I have done some research on this issue. The "correct" behavior seems to be to match what Windows would do in cmd.exe. That would provide the users with the least surprise.
Michael Kaplan's guidance[1] on file names seems to also describe how Windows handles names of environment variables: > For the uppercasing operation, you can use CharUpper, CharUpperBuff, or > LCMapString > with the LCMAP_UPPERCASE flag (and *without* the LCMAP_LINGUISTIC_CASING > flag!). > (emphasis in the original) To test whether the same applied to names of environment variables as well, I set the user locale to Turkish, and verified that iş was mapped to IŞ and not İŞ on a computer where the OS code page was not Turkish. The most straightforward function to use is thus CharUpperBuffW[2]. [1]: http://archives.miloush.net/michkap/archive/2005/10/17/481600.html [2]: https://msdn.microsoft.com/en-us/library/windows/desktop/ms647475(v=vs.85).aspx
