#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 Related C preprocessor tutorial |
