on a scalar object is unsequenced relative to either a different 

side effect on the same scalar object or a value computation using 

the value of the same scalar object (6.5). 

36 An exceptional condition occurs during the evaluation of an expression 

(6.5). 

37 An object has its stored value accessed other than by an lvalue of an 

allowable type (6.5). 

38 For a call to a function without a function prototype in scope, the 

number of arguments does not equal the number of parameters 

(6.5.2.2). 

39 For call to a function without a function prototype in scope where the 

function is defined with a function prototype, either the prototype 

ends with an ellipsis or the types of the arguments after promotion are 

not compatible with the types of the parameters (6.5.2.2). 

40 For a call to a function without a function prototype in scope where 

the function is not defined with a function prototype, the types of the 

arguments after promotion are not compatible with those of the parameters 

after promotion (with certain exceptions) (6.5.2.2). 

41 A function is defined with a type that is not compatible with the type 

(of the expression) pointed to by the expression that denotes the 

called function (6.5.2.2). 

42 A member of an atomic structure or union is accessed (6.5.2.3). 

43 The operand of the unary * operator has an invalid value (6.5.3.2). 

44 A pointer is converted to other than an integer or pointer type (6.5.4). 

EXP39-C 

EXP30-C 

INT32-C 

DCL40- 

C,EXP39-C 

EXP37-C 

EXP37-C 

EXP37-C 

DCL40-C, 

EXP37-C 

45 The value of the second operand of the / or % operator is zero (6.5.5). INT33-C 





46 Addition or subtraction of a pointer into, or just beyond, an array object 

and an integer type produces a result that does not point into, or 

just beyond, the same array object (6.5.6). 

47 Addition or subtraction of a pointer into, or just beyond, an array object 

and an integer type produces a result that points just beyond the 

array object and is used as the operand of a unary * operator that is 

evaluated (6.5.6). 

48 Pointers that do not point into, or just beyond, the same array object 

are subtracted (6.5.6). 

49 An array subscript is out of range, even if an object is apparently accessible 

with the given subscript (as in the lvalue expression 

a[1][7] given the declaration int a[4][5]) (6.5.6). 

50 The result of subtracting two pointers is not representable in an object 

of type ptrdiff_t (6.5.6). 

51 An expression is shifted by a negative number or by an amount 

greater than or equal to the width of the promoted expression (6.5.7). 

52 An expression having signed promoted type is left-shifted and either 

the value of the expression is negative or the result of shifting would 

not be representable in the promoted type (6.5.7). 

53 Pointers that do not point to the same aggregate or union (nor just beyond 

the same array object) are compared using relational operators 

(6.5.8). 

54 An object is assigned to an inexactly overlapping object or to an exactly 

overlapping object with incompatible type (6.5.16.1). 

55 An expression that is required to be an integer constant expression 

does not have an integer type; has operands that are not integer constants, 

enumeration constants, character constants, sizeof expressions 

whose results are integer constants, or immediately-cast floating 

constants; or contains casts (outside operands to sizeof operators) 

other than conversions of arithmetic types to integer types (6.6). 

56 A constant expression in an initializer is not, or does not evaluate to, 

one of the following: an arithmetic constant expression, a null pointer 

constant, an address constant, or an address constant for an object 

type plus or minus an integer constant expression (6.6). 

57 An arithmetic constant expression does not have arithmetic type; has 

operands that are not integer constants, floating constants, enumeration 

constants, character constants, or sizeof expressions; or contains 

casts (outside operands to sizeof operators) other than conversions 

of arithmetic types to arithmetic types (6.6). 

ARR30-C 

ARR30-C 

ARR36-C 

ARR30-C 

INT34-C 

ARR36-C 





58 The value of an object is accessed by an array-subscript [], memberaccess 

. or ->, address &, or indirection * operator or a pointer cast 

in creating an address constant (6.6). 

59 An identifier for an object is declared with no linkage and the type of 

the object is incomplete after its declarator, or after its init-declarator 

if it has an initializer (6.7). 

60 A function is declared at block scope with an explicit storage-class 

specifier other than extern (6.7.1). 

61 A structure or union is defined as containing no named members 

(6.7.2.1). 

62 An attempt is made to access, or generate a pointer to just past, a 

flexible array member of a structure when the referenced object provides 

no elements for that array (6.7.2.1). 

63 When the complete type is needed, an incomplete structure or union 

type is not completed in the same scope by another declaration of the 

tag that defines the content (6.7.2.3). 

64 An attempt is made to modify an object defined with a const-qualified 

type through use of an lvalue with non-const-qualified type 

(6.7.3). 

65 An attempt is made to refer to an object defined with a volatilequalified 

type through use of an lvalue with non-volatile-qualified 

type (6.7.3). 

66 The specification of a function type includes any type qualifiers 

(6.7.3). 

67 Two qualified types that are required to be compatible do not have 

the identically qualified version of a compatible type (6.7.3). 

68 An object which has been modified is accessed through a restrictqualified 

pointer to a const-qualified type, or through a restrictqualified 

pointer and another pointer that are not both based on the 

same object (6.7.3.1). 

69 A restrict-qualified pointer is assigned a value based on another 

restricted pointer whose associated block neither began execution before 

the block associated with this pointer, nor ended before the assignment 

(6.7.3.1). 

70 A function with external linkage is declared with an inline function 

specifier, but is not also defined in the same translation unit (6.7.4). 

71 A function declared with a _Noreturn function specifier returns to 

its caller (6.7.4). 

ARR30-C 

EXP05-C, 

EXP40-C 

EXP32-C 

EXP43-C 





72 The definition of an object has an alignment specifier and another 

declaration of that object has a different alignment specifier (6.7.5). 

73 Declarations of an object in different translation units have different 

alignment specifiers (6.7.5). 

74 Two pointer types that are required to be compatible are not identically 

qualified, or are not pointers to compatible types (6.7.6.1). 

75 The size expression in an array declaration is not a constant expression 

and evaluates at program execution time to a nonpositive value 

(6.7.6.2). 

76 In a context requiring two array types to be compatible, they do not 

have compatible element types, or their size specifiers evaluate to unequal 

values (6.7.6.2). 

77 A declaration of an array parameter includes the keyword static 

within the [ and ] and the corresponding argument does not provide 

access to the first element of an array with at least the specified number 

of elements (6.7.6.3). 

78 A storage-class specifier or type qualifier modifies the keyword void 

as a function parameter type list (6.7.6.3). 

79 In a context requiring two function types to be compatible, they do 

not have compatible return types, or their parameters disagree in use 

of the ellipsis terminator or the number and type of parameters (after 

default argument promotion, when there is no parameter type list or 

when one type is specified by a function definition with an identifier 

list) (6.7.6.3). 

80 The value of an unnamed member of a structure or union is used 

(6.7.9). 

81 The initializer for a scalar is neither a single expression nor a single 

expression enclosed in braces (6.7.9). 

82 The initializer for a structure or union object that has automatic storage 

duration is neither an initializer list nor a single expression that 

has compatible structure or union type (6.7.9). 

83 The initializer for an aggregate or union, other than an array initialized 

by a string literal, is not a brace-enclosed list of initializers for its 

elements or members (6.7.9). 

84 An identifier with external linkage is used, but in the program there 

does not exist exactly one external definition for the identifier, or the 

identifier is not used and there exist multiple external definitions for 

the identifier (6.9). 

85 A function definition includes an identifier list, but the types of the 

parameters are not declared in a following declaration list (6.9.1). 

ARR32-C 





86 An adjusted parameter type in a function definition is not an object 

type (6.9.1). 

87 A function that accepts a variable number of arguments is defined 

without a parameter type list that ends with the ellipsis notation 

(6.9.1). 

88 The } that terminates a function is reached, and the value of the function 

call is used by the caller (6.9.1). 

89 An identifier for an object with internal linkage and an incomplete 

type is declared with a tentative definition (6.9.2). 

90 The token defined is generated during the expansion of a #if or 

#elif preprocessing directive, or the use of the defined unary operator 

does not match one of the two specified forms prior to macro replacement 

(6.10.1). 

91 The #include preprocessing directive that results after expansion 

does not match one of the two header name forms (6.10.2). 

92 The character sequence in an #include preprocessing directive does 

not start with a letter (6.10.2). 

93 There are sequences of preprocessing tokens within the list of macro 

arguments that would otherwise act as preprocessing directives 

(6.10.3). 

94 The result of the preprocessing operator # is not a valid character 

string literal (6.10.3.2). 

95 The result of the preprocessing operator ## is not a valid preprocessing 

token (6.10.3.3). 

96 The #line preprocessing directive that results after expansion does 

not match one of the two well-defined forms, or its digit sequence 

specifies zero or a number greater than 2147483647 (6.10.4). 

97 A non-STDC#pragma preprocessing directive that is documented as 

causing translation failure or some other form of undefined behavior 

is encountered (6.10.6). 

98 A #pragma STDC preprocessing directive does not match one of the 

well-defined forms (6.10.6). 

99 The name of a predefined macro, or the identifier defined, is the subject 

of a #define or #undef preprocessing directive (6.10.8). 

100 An attempt is made to copy an object to an overlapping object by use 

of a library function, other than as explicitly allowed (e.g., memmove) 

(clause 7). 

MSC37-C 

PRE32-C 





101 A file with the same name as one of the standard headers, not provided 

as part of the implementation, is placed in any of the standard 

places that are searched for included source files (7.1.2). 

102 A header is included within an external declaration or definition 

(7.1.2). 

103 A function, object, type, or macro that is specified as being declared 

or defined by some standard header is used before any header that declares 

or defines it is included (7.1.2). 

104 A standard header is included while a macro is defined with the same 

name as a keyword (7.1.2). 

105 The program attempts to declare a library function itself, rather than 

via a standard header, but the declaration does not have external linkage 

(7.1.2). 

106 The program declares or defines a reserved identifier, other than as 

allowed by 7.1.4 (7.1.3). 

107 The program removes the definition of a macro whose name begins 

with an underscore and either an uppercase letter or another underscore 

(7.1.3). 

108 An argument to a library function has an invalid value or a type not 

expected by a function with a variable number of arguments (7.1.4). 

109 The pointer passed to a library function array parameter does not 

have a value such that all address computations and object accesses 

are valid (7.1.4). 

110 The macro definition of assert is suppressed in order to access an 

actual function (7.2). 

111 The argument to the assert macro does not have a scalar type (7.2). 

112 The CX_LIMITED_RANGE, FENV_ACCESS, or FP_CONTRACT pragma 

is used in any context other than outside all external declarations or 

preceding all explicit declarations and statements inside a compound 

statement (7.3.4, 7.6.1, 7.12.2). 

113 The value of an argument to a character handling function is neither 

equal to the value of EOF nor representable as an unsigned char 

(7.4). 

114 A macro definition of errno is suppressed in order to access an actual 

object, or the program defines an identifier with the name errno 

(7.5). 

115 Part of the program tests floating-point status flags, sets floatingpoint 

control modes, or runs under non-default mode settings, but 

DCL37-C 

ARR30-C, 

ARR38-C 

MSC38-C 

STR37-C 

DCL37-C, 

MSC38-C 





was translated with the state for the FENV_ACCESS pragma "off" 

(7.6.1). 

116 The exception-mask argument for one of the functions that provide 

access to the floating-point status flags has a nonzero value not obtained 

by bitwise OR of the floating-point exception macros (7.6.2). 

117 The fesetexceptflag function is used to set floating-point status 

flags that were not specified in the call to the fegetexceptflag 

function that provided the value of the corresponding fexcept_t object 

(7.6.2.4). 

118 The argument to fesetenv or feupdateenv is neither an object set 

by a call to fegetenv or feholdexcept, nor is it an environment 

macro (7.6.4.3, 7.6.4.4). 

119 The value of the result of an integer arithmetic or conversion function 

cannot be represented (7.8.2.1, 7.8.2.2, 7.8.2.3, 7.8.2.4, 7.22.6.1, 

7.22.6.2, 7.22.1). 

120 The program modifies the string pointed to by the value returned by 

the setlocale function (7.11.1.1). 

121 The program modifies the structure pointed to by the value returned 

by the localeconv function (7.11.2.1). 

122 A macro definition of math_errhandling is suppressed or the program 

defines an identifier with the name math_errhandling 

(7.12). 

123 An argument to a floating-point classification or comparison macro is 

not of real floating type (7.12.3, 7.12.14). 

124 A macro definition of setjmp is suppressed in order to access an actual 

function, or the program defines an external identifier with the 

name setjmp (7.13). 

125 An invocation of the setjmp macro occurs other than in an allowed 

context (7.13.2.1). 

126 The longjmp function is invoked to restore a nonexistent environment 

(7.13.2.1). 

127 After a longjmp, there is an attempt to access the value of an object 

of automatic storage class with non-volatile-qualified type, local 

to the function containing the invocation of the corresponding setjmp 

macro, that was changed between the setjmp invocation and 

longjmp call (7.13.2.1). 

128 The program specifies an invalid pointer to a signal handler function 

(7.14.1.1). 

ERR07-C 

ENV30-C 

ENV30-C 

MSC38-C 

MSC38-C 

MSC22-C 

MSC22-C 

MSC22-C 





129 A signal handler returns when the signal corresponded to a computational 

exception (7.14.1.1). 

130 A signal handler called in response to SIGFPE, SIGILL, SIGSEGV, or 

any other implementation-defined value corresponding to a computational 

exception returns (7.14.1.1). 

131 A signal occurs as the result of calling the abort or raise function, 

and the signal handler calls the raise function (7.14.1.1). 

SIG31-C 

SIG35-C 

SIG30-C, 

SIG31-C 

132 A signal occurs other than as the result of calling the abort or raise 

function, and the signal handler refers to an object with static or 

thread storage duration that is not a lock-free atomic object other than 

by assigning a value to an object declared as volatile sig_atomic_t, or 

calls any function in the standard library other than the abort function, 

the _Exit function, the quick_exit function, or the signal function 

(for the same signal number) (7.14.1.1). 

133 The value of errno is referred to after a signal occurred other than as 

the result of calling the abort or raise function and the corresponding 

signal handler obtained a SIG_ERR return from a call to the signal 

function (7.14.1.1). 

134 A signal is generated by an asynchronous signal handler (7.14.1.1). 

135 The signal function is used in a multi-threaded program (7.14.1.1). 

136 A function with a variable number of arguments attempts to access its 

varying arguments other than through a properly declared and initialized 

va_list object, or before the va_start macro is invoked 

(7.16, 7.16.1.1, 7.16.1.4). 

137 The macro va_arg is invoked using the parameter ap that was 

passed to a function that invoked the macro va_arg with the same 

parameter (7.16). 

138 A macro definition of va_start, va_arg, va_copy, or va_end is 

suppressed in order to access an actual function, or the program defines 

an external identifier with the name va_copy or va_end 

(7.16.1). 

139 The va_start or va_copy macro is invoked without a corresponding 

invocation of the va_end macro in the same function, or vice 

versa (7.16.1, 7.16.1.2, 7.16.1.3, 7.16.1.4). 

140 The type parameter to the va_arg macro is not such that a pointer to 

an object of that type can be obtained simply by postfixing a * 

(7.16.1.1). 

SIG31-C 

ERR32-C 

CON37-C 

MSC38-C 





141 The va_arg macro is invoked when there is no actual next argument, 

or with a specified type that is not compatible with the promoted type 

of the actual next argument, with certain exceptions (7.16.1.1). 

142 The va_copy or va_start macro is called to initialize a va_list 

that was previously initialized by either macro without an intervening 

invocation of the va_end macro for the same va_list (7.16.1.2, 

7.16.1.4). 

143 The parameter parmN of a va_start macro is declared with the register 

storage class, with a function or array type, or with a type that is 

not compatible with the type that results after application of the default 

argument promotions (7.16.1.4). 

144 The member designator parameter of an offsetof macro is an invalid 

right operand of the . operator for the type parameter, or designates 

a bit-field (7.19). 

145 The argument in an instance of one of the integer-constant macros is 

not a decimal, octal, or hexadecimal constant, or it has a value that 

exceeds the limits for the corresponding type (7.20.4). 

146 A byte input/output function is applied to a wide-oriented stream, or a 

wide character input/output function is applied to a byte-oriented 

stream (7.21.2). 

147 Use is made of any portion of a file beyond the most recent wide 

character written to a wide-oriented stream (7.21.2). 

148 The value of a pointer to a FILE object is used after the associated 

file is closed (7.21.3). 

149 The stream for the fflush function points to an input stream or to an 

update stream in which the most recent operation was input 

(7.21.5.2). 

150 The string pointed to by the mode argument in a call to the fopen 

function does not exactly match one of the specified character sequences 

(7.21.5.3). 

151 An output operation on an update stream is followed by an input operation 

without an intervening call to the fflush function or a file 

positioning function, or an input operation on an update stream is followed 

by an output operation with an intervening call to a file positioning 

function (7.21.5.3). 

152 An attempt is made to use the contents of the array that was supplied 

in a call to the setvbuf function (7.21.5.6). 

153 There are insufficient arguments for the format in a call to one of the 

formatted input/output functions, or an argument does not have an appropriate 

type (7.21.6.1, 7.21.6.2, 7.29.2.1, 7.29.2.2). 

DCL10-C 

FIO42-C, 

FIO46-C 

FIO39-C 

FIO47-C 





154 The format in a call to one of the formatted input/output functions or 

to the strftime or wcsftime function is not a valid multibyte character 

sequence that begins and ends in its initial shift state (7.21.6.1, 

7.121.6.2, 7.27.3.5, 7.229.2.1, 7.29.2.2, 7.29.5.1). 

155 In a call to one of the formatted output functions, a precision appears 

with a conversion specifier other than those described (7.21.6.1, 

7.29.2.1). 

156 A conversion specification for a formatted output function uses an asterisk 

to denote an argument-supplied field width or precision, but the 

corresponding argument is not provided (7.21.6.1, 7.29.2.1). 

157 A conversion specification for a formatted output function uses a # or 

0 flag with a conversion specifier other than those described 

(7.21.6.1, 7.29.2.1). 

158 A conversion specification for one of the formatted input/output functions 

uses a length modifier with a conversion specifier other than 

those described (7.21.6.1, 7.21.6.2, 7.29.2.1, 7.29.2.2). 

159 An s conversion specifier is encountered by one of the formatted output 

functions, and the argument is missing the null terminator (unless 

a precision is specified that does not require null termination) 

(7.21.6.1, 7.29.2.1). 

160 An n conversion specification for one of the formatted input/output 

functions includes any flags, an assignment-suppressing character, a 

field width, or a precision (7.21.6.1, 7.21.6.2, 7.29.2.1, 7.29.2.2). 

161 A % conversion specifier is encountered by one of the formatted input/output 

functions, but the complete conversion specification is not 

exactly %% (7.21.6.1, 7.21.6.2, 7.29.2.1, 7.29.2.2). 

162 An invalid conversion specification is found in the format for one of 

the formatted input/output functions, or the strftime or wcsftime 

function (7.21.6.1, 7.21.6.2, 7.27.3.5, 7.29.2.1, 7.29.2.2, 7.29.5.1). 

163 The number of characters or wide characters transmitted by a formatted 

output function (or written to an array, or that would have been 

written to an array) is greater than INT_MAX (7.21.6.1, 7.29.2.1). 

164 The number of input items assigned by a formatted input function is 

greater than INT_MAX (7.21.6.2, 7.29.2.2). 

165 The result of a conversion by one of the formatted input functions 

cannot be represented in the corresponding object, or the receiving 

object does not have an appropriate type (7.21.6.2, 7.29.2.2). 

FIO47-C 

FIO47-C 

FIO47-C 

FIO47-C 

FIO47-C 





166 A c, s, or [ conversion specifier is encountered by one of the formatted 

input functions, and the array pointed to by the corresponding argument 

is not large enough to accept the input sequence (and a null 

terminator if the conversion specifier is s or [) (7.21.6.2, 7.29.2.2). 

167 A c, s, or [ conversion specifier with an l qualifier is encountered by 

one of the formatted input functions, but the input is not a valid 

multibyte character sequence that begins in the initial shift state 

(7.21.6.2, 7.29.2.2). 

168 The input item for a %p conversion by one of the formatted input 

functions is not a value converted earlier during the same program 

execution (7.21.6.2, 7.29.2.2). 

169 The vfprintf, vfscanf, vprintf, vscanf, vsnprintf, 

vsprintf, vsscanf, vfwprintf, vfwscanf, vswprintf, vswscanf, 

vwprintf, or vwscanf function is called with an improperly 

initialized va_list argument, or the argument is used (other than in 

an invocation of va_end) after the function returns (7.21.6.8, 

7.21.6.9, 7.21.6.10, 7.21.6.11, 7.21.6.12, 7.21.6.13, 7.21.6.14, 

7.29.2.5, 7.29.2.6, 7.29.2.7, 7.29.2.8, 7.29.2.9, 7.29.2.10). 

170 The contents of the array supplied in a call to the fgets, gets, or 

fgetws function are used after a read error occurred (7.21.7.2, 

7.21.7.7, 7.293.2). 

171 The file position indicator for a binary stream is used after a call to 

the ungetc function where its value was zero before the call 

(7.21.7.11). 

172 The file position indicator for a stream is used after an error occurred 

during a call to the fread or fwrite function (7.21.8.1, 7.21.8.2). 

173 A partial element read by a call to the fread function is used 

(7.21.8.1). 

174 The fseek function is called for a text stream with a nonzero offset 

and either the offset was not returned by a previous successful call to 

the ftell function on a stream associated with the same file or 

whence is not SEEK_SET (7.21.9.2). 

175 The fsetpos function is called to set a position that was not returned 

by a previous successful call to the fgetpos function on a stream associated 

with the same file (7.21.9.3). 

176 A non-null pointer returned by a call to the calloc, malloc, or realloc 

function with a zero requested size is used to access an object 

(7.22.3). 

177 The value of a pointer that refers to space deallocated by a call to the 

free or realloc function is used (7.22.3). 

FIO40-C 

MEM04-C 

MEM30-C 





178 The alignment requested of the aligned_alloc function is not 

valid or not supported by the implementation, or the size requested is 

not an integral multiple of the alignment (7.22.3.1). 

179 The pointer argument to the free or realloc function does not 

match a pointer earlier returned by calloc, malloc, or realloc, or 

the space has been deallocated by a call to free or realloc 

(7.22.3.3, 7.22.3.5). 

180 The value of the object allocated by the malloc function is used 

(7.22.3.4). 

181 The values of any bytes in a new object allocated by the realloc 

function beyond the size of the old object are used (7.22.3.5). 

182 The program calls the exit or quick_exit function more than once, or 

calls both functions (7.22.4.4, 7.22.4.7). 

183 During the call to a function registered with the atexit or 

at_quick_exit function, a call is made to the longjmp function that 

would terminate the call to the registered function (7.22.4.4, 

7.22.4.7). 

184 The string set up by the getenv or strerror function is modified 

by the program (7.22.4.6, 7.24.6.2). 

185 A signal is raised while the quick_exit function is executing 

(7.22.4.7). 

186 A command is executed through the system function in a way that is 

documented as causing termination or some other form of undefined 

behavior (7.22.4.8). 

187 A searching or sorting utility function is called with an invalid pointer 

argument, even if the number of elements is zero (7.22.5). 

188 The comparison function called by a searching or sorting utility function 

alters the contents of the array being searched or sorted, or returns 

ordering values inconsistently (7.22.5). 

189 The array being searched by the bsearch function does not have its 

elements in proper order (7.22.5.1). 

190 The current conversion state is used by a multibyte/wide character 

conversion function after changing the LC_CTYPE category (7.22.7). 

191 A string or wide string utility function is instructed to access an array 

beyond the end of an object (7.24.1, 7.29.4). 

192 A string or wide string utility function is called with an invalid 

pointer argument, even if the length is zero (7.24.1, 7.29.4). 

193 The contents of the destination array are used after a call to the 

strxfrm, strftime, wcsxfrm, or wcsftime function in which the 

MEM34-C 

EXP33-C 

ENV32- 

C,ERR04-C 

ENV32-C 

ENV30-C 





specified length was too small to hold the entire null-terminated result 

(7.24.4.5, 7.27.3.5, 7.29.4.4.4, 7.29.5.1). 

194 The first argument in the very first call to the strtok or wcstok is a 

null pointer (7.24.5.8, 7.29.4.5.7). 

195 The type of an argument to a type-generic macro is not compatible 

with the type of the corresponding parameter of the selected function 

(7.25). 

196 A complex argument is supplied for a generic parameter of a type-generic 

macro that has no corresponding complex function (7.25). 

197 At least one member of the broken-down time passed to asctime contains 

a value outside its normal range, or the calculated year exceeds 

four digits or is less than the year 1000 (7.27.3.1). 

198 The argument corresponding to an s specifier without an l qualifier 

in a call to the fwprintf function does not point to a valid multibyte 

character sequence that begins in the initial shift state (7.29.2.11). 

199 In a call to the wcstok function, the object pointed to by ptr does 

not have the value stored by the previous call for the same wide string 

(7.29.4.5.7). 

200 An mbstate_t object is used inappropriately (7.29.6). EXP33-C 

201 The value of an argument of type wint_t to a wide character classification 

or case mapping function is neither equal to the value of 

WEOF nor representable as a wchar_t (7.30.1). 

202 The iswctype function is called using a different LC_CTYPE category 

from the one in effect for the call to the wctype function that 

returned the description (7.30.2.2.1). 

203 The towctrans function is called using a different LC_CTYPE category 

from the one in effect for the call to the wctrans function that 

returned the description (7.30.3.2.1). 




Appendix D: Unspecified Behavior 

According to the C Standard, Annex J, J.1 [ISO/IEC 9899:2011], the behavior of a program is unspecified 

in the circumstances outlined the following table. The descriptions of unspecified behaviors 

in the “Description” column are direct quotes from the standard. The parenthesized numbers 

refer to the subclause of the C Standard (C11) that identifies the unspecified behavior. The 

“Guideline” column in the table identifies the coding practices that address the specific case of 

unspecified behavior (USB). 

USB Description Guideline 

1 The manner and timing of static initialization (5.1.2). 

2 The termination status returned to the hosted environment if the return 

type of main is not compatible with int (5.1.2.2.3). 

3 The values of objects that are neither lock-free atomic objects nor of 

type volatile sig_atomic_t and the state of the floating-point 

environment when the processing of the abstract machine is interrupted 

by receipt of a signal (5.1.2.3). 

4 The behavior of the display device if a printing character is written 

when the active position is at the final position of a line (5.2.2). 

5 The behavior of the display device if a backspace character is written 

when the active position is at the initial position of a line 5.2.2). 

6 The behavior of the display device if a horizontal tab character is 

written when the active position is at or past the last defined horizontal 

tabulation position (5.2.2). 

7 The behavior of the display device if a vertical tab character is written 

when the active position is at or past the last defined vertical tabulation 

position (5.2.2). 

8 How an extended source character that does not correspond to a universal 

character name counts toward the significant initial characters 

in an external identifier (5.2.4.1). 

9 Many aspects of the representations of types (6.2.6). 

10 The value of padding bytes when storing values in structures or unions 

(6.2.6.1). 

11 The values of bytes that correspond to union members other than the 

one last stored into (6.2.6.1). 

12 The representation used when storing a value in an object that has 

more than one object representation for that value (6.2.6.1). 

13 The values of any padding bits in integer representations (6.2.6.2). 

EXP39-C 





14 Whether certain operators can generate negative zeros and whether a 

negative zero becomes a normal zero when stored in an object 

(6.2.6.2). 

15 Whether two string literals result in distinct arrays (6.4.5). 

16 The order in which subexpressions are evaluated and the order in 

which side effects take place, except as specified for the function-call 

(), &&, ||, ?:, and comma operators (6.5). 

17 The order in which the function designator, arguments, and subexpressions 

within the arguments are evaluated in a function call 

(6.5.2.2). 

18 The order of side effects among compound literal initialization list 

expressions (6.5.2.5). 

19 The order in which the operands of an assignment operator are evaluated 

(6.5.16). 

20 The alignment of the addressable storage unit allocated to hold a bitfield 

(6.7.2.1). 

21 Whether a call to an inline function uses the inline definition or the 

external definition of the function (6.7.4). 

22 Whether or not a size expression is evaluated when it is part of the 

operand of a sizeof operator and changing the value of the size expression 

would not affect the result of the operator (6.7.6.2). 

23 The order in which any side effects occur among the initialization list 

expressions in an initializer (6.7.9). 

24 The layout of storage for function parameters (6.9.1). 

25 When a fully expanded macro replacement list contains a functionlike 

macro name as its last preprocessing token and the next preprocessing 

token from the source file is a (, and the fully expanded replacement 

of that macro ends with the name of the first macro and the 

next preprocessing token from the source file is again a (, whether 

that is considered a nested replacement (6.10.3). 

26 The order in which # and ## operations are evaluated during macro 

substitution (6.10.3.2, 6.10.3.3). 

27 The state of the floating-point status flags when execution passes 

from a part of the program translated with FENV_ACCESS "off" to a 

part translated with FENV_ACCESS "on" (7.6.1). 

28 The order in which feraiseexcept raises floating-point exceptions, 

except as stated in F.8.6 (7.6.2.3). 

29 Whether math_errhandling is a macro or an identifier with external 

linkage (7.12). 

EXP30-C 

EXP44-C 

DCL37-C 





30 The results of the frexp functions when the specified value is not a 

floating-point number (7.12.6.4). 

31 The numeric result of the ilogb functions when the correct value is 

outside the range of the return type (7.12.6.5, F.10.3.5). 

32 The result of rounding when the value is out of range (7.12.9.5, 

7.12.9.7, F.10.6.5). 

33 The value stored by the remquo functions in the object pointed to by 

quo when y is zero (7.12.10.3). 

34 Whether a comparison macro argument that is represented in a format 

wider than its semantic type is converted to the semantic type 

(7.12.14). 

35 Whether setjmp is a macro or an identifier with external linkage 

(7.13). 

36 Whether va_copy and va_end are macros or identifiers with external 

linkage (7.16.1). 

37 The hexadecimal digit before the decimal point when a non-normalized 

floating-point number is printed with an a or A conversion specifier 

(7.21.6.1, 7.29.2.1). 

38 The value of the file position indicator after a successful call to the 

ungetc function for a text stream, or the ungetwc function for any 

stream, until all pushed-back characters are read or discarded 

(7.21.7.10, 7.29.3.10). 

39 The details of the value stored by the fgetpos function (7.21.9.1). 

40 The details of the value returned by the ftell function for a text 

stream (7.21.9.4). 

41 Whether the strtod, strtof, strtold, wcstod, wcstof, and 

wcstold functions convert a minus-signed sequence to a negative 

number directly or by negating the value resulting from converting 

the corresponding unsigned sequence (7.22.1.3, 7.29.4.1.1). 

42 The order and contiguity of storage allocated by successive calls to 

the calloc, malloc, and realloc functions (7.22.3). 

43 The amount of storage allocated by a successful call to the calloc, 

malloc, and realloc function when 0 bytes was requested (7.22.3). 

44 Whether a call to the atexit function that does not happen before 

the exit function is called will succeed (7.22.4.2). 

45 Whether a call to the at_quick_exit function that does not happen 

before the quick_exit function is called will succeed (7.22.4.3). 

46 Which of two elements that compare as equal is matched by the 

bsearch function (7.22.5.1). 

DCL37-C 

DCL37-C 

MEM04-C 





47 The order of two elements that compare as equal in an array sorted by 

the qsort function (7.22.5.2). 

48 The encoding of the calendar time returned by the time function 

(7.27.2.4). 

49 The characters stored by the strftime or wcsftime function if any 

of the time values being converted is outside the normal range 

(7.27.3.5, 7.29.5.1). 

50 Whether an encoding error occurs if a wchar_t value that does not 

correspond to a member of the extended character set appears in the 

format string for a function in 7.29.2 or 7.29.5 and the specified semantics 

do not require that value to be processed by wcrtomb 

(7.29.1). 

51 The conversion state after an encoding error occurs (7.28.1.1, 

7.28.1.2, 7.28.1.3, 7.28.1.4, 7.29.6.3.2, 7.29.6.3.3, 7.29.6.4.1, 

7.29.6.4.2). 

52 The resulting value when the “invalid” floating-point exception is 

raised during IEC 60559 floating to integer conversion (F.4). 

53 Whether conversion of non-integer IEC 60559 floating values to integer 

raises the “inexact” floating-point exception (F.4). 

54 Whether or when library functions in raise the “inexact” 

floating-point exception in an IEC 60559 conformant implementation 

(F.10). 

55 Whether or when library functions in raise an undeserved 

“underflow” floating-point exception in an IEC 60559 conformant 

implementation (F.10). 

56 The exponent value stored by frexp for a NaN or infinity (F.10.3.4). 

57 The numeric result returned by the lrint, llrint, lround, and 

llround functions if the rounded value is outside the range of the return 

type (F.10.6.5, F.10.6.7). 

58 The sign of one part of the complex result of several math functions 

for certain exceptional values in IEC 60559 compatible implementations 

(G.6.1.1, G.6.2.2, G.6.2.3, G.6.2.4, G.6.2.5, G.6.2.6, G.6.3.1, 

G.6.4.2). 

MSC05-C

SEI CERT C Coding Standard

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