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.

#ifdef directive syntax
.----------------.
V |
>>-#--ifdef--identifier----token_sequence-+--------------------->
>--newline_character-------------------------------------------><
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