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defined                  defined Preprocessor Operator

 defined(identifier)

    This preprocessor-only operator tests whether the identifier argument
    is currently defined as a macro. The expression evaluates to true
    (non-zero) or  false (zero). The operator is used with #if (instead
    of nested #ifdef or #ifndef directives).

      Notes:    The operator may occur multiple times in a boolean
                expression as part of the #if and #elif directives. For
                example, #if defined(VERSION3) && !defined(DEBUG) ...
                #elif defined(DEBUG) ...

                Any construct involving defined can be rewritten using
                #ifdef and #ifndef.

                Although defined is not a reserved word in C, it is
                strongly suggested you not use it as the name of an
                identifier.