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#if                      Conditional Compilation by Expression

 #if  constant-expression

    The #if directive, along with #else, #elif and #endif, controls
    compilation in a selective manner.  These directives are used to
    identify a group of source lines which are included or excluded from
    the compilation based on one or more criteria.  If constant-
    expression has the value true (non-zero), the group of lines
    following the #if is included in the compilation.

      Notes:    constant-expression may not contain the sizeof operator,
                type casts or enumeration constants.

                If constant-expression contains a macro which is
                currently undefined, that macro is treated as though it
                were defined with a value of zero.

                #if directives can be nested up to 32 levels deep.

                Although comments may not be nested, you can use
                #if 0 ... #endif to disable a block of code containing
                comments.