OpenGL forward compatible context initialization

OpenGL forward compatible context initialization

Tim Spain

Member #12,201

August 2010

Salutations!

I'm new to Allegro and am trying to use it to create an OpenGL 3+ context. Which works, using the code below:

#SelectExpand
  1#include <GL/gl.h>
  2#include <allegro5/allegro.h>
  3#include <stdio.h>
  4#include <stdlib.h>
  5
  6int main (int argc, char** argv)
  7{
  8  if (!al_init())
  9    exit(1);
 10
 11  al_set_new_display_flags( ALLEGRO_WINDOWED |
 12          ALLEGRO_OPENGL |
 13          ALLEGRO_NOFRAME );
 14
 15  ALLEGRO_DISPLAY* dpy = al_create_display(1, 1);
 16  al_set_new_display_flags(0);
 17
 18  al_set_new_display_flags( ALLEGRO_WINDOWED
 19        | ALLEGRO_OPENGL_3_0
 20      /*| ALLEGRO_OPENGL_FORWARD_COMPATIBLE*/ );
 21
 22  ALLEGRO_DISPLAY* dpy3 = al_create_display(800, 600);
 23
 24  al_destroy_display(dpy);
 25
 26  printf("OpenGL version: %s\n", glGetString(GL_VERSION) );
 27  printf("GLSL   version: %s\n", glGetString(GL_SHADING_LANGUAGE_VERSION) );
 28
 29  al_destroy_display(dpy3);
 30
 31  return 0;
 32}

This compiles, links and returns:

OpenGL version: 3.0.0 NVIDIA 256.44
GLSL   version: 1.30 NVIDIA via Cg compiler

However, when I try to initialize a forward compatible context by uncommenting ALLEGRO_OPENGL_FORWARD_COMPATIBLE, the program crashes with the output

opengl3.3.exe: /home/tim/ccpp/allegro-4.9.21/src/opengl/extensions.c:153: print_extensions: Assertion `extension' failed.
Aborted

where the assertion is checking for an empty string of OpenGL extensions.

So:

What am I doing wrong? What is the correct way to request a forward compatible context?
Or is it Allegro that is doing something wrong?

Also, I have two other questions that I wouldn't mind an answer to:

Is it necessary to do both the two al_create_display()s? On my installation it crashes if I do not first create the OpenGL 2 context. Is this the expected behaviour for allegro?
How do I request a OpenGL version of, for example, 3.3? The glxinfo output claims to be capable of that.

Speaking of which, I am using Allegro 4.9.21 (from sourceforge 2010-08-17), built with GCC 4.3.3 on Ubuntu 9.4 (Jaunty) with the Nvidia 256.44 drivers.

Any and all answers are greatly appreciated!

Peter Wang

Member #23

April 2000

The assertion will be because Allegro internally calls glGetString(GL_EXTENSIONS); which is deprecated in OpenGL 3. So you aren't doing anything wrong, it's a bug. You shouldn't need to create two displays either.

We'll need to think about the API for requesting later OpenGL versions as well.

You're probably the first to try OpenGL 3 support...

Tim Spain

Member #12,201

August 2010

Thanks for the prompt reply, Peter.

It is good to know that I am not doing it wrong...