Solaris/Illumos has a thread-safe setenv()/unsetenv()/putenv(). There is no reason not to have it be thread-safe, though it must leak (Solaris/Illumos retains

Solaris/Illumos has a thread-safe setenv()/unsetenv()/putenv(). There is no reason not to have it be thread-safe, though it must leak (Solaris/Illumos retains the references to the deleted envs so as to fool memory debuggers into thinking they are still referenced and so quiet what would be essentially false positives).