I think you're confusing setting the environment before running a process, with setting the environment _within_ the process. If you're running a shell session, or even a compiled process which is just a "runner" for some other process - then certainly, we all do "export SOME_SETTING=value" and run things. But if you're writing a C library, which could well be used in a multi-threaded environment - you don't need to "adjust" anything, and should not invoke setenv. If your library is not pleased with the settings of another library, then it should start returning errors, or even exit() if you're a violent kind of a guy - but not setenv().