PRE32-C. Do not use preprocessor directives inside macro arguments

The arguments to a macro should not include preprocessor directives such as #define, #ifdef, and #include.  Doing so is undefined behavior according to section 6.10.3.11 of the C99 standard [[ISO/IEC 9899:1999]]:

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.

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.  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], the author uses preprocessor directives to specify platform-specific arguments to memcpy().  However, if memcpy() is implemented using a macro, the code will result in undefined behavior.  For example, this code will compile using GCC version 3.3 and later, but will not compile using GCC versions prior to 3.3 if memcpy() is a macro.

   memcpy(dest, src,
#ifdef PLATFORM1
	 12
#else
	 24
#endif
	);

Compliant Code Example

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

#ifdef PLATFORM1
   memcpy(dest, src, 12);
#else
   memcpy(dest, src, 24);
#endif

Risk Assessment

Improper use of macros may result in undefined behavior.

References

[GCC Bugs] "Non-bugs"
[[ISO/IEC 9899:1999]] Section 6.10.3.11, "Macro replacement"