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Else

The #else directive can be added to a conditional to provide alternative text to be used if the condition fails. This is what it looks like:

     #if expression
     text-if-true
     #else /* Not expression */
     text-if-false
     #endif /* Not expression */
     

If expression is nonzero, the text-if-true is included and the text-if-false is skipped. If expression is zero, the opposite happens.

You can use #else with #ifdef and #ifndef, too.