The #ifdef directive checks for the existence of macro definitions.
If the identifier specified is defined as a macro, the lines of code that immediately follow the condition are passed on to the compiler. You must use the #endif directive to end the conditional compilation directive.
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