Unlock the mutex.
int SDL_UnlockMutex(SDL_mutex * mutex);
the mutex to unlock.
Returns 0, or -1 on error.
It is legal for the owning thread to lock an already-locked mutex. It must unlock it the same number of times before it is actually made available for other threads in the system (this is known as a "recursive mutex").
It is an error to unlock a mutex that has not been locked by the current thread, and doing so results in undefined behavior.
It is also an error to unlock a mutex that isn't locked at all.
This function is available since SDL 2.0.0.