The documentation for setenv:
Caveats: POSIX.1 does not require setenv() or unsetenv() to be reentrant.
...
Interface: setenv(), unsetenv()
Attribute: Thread safety
Value: MT-Unsafe const:env
Libraries that are thread-safe DO provide that documentation. One assumes that
libraries that don't provide that documentation are not thread=safe.
GnuTLS docs: The GnuTLS library is thread safe by design, meaning that objects of the library such as TLS sessions, can be safely divided across threads as long as a single thread accesses a single object.
