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The arguments to a macro should must not include preprocessor directives, such as #define, #ifdef, and #include. Doing so results in undefined behavior, according to the C Standard, 6.10.5, paragraph 11 [ISO/IEC 9899:2024]:

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   Doing so is undefined behavior.  This includes using preprocessor directives in arguments to a any function where it is unknown whether or not the function is implemented using a macro.   Examples include 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

Wiki MarkupIn this noncompliant code example \[ [Non-bugs in GCC C|http://gcc.gnu.org/bugs.html#nonbugs_c]\], the author is attempting to specify an argument to {{memcpy()}} depending on the current platform by using preprocessor directives within the function call.  However, if {{memcpy()}} is implemented using a macro, the code will result in undefined 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 \[[Non-bugs in GCC C|http://gcc.gnu.org/bugs.html#nonbugs_c]\], 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
  /* ... */

References

}

Risk Assessment

Including preprocessor directives in macro arguments is undefined behavior.

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 Added Wiki Markup\[[Non-bugs in GCC C|http://gcc.gnu.org/bugs.html#nonbugs_c]\]