The
C Preprocessor, a
program notionally run before the
C compiler cc. The
preprocessor removes
comments, arranges for
#includes to be included, deals with
conditional compilation via the
#if,
#ifdef,
#endif,
#else and
#elif directives, and expands
#define macros. However,
sizeof is
not expanded in time to be used for conditional compilation.
By the "as if" rule, an ANSI C standard compiler need not in fact invoke a separate cpp program. If you need to run just the preprocessor, it may often be invoked using your C compiler's -E flag.