I believe that CVI is no different from any other C compiler in this regard. Maximum length is 63 characters for macros and internal names, and 31 characters from external names. You should be able to use longer names, but keep in mind that the compiler will only look at the first 63/31 characters so it won't be able to tell the difference between something like ReallyIncrediblyLongFunctionNameWithLotsOfCharactersThatTakesWayTooLongToType1() and ReallyIncrediblyLongFunctionNameWithLotsOfCharactersThatTakesWayTooLongToType2().