### #ifdef Preprocessor Directive

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