The #warning directive (IBM extension)

A preprocessor warning directive causes the preprocessor to generate a warning message but allows compilation to continue. The argument to #warning is not subject to macro expansion.

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#warning directive syntax

               .--------------------.   
               V                    |   
>>-#--warning----preprocessor_token-+--------------------------><

The preprocessor #warning directive is a language extension provided to facilitate handling programs developed with GNU C. The IBM implementation preserves multiple white spaces.