Uint32 fps_lasttime = SDL_GetTicks(); //the last recorded time.
Uint32 fps_current; //the current FPS.
Uint32 fps_frames = 0; //frames passed since the last recorded fps.

while (!quit) {
  if (update()) {
    fps_frames++;
    if (fps_lasttime < SDL_GetTicks() - 1000) {
      fps_lasttime = SDL_GetTicks();
      fps_current = fps_frames;
      fps_frames = 0;
    }
    printf("Current FPS: %d\n", fps_current);
  }
}