Conditional Compilation

Conditional compilation is useful for things like machine-dependencies, debugging, and for setting certain options at compile-time. Beware of conditional compilation. Various controls can easily combine in unforeseen ways. If you #ifdef machine dependencies, make sure that when no machine is specified, the result is an error, not a default machine. (Use ``#error'' and indent it so it works with older compilers.) If you #ifdef optimizations, the default should be the unoptimized code rather than an uncompilable program. Be sure to test the unoptimized code.

Note that the text inside of an #ifdeffed section may be scanned (processed) by the compiler, even if the #ifdef is false. Thus, even if the #ifdeffed part of the file never gets compiled (e.g., ),"#ifdefCOMMENT" it cannot be arbitrary text.

Put #ifdefs in header files instead of source files when possible. Use the #ifdefs to define macros that can be used uniformly in the code. For instance, a header file for checking memory allocation might look like (omitting definitions for REALLOC and FREE):

#ifdef DEBUG
	extern void *mm_malloc();
#	define MALLOC(size) (mm_malloc(size))
#else
	extern void *malloc();
#	define MALLOC(size) (malloc(size))
#endif

Conditional compilation should generally be on a feature-by-feature basis. Machine or operating system dependencies should be avoided in most cases.

#ifdef 4BSD
	long t = time ((long *)NULL);
#endif