Actually I don't believe that's the case. The getenv function as described by ISO C cannot be safely used in a program that only uses getenv, if that program uses ISO C threads, and more than one threat calls getenv without synchronizing with the others.
I don't think POSIX fixes this: it doesn't specify that the environ array is protected against concurrent access.
If two threads call getenv right around the same time, one of them could invalidate the environ array just as the other one has started to traverse it.
If you want to be safe, copy the environment to a different data structure on program startup. Then have all your threads refer to that data structure.
Hmm, I'm apparently correct for C++11, where calling getenv only is thread safe, but that's not guaranteed by earlier standards (or, as far as I can tell, by C or POSIX).
I don't think POSIX fixes this: it doesn't specify that the environ array is protected against concurrent access.
If two threads call getenv right around the same time, one of them could invalidate the environ array just as the other one has started to traverse it.
If you want to be safe, copy the environment to a different data structure on program startup. Then have all your threads refer to that data structure.