Testers who have their own games, or other people who tested your game? If your game is not working on your computer then it’s not likely to work elsewhere. The only thing I can think of off the top of my head is that if your executable entry point spawns a subprocess it’s possible that the subprocess will not have access to the environment variables from the parent process.
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Testers with my game, yes. Just wanted to make sure it's not an issue with my machine.
Are the environment variables supposed to be exclusive to the process and thus not visible through, say, the Windows command prompt? In that case I have been approaching the whole process in the wrong way (treated the evnironment variables as a global thing).