#ifndef
#ifndef <token>
/* code */
#else
/* code to include if the token is defined */
#endif
#ifndef checks whether the given token has been #defined earlier in the file or
in an included file; if not, it includes the code between it and the closing
#else or, if no #else is present, #endif statement. #ifndef is often used to
make header files
idempotent by
defining a token once the file has been included and checking that the token
was not set at the top of that file.
#ifndef _INCL_GUARD
#define _INCL_GUARD
#endif
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