Vsync-Dependent Game Loop

no-reply@allegro.cc (DragonDePlatino) — Wed, 14 Dec 2016 03:21:09 +0000

I recently started C programming so I decided to check out allegro 5. I've statically linked everything and confirmed allegro is correctly installed by running some examples.

I want to create a simple game loop tied to vsync but I cannot do so. I've tried every combination of these I can think of but it always crashes the moment an event is found. I've tried debugging but it fails with [Inferior 1 (process 2552) exited with code 03] every time I try to add an event loop.

To be clear I do not want delta time, I do not want to use timers and I do not want to pause my program's execution to wait for events. I want an SDL-style event loop where I continually render at 60 FPS regardless if there are any events in the queue. I've done this in the past with SDL 2.0, SFML and CSFML but I cannot do it with allegro.

#SelectExpand
  1#include 
  2#include 
  3
  4static ALLEGRO_DISPLAY*    display;
  5static ALLEGRO_EVENT_QUEUE* window_queue;
  6
  7static void init(void);
  8static void quit(void);
  9static void loop(void);
 10
 11int main(void)
 12{
 13  init();
 14  loop();
 15  quit();
 16}
 17
 18void init(void) {
 19  if (!al_init()) {
 20    puts("Failed to initialize allegro!");
 21    exit(1);
 22  }
 23
 24  al_set_new_display_option(ALLEGRO_VSYNC, 1, ALLEGRO_REQUIRE);
 25  display = al_create_display(640, 480);
 26  if (!display) {
 27    puts("Failed to create display!");
 28    exit(1);
 29  }
 30
 31  window_queue = al_create_event_queue();
 32  if (!window_queue) {
 33    puts("Failed to create event queue!");
 34    al_destroy_display(display);
 35    exit(1);
 36  }
 37  al_register_event_source(window_queue, al_get_display_event_source(display));
 38}
 39
 40void quit(void) {
 41  al_destroy_display(display);
 42  al_destroy_event_queue(window_queue);
 43}
 44
 45void loop(void) {
 46  bool window_open = true;
 47  ALLEGRO_EVENT* event = NULL;
 48  int value = 0;
 49
 50  while (window_open) {
 51    while (al_get_next_event(window_queue, event)) {
 52      if (event->type == ALLEGRO_EVENT_DISPLAY_CLOSE) {
 53        window_open = false;
 54      }
 55    }
 56
 57    value = (value + 1) % 256;
 58    al_clear_to_color(al_map_rgb(value, value, value));
 59    al_flip_display();
 60  }
 61}

And yes, I am aware that my game's speed will be tied to framerate and that the game will not even boot if the user does not have vsync.

no-reply@allegro.cc (Edgar Reynaldo) — Wed, 14 Dec 2016 03:58:18 +0000

DragonDePlatino said:

 ALLEGRO_EVENT* event = NULL;
 int value = 0;
 while (window_open) {
  while (al_get_next_event(window_queue, event)) 
   {

You're passing NULL to al_get_next_event. You need to declare an ALLEGRO_EVENT and pass its address not an empty pointer.

no-reply@allegro.cc (DragonDePlatino) — Wed, 14 Dec 2016 04:19:43 +0000

Hey, I got it working! Keyboard input too! Thanks for your help.

#SelectExpand
  1#include 
  2#include 
  3
  4static ALLEGRO_DISPLAY*    display;
  5static ALLEGRO_EVENT_QUEUE* event_queue;
  6
  7static void init(void);
  8static void quit(void);
  9static void loop(void);
 10
 11int main(void)
 12{
 13  init();
 14  loop();
 15  quit();
 16}
 17
 18void init(void) {
 19  if (!al_init()) {
 20    puts("Failed to initialize allegro!");
 21    exit(1);
 22  }
 23
 24  if(!al_install_keyboard()) {
 25    puts("Failed to initialize keyboard!");
 26    exit(1);
 27  }
 28
 29  al_set_new_display_option(ALLEGRO_VSYNC, 1, ALLEGRO_REQUIRE);
 30  display = al_create_display(640, 480);
 31  if (!display) {
 32    puts("Failed to create display!");
 33    exit(1);
 34  }
 35
 36  event_queue = al_create_event_queue();
 37  if (!event_queue) {
 38    puts("Failed to create event queue!");
 39    al_destroy_display(display);
 40    exit(1);
 41  }
 42  al_register_event_source(event_queue, al_get_display_event_source(display));
 43  al_register_event_source(event_queue, al_get_keyboard_event_source());
 44}
 45
 46void quit(void) {
 47  al_destroy_display(display);
 48  al_destroy_event_queue(event_queue);
 49}
 50
 51void loop(void) {
 52  bool window_open = true;
 53  ALLEGRO_EVENT event;
 54  int value = 0;
 55
 56  while (window_open) {
 57    while (al_get_next_event(event_queue, &event)) {
 58      if (event.type == ALLEGRO_EVENT_DISPLAY_CLOSE) {
 59        window_open = false;
 60      } else if (event.type == ALLEGRO_EVENT_KEY_DOWN) {
 61        switch (event.keyboard.keycode) {
 62        case ALLEGRO_KEY_ESCAPE:
 63          window_open = false;
 64        }
 65      }
 66    }
 67
 68    value = (value + 1) % 256;
 69    al_clear_to_color(al_map_rgb(value, value, value));
 70    al_flip_display();
 71  }
 72}