Joystick example / 01-joystick-polling

Mathias · April 4, 2025, 4:13pm

Why are one and wing initialized with 640 and 480, even though they are later queried with SDL_GetWindowSize.

SDL_AppResult SDL_AppIterate(void *appstate)
{
    int winw = 640, winh = 480;
...
    SDL_GetWindowSize(window, &winw, &winh);

github.com/libsdl-org/SDL

examples/input/01-joystick-polling/joystick-polling.c

main

/*
 * This example code looks for the current joystick state once per frame,
 * and draws a visual representation of it.
 *
 * This code is public domain. Feel free to use it for any purpose!
 */

/* Joysticks are low-level interfaces: there's something with a bunch of
   buttons, axes and hats, in no understood order or position. This is
   a flexible interface, but you'll need to build some sort of configuration
   UI to let people tell you what button, etc, does what. On top of this
   interface, SDL offers the "gamepad" API, which works with lots of devices,
   and knows how to map arbitrary buttons and such to look like an
   Xbox/PlayStation/etc gamepad. This is easier, and better, for many games,
   but isn't necessarily a good fit for complex apps and hardware. A flight
   simulator, a realistic racing game, etc, might want this interface instead
   of gamepads. */

/* SDL can handle multiple joysticks, but for simplicity, this program only
   deals with the first stick it sees. */

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flowCRANE · April 4, 2025, 8:14pm

Because SDL_GetWindowSize() can fail and if so, the values of both variables would not be set by this function. And this would cause further calculations to be based on uninitialized variables, and therefore on garbage data, which could lead to the application crashing.

flowCRANE · April 5, 2025, 3:09pm

One more note — it seems that if SDL is initialized and the window context is created correctly, there is no need to check the result of the SDL_GetWindowSize() function, because in such a case it will never fail (it only copies data from the window object field to the pointers given in the parameters).

github.com/libsdl-org/SDL

src/video/SDL_video.c

33f90f2e4


      
          bool SDL_GetWindowSize(SDL_Window *window, int *w, int *h)
          {
              CHECK_WINDOW_MAGIC(window, false);
              if (w) {
                  *w = window->w;
              }
              if (h) {
                  *h = window->h;
              }
              return true;
          }