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Message: [QUOTE="Michael Wojcik, post: 2446979"] It's my understanding that this invokes undefined behavior. Whether it's a problem for any actual existing implementation is another question, of course. Redefining a keyword is only "legal" because they're only reserved in translation phases 7 and 8 (C99 6.4.1#2). (This is a "shall" requirement, so violating it causes UB, by 4#2.) Since macros are expanded in translation phase 4, by the time phase 7 is reached, macros that have the same names as keywords (eg "#define if foo") no longer appear as identifiers in the translation unit. No such special rule exists for other reserved identifiers, as far as I can see. [/QUOTE]