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
.----------------.
V |
>>-#--ifndef--identifier----token_sequence-+-------------------->
>--newline_character-------------------------------------------><
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