If the condition specified in the #if, #ifdef, or #ifndef directive evaluates to 0, and the conditional compilation directive contains a preprocessor #else directive, the source text located between the preprocessor #else directive and the preprocessor #endif directive is selected by the preprocessor to be passed on to the compiler.
Conditional
Compilation Directives
Preprocessor
Directives
#elif Preprocessor Directive
#endif Preprocessor
Directive
#if Preprocessor
Directive
#ifdef Preprocessor
Directive
#ifndef Preprocessor
Directive
List of Preprocessor
Directives
