The #ifndef directive

The #ifndef directive checks whether a macro is not defined.

If the identifier specified is not defined as a macro, the lines of code immediately follow the condition are passed on to the compiler.

#ifndef directive syntax

Read syntax diagramSkip visual syntax diagram#ifndefidentifier
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