Lock the mutex.
int SDL_LockMutex(SDL_mutex * mutex);
the mutex to lock
Return 0, or -1 on error.
This will block until the mutex is available, which is to say it is in the unlocked state and the OS has chosen the caller as the next thread to lock it. Of all threads waiting to lock the mutex, only one may do so at a time.
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").
This function is available since SDL 2.0.0.