SDL_mutexP- Lock a mutex
int SDL_mutexP(SDL_mutex *mutex);
Locks the mutex, which was previously created with
SDL_!CreateMutex?. If the mutex is already locked then
SDL_mutexP will not return until it is
unlocked. Returns 0 on success, or -1
on an error.
SDL also defines a macro #define SDL_!LockMutex?(m)