See if a semaphore has a positive value and decrement it if it does.
int SDL_SemTryWait(SDL_sem * sem);
the semaphore to wait on
Returns 0 if the wait succeeds,
SDL_MUTEX_TIMEDOUT if the wait would block, or a negative error code on failure; call SDL_GetError() for more information.
This function checks to see if the semaphore pointed to by
sem has a positive value and atomically decrements the semaphore value if it does. If the semaphore doesn't have a positive value, the function immediately returns SDL_MUTEX_TIMEDOUT.
This function is available since SDL 2.0.0.