The #else directive

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 lines of code located between the preprocessor #else directive and the preprocessor #endif directive is selected by the preprocessor to be passed on to the compiler.

#else directive syntax
 
            .----------------.
            V                |
>>-#--else----token_sequence-+--newline_character--------------><