A lot of comments in here seem to be saying that "setenv is rarely used, so we can make setenv threadsafe even if it is costly", but I think that's missing the

A lot of comments in here seem to be saying that "setenv is rarely used, so we can make setenv threadsafe even if it is costly", but I think that's missing the point. The thread-safety is in the operators getenv/setenv (and putenv and unsetenv). Thread safety has to apply to all operators, and the functionality of getenv() (which is by far the most commonly used of these operators) is what has to be fixed. You simply cannot make setenv() thread-safe so long as getenv() has its current interface. You need to make getenv() safe first; ideally with a getenv_r() call that fills in a user-supplied buffer. From that point making the rest of the calls thread safe is trivial.