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.
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