gmtime_r should be thread-safe, but it calls tzset_internal which calls getenv("TZ") which is non-thread-safe. a multi-threaded application may modify the TZ env var through the environ pointer if there is no concurrent access to this env var in other threads. gmtime_r is not specified to access TZ, if it does, that can introduce a data race in a conforming application and gmtime_r may misbehave if it observes TZ in an inconsistent state.
Dup of 14023 *** This bug has been marked as a duplicate of bug 14023 ***
getenv is thread-safe (so long as other threads aren't modifying the environment), so if getenv() was the problem, this bug would be invalid. But tzset_internal writes globals, and is definitely not thread-safe.
i used the wrong words there. getenv is thread-safe, but an application is allowed to modify an env var concurrently with a thread-safe function except if the function is specified to access the environment. gmtime_r is not specified to access the env, so it is not allowed to call getenv. (i.e. unconditionally thread-safe functions are not allowed to call conditionally thread-safe ones.)