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SDL Wiki

SDL_AddTimer

Call a callback function at a future time.

Syntax

SDL_TimerID SDL_AddTimer(Uint32 interval,
                         SDL_TimerCallback callback,
                         void *param);

Function Parameters

interval

the timer delay, in milliseconds, passed to callback

callback

the SDL_TimerCallback function to call when the specified interval elapses

param

a pointer that is passed to callback

Return Value

Returns a timer ID or 0 if an error occurs; call SDL_GetError() for more information.

Remarks

If you use this function, you must pass SDL_INIT_TIMER to SDL_Init().

The callback function is passed the current timer interval and the user supplied parameter from the SDL_AddTimer() call and should return the next timer interval. If the value returned from the callback is 0, the timer is canceled.

The callback is run on a separate thread.

Timers take into account the amount of time it took to execute the callback. For example, if the callback took 250 ms to execute and returned 1000 (ms), the timer would only wait another 750 ms before its next iteration.

Timing may be inexact due to OS scheduling. Be sure to note the current time with SDL_GetTicks() or SDL_GetPerformanceCounter() in case your callback needs to adjust for variances.

Code Examples

/* Start the timer; the callback below will be executed after the delay */

Uint32 delay = (33 / 10) * 10;  /* To round it down to the nearest 10 ms */
SDL_TimerID my_timer_id = SDL_AddTimer(delay, my_callbackfunc, my_callback_param);

...

Uint32 my_callbackfunc(Uint32 interval, void *param)
{
    SDL_Event event;
    SDL_UserEvent userevent;

    /* In this example, our callback pushes an SDL_USEREVENT event
    into the queue, and causes our callback to be called again at the
    same interval: */

    userevent.type = SDL_USEREVENT;
    userevent.code = 0;
    userevent.data1 = NULL;
    userevent.data2 = NULL;

    event.type = SDL_USEREVENT;
    event.user = userevent;

    SDL_PushEvent(&event);
    return(interval);
}

Note that it is possible to avoid the multithreading problems with SDL timers by giving to userevent.data1 the address of a function you want to be executed and to userevent.data2 its params, and then deal with it in the event loop.

/* with the same code as before: */
Uint32 my_callbackfunc(Uint32 interval, void *param)
{
    SDL_Event event;
    SDL_UserEvent userevent;

    /* In this example, our callback pushes a function
    into the queue, and causes our callback to be called again at the
    same interval: */

    userevent.type = SDL_USEREVENT;
    userevent.code = 0;
    userevent.data1 = &my_function;
    userevent.data2 = param;

    event.type = SDL_USEREVENT;
    event.user = userevent;

    SDL_PushEvent(&event);
    return(interval);
}

/* Now the event loop */
SDL_Event event;
while (SDL_PollEvent (&event))
{
    switch(event.type)
    {
        case SDL_USEREVENT: {
            /* and now we can call the function we wanted to call in the timer but couldn't because of the multithreading problems */
            void (*p) (void*) = event.user.data1;
            p(event.user.data2);
            break;
        }
        /* ... */
    }
}
SDL_RemoveTimer

CategoryAPI, CategoryTimer


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