#elif constant-expression

The #elif directive marks a region of conditional compilation. Every #elif must be paired with an introductory directive: #if, #ifdef, or #ifndef. If the initial condition was false (0), and every subsequent #elif condition is false, and constant-expression is true (nonzero), subsequent statements are compiled until the next #elif, #else, or #endif directive is reached for this level of nesting.

See Also

#else directive, #if directive