PRE13-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 [[ISO/IEC 9899:1999]].  This includes using preprocessor directives in arguments to a function where it is unknown whether or not the function is implemented using a macro.  Examples include standard library functions such as memcpy(), printf(), and assert().

Noncompliant Code Example

In this noncompliant code example [GCC Bugs], 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 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.

   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, "Macro replacement"