Yes it is, if you program application software for Linux (and your other two supported platforms are Windows and macOS, as is the case for OP and their colleagu

Yes it is, if you program application software for Linux (and your other two supported platforms are Windows and macOS, as is the case for OP and their colleague who appear to work for Valve, supporting Steam) And the reason the colleague dubs it the worst has nothing to do with its specification in POSIX (which doesn't require the function to be thread-safe but also doesn't prevent it from being made thread-safe), but rather its specific implementation in glibc, which is the C library in use on all the Linux distributions that Valve support Steam on and all are equally dubbed as "Linux"