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Wiki MarkupThe arguments to a macro should must not include preprocessor directives , such as {{\#define}}, {{\#ifdef}}, and {{\#include}}. &nbsp; Doing so is [undefined behavior|BB. Definitions#undefined behavior] according to section so results in undefined behavior, according to the C Standard, 6.10.35, paragraph 11 of the C99 standard \[ [ISO/IEC 9899:1999|AA. Bibliography#ISO/IEC 9899-1999]\] (see also [undefined behavior 87 | CC. Undefined Behavior#ub_87] of Annex J): <blockquote><p>The sequence of preprocessing tokens bounded by the outside-most matching parentheses forms the list of arguments for the function-like macro.&nbsp; The individual arguments within the list are separated by comma preprocessing tokens, but comma preprocessing tokens between matching inner parentheses do not separate arguments.&nbsp; <strong>If there are sequences of preprocessing tokens within the list of arguments that would otherwise act as preprocessing directives, the behavior is undefined.</strong></p></blockquote>The scope of this rule includes using preprocessor directives in arguments to a function where it is unknown whether or not the function is implemented using a macro.&nbsp; For example, standard library functions such as {{memcpy()}}, {{printf()}}, and {{assert()}} may be implemented as macros.

Noncompliant Code Example

In this noncompliant code example \[[GCC Bugs|http://gcc.gnu.org/bugs.html#nonbugs_c]\], the author uses preprocessor directives to specify platform-specific arguments to {{memcpy()}}. However, if {{memcpy()}} is implemented using a macro, the code results in undefined behavior. 

2024]:

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.

See also undefined behavior 93.

This rule also applies to the use of preprocessor directives in arguments to any function where it is unknown whether or not the function is implemented using a macro. This includes all standard library functions, such as memcpy(), printf(), and assert(), because any standard library function may be implemented as a macro. (C24, 7.1.4, paragraph 1).

Noncompliant Code Example

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 <string.h>
 
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
  );
  /* ... */
}

Compliant

...

Solution

In this compliant solution [GCC Bugs], the appropriate call to memcpy() is determined outside the function call: Wiki MarkupIn this compliant solution \[[GCC Bugs|http://gcc.gnu.org/bugs.html#nonbugs_c]\], the appropriate call to {{memcpy()}} is determined outside the function call.

#include <string.h>

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
  /* ... */
}

Risk Assessment

Improper use of macros may result in Including preprocessor directives in macro arguments is undefined behavior.

References

\[[GCC Bugs|http://gcc.gnu.org/bugs.html#nonbugs_c]\] "Non-bugs"
\[[ISO/IEC 9899:1999|AA. Bibliography#ISO/IEC 9899-1999]\] Section 6.10.3.1, "Argument substitution," paragraph 11

Automated Detection

Related Vulnerabilities

Search for vulnerabilities resulting from the violation of this rule on the CERT website.

Bibliography

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Image Added Image Added Image AddedPRE31-C. Avoid side-effects in arguments to unsafe macros      01. Preprocessor (PRE)      02. Declarations and Initialization (DCL)