If you unlock an already unlocked mutex, is the behavior undefined?

If you unlock an already unlocked mutex, is the behavior unsafe, safe, or undefined?

The purpose of the question is related to the following code, where I don't know if it would be better to unlock the mutexes within the if block, or just outside the if block.

    // This chunk of code makes dual locking semi-autonomous.
    int c_lckd = 0, q_lckd = 0;
    if (pthread_mutex_trylock(&crunch_mutex) == 0) c_lckd = 1;
    if (pthread_mutex_trylock(&queue_mutex) == 0) q_lckd = 1;
    if (q_lckd && !c_lckd) { QUEUE_UNLOCK; q_lckd = 0; }
    else if (c_lckd && !q_lckd) { CRUNCH_UNLOCK; c_lckd = 0; }

    if (c_lckd && q_lckd) {
      printf("cr = %d, max = %d, cnt = %d\n",
        crunching, max_crunching, queue_count(conn_queue));
      if (crunching < max_crunching && queue_count(conn_queue)) {
        pthread_t tid =
          pthread_create(
            &tid,
            NULL,
            crunch_conn,
            (void *)queue_dequeue(conn_queue)
          );
        crunching++;
      }
      CRUNCH_UNLOCK QUEUE_UNLOCK
    }

Thanks, Chenz

For pthreads it will result in undefined behaviour if the mutex type is PTHREAD_MUTEX_NORMAL or PTHREAD_MUTEX_DEFAULT (with non-robust implementation), or will return an error if the mutex type is PTHREAD_MUTEX_RECURSIVE or PTHREAD_MUTEX_ERRORCHECK

From the man page for pthread_mutex_unlock:

If a thread attempts to unlock a mutex that it has not locked or a mutex which is unlocked, pthread_mutex_unlock() shall behave as described in the Unlock When Not Owner column of the following table.

Mutex Type	Robustness	Unlock When Not Owner
NORMAL	non-robust	undefined behavior
NORMAL	robust	error returned
ERRORCHECK	either	error returned
RECURSIVE	either	error returned
DEFAULT	non-robust	undefined behavior
DEFAULT	robust	error returned

Other mutexes will have their own behviour. As others have stated, it's best to read the manual for whichever mutex you're using.