no-reply@allegro.cc (tobing) — Wed, 09 Aug 2017 14:11:45 +0000

Hi, today I debugged into an issue and here's what I found.

Windows 10, Intel graphics.

The laptop has three displays which are enumerated in wsystem.c, and unfortunately only one has the flag DISPLAY_DEVICE_ATTACHED_TO_DESKTOP set. So the reported number of display drivers is 1, but the adapter 0 is not connected to the desktop, which results in win_get_monitor_info reporting a size of 0,0.

So there is an implicit assumption: all enumerated displays which are connected to the desktop are listed first.

I'm not sure about the linux part, but on windows well it's not always true.

As a side note, this happens when I set the laptop to sleep and take put it back into the docking station which is connected to a 4k monitor...

The solution is to use the same logic within win_get_monitor_info as is used in win_get_num_video_adapters to find the nth display that is connected to a desktop.

Here's the updated code (on recent (liballeg/master) wsystem.c line 484ff:

#SelectExpand
  1/* new function */
  2static int win_find_nth_adapter_with_desktop(DISPLAY_DEVICE* pdd, int adapter)
  3{
  4    int count = 0;
  5    int c = 0;
  6
  7    while(EnumDisplayDevices(NULL, count, pdd, 0)) {
  8        if(pdd->StateFlags & DISPLAY_DEVICE_ATTACHED_TO_DESKTOP) {
  9            if(c == adapter)
 10                return true;
 11            c++;
 12        }
 13        count++;
 14    }
 15
 16    return false;
 17}
 18
 19static bool win_get_monitor_info(int adapter, ALLEGRO_MONITOR_INFO *info)
 20{
 21   DISPLAY_DEVICE dd;
 22   DEVMODE dm;
 23
 24   memset(&dd, 0, sizeof(dd));
 25   dd.cb = sizeof(dd);
 26   if(!win_find_nth_adapter_with_desktop(&dd, adapter)) /* use new function */
 27       return false;
 28
 29   memset(&dm, 0, sizeof(dm));
 30   dm.dmSize = sizeof(dm);
 31   if (!EnumDisplaySettings(dd.DeviceName, ENUM_CURRENT_SETTINGS, &dm)) {
 32      return false;
 33   }
 34...

I'll try to format a patch as well.

Edit: patch file attached.

no-reply@allegro.cc (SiegeLord) — Fri, 11 Aug 2017 10:31:34 +0000

Thanks a lot! I tried to reproduce this issue, but I couldn't so thanks a lot for your work and fix. The logic seems to make sense and it doesn't seem to break things for me.

no-reply@allegro.cc (tobing) — Fri, 11 Aug 2017 19:31:45 +0000

Thanks for submitting!

bug about multiple displays and first one is not connected to desktop