#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
