### #indef Preprocessor Directive

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