The #ifdef directive checks for the existence of macro definitions.
If the identifier specified is defined as a macro, the tokens that immediately follow the condition are passed on to the compiler.
The following example defines MAX_LEN to be 75 if EXTENDED is defined for the preprocessor. Otherwise, MAX_LEN is defined to be 50.
#ifdef EXTENDED # define MAX_LEN 75 #else # define MAX_LEN 50 #endif
Conditional
Compilation Directives
Preprocessor
Directives
#elif Preprocessor Directive
#endif Preprocessor
Directive
#if Preprocessor
Directive
#ifndef Preprocessor
Directive
List of Preprocessor
Directives
