SUSv2 says that the _toupper()/_tolower() macros are the same as toupper()/tolower() except the argument must be an opposite-case letter. Failure modes are undefined. Later POSIX specs clarifies that it's the application's responsibility to ensure to correct input to these macros. Therefore I think a simple _to -> to alias to implement this doesn't violate the specs, they will just return correct results for the undefined range of inputs.