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.

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