From: ao950 AT FreeNet DOT Carleton DOT CA (Paul Derbyshire)
Newsgroups: comp.os.msdos.djgpp
Subject: Re: Allegro: Minor nuiscances in 2.2
Date: 26 May 1997 13:36:27 GMT
Organization: The National Capital FreeNet
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Reply-To: ao950 AT FreeNet DOT Carleton DOT CA (Paul Derbyshire)
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Shawn Hargreaves (Shawn AT talula DOT demon DOT co DOT uk) writes:
> Paul Derbyshire writes:
>>The first is that calling vsync() in text mode fails. (This seems like it
>>should be expected). The problem is it fails rather unpredictably and
> 
> I'm sorry, but I don't think that really qualifies as a bug! In fact,
> almost any graphics function will crash if you call it while in text
> mode. The vsync() routine is vectored through a pointer in the
> GFX_DRIVER structure, but when gfx_driver is NULL this will end up
> referencing an address somewhere down in low memory. To be precise,
> you'll be calling whatever code is pointed to by the 32 bit int at
> memory address #20, which is liable to do all sorts of horrible
> things...
> 
> The only graphics related functions that you can safely call from text
> mode are the palette fades. I added a special case to deal with those,
> because I wanted to be able to fade out the screen when starting a
> program, but in the interests of efficiency I'm not going to add similar
> checks to any other functions. It's up to you to make sure you are in a
> suitable screen mode before you call them...

Aha. It did look like it was jumping into never-never-land. The call
sequence looked like this:

crt0_startup+19
main+12
dua_execute+54
intro_sequence+77
0x00000133 (???)

The highlighted line in intro_sequence being vsync().
Perhaps gfx_text should not set the driver to null, but a pointer to a RET...
or does that raise hairy problems with other aspects of implementation?

>>The second, is that if you have a mouse pointer up, then exit the program
>>with something like
>>
>>set_gfx_mode (GFX_TEXT,80,25,0,0);
>>exit(0);
>>
>>all you get is a silent hang... or so it seems, until you blind-type
>>something like MODE CO80 or EXIT.
> 
> Interesting :-)  Can I have some source that reproduces this, please?

Well, try this... it showed up in a large program I'm doing, but
essentially, it did this:

#include <stdio.h>
#include <conio.h>
#include <allegro.h>

// Save me as mousecra.c and compile with gcc mousecra.c -o mousecra.exe
// -lalleg, and run mousecra...
int main (int argc, char *argv[]) {
  (void)argc;(void)argv; // Pre-empt warnings
  allegro_init();
  install_timer();
  install_keyboard(); // probably not important but it was there
  install_mouse();
  set_gfx_mode (GFX_AUTODETECT,640,480,0,0);
  show_mouse(screen);
  set_gfx_mode (GFX_TEXT,80,25,0,0);
  clrscr();
  gotoxy(1,1);
  printf ("Can you see this? Try MODE CO80 if not.\n");
  return 0;
}

--
    .*.  Where feelings are concerned, answers are rarely simple [GeneDeWeese]
 -()  <  When I go to the theater, I always go straight to the "bag and mix"
    `*'  bulk candy section...because variety is the spice of life... [me]
Paul Derbyshire ao950 AT freenet DOT carleton DOT ca, http://chat.carleton.ca/~pderbysh