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The arguments to a macro should must not include preprocessor directives, such as #define, #ifdef, and #include. Doing so is results in undefined behavior, according to subclause the C Standard, 6.10.35,  paragraph paragraph 11 , of the C Standard [ISO/IEC 9899:20112024]:

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 of Annex J.)

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

Noncompliant Code Example

In this noncompliant code example [GCC Bugs], the author 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

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Solution

In this compliant solution [GCC Bugs], 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.

Automated Detection

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Related Vulnerabilities

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

Bibliography

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