SDL_AddTimer man page
SDL_AddTimer — Add a timer which will call a callback after the specified number of milliseconds has elapsed.
SDL_TimerID SDL_AddTimer(Uint32 interval, SDL_NewTimerCallback callback, void *param);
/* type definition for the "new" timer callback function */ typedef Uint32 (*SDL_NewTimerCallback)(Uint32 interval, void *param);
Adds a callback function to be run after the specified number of milliseconds has elapsed. The callback function is passed the current timer interval and the user supplied parameter from the SDL_AddTimer call and returns the next timer interval. If the returned value from the callback is the same as the one passed in, the periodic alarm continues, otherwise a new alarm is scheduled.
To cancel a currently running timer call SDL_RemoveTimer with the timer ID returned from SDL_AddTimer.
The timer callback function may run in a different thread than your main program, and so shouldn't call any functions from within itself. You may always call SDL_PushEvent, however.
The granularity of the timer is platform-dependent, but you should count on it being at least 10 ms as this is the most common number. This means that if you request a 16 ms timer, your callback will run approximately 20 ms later on an unloaded system. If you wanted to set a flag signaling a frame update at 30 frames per second (every 33 ms), you might set a timer for 30 ms (see example below). If you use this function, you need to pass SDL_INIT_TIMER to SDL_Init.
Returns an ID value for the added timer or NULL if there was an error.
my_timer_id = SDL_AddTimer((33/10)*10, my_callbackfunc, my_callback_param);