The #ifndef directive checks for the existence of macro definitions.
If the identifier specified is not defined as a macro, the tokens that immediately follow the condition are passed on to the compiler.
An identifier must follow the #ifndef keyword. The following example defines MAX_LEN to be 50 if EXTENDED is not defined for the preprocessor. Otherwise, MAX_LEN is defined to be 75.
#ifndef EXTENDED # define MAX_LEN 50 #else # define MAX_LEN 75 #endif
Conditional
Compilation Directives
Preprocessor
Directives
#elif Preprocessor Directive
#endif Preprocessor
Directive
#if Preprocessor
Directive
#ifdef Preprocessor
Directive
List of Preprocessor
Directives
