Win32: taskswitch using alt-tab locks alt key

SDL Development
1

Hi,

I’ve encountered a problem with SDL on Win32 (Windows 2000 SP2 and
using the 1.2.5 SDL.DLL from libsdl.org). When running an SDL app and
using alt-tab to switch to another task, then switching back by
pressing alt-tab again, SDL sometimes doesn’t notice that the alt-key
has been released.

I’ve written a simple test program. It displays the keys that are
pressed. A sample session is as follows:

  • run program
  • press ‘d’ for example , the programm prints ‘d’
  • press ‘alt-d’ for example , the programm prints ‘d alt’
  • now press alt-tab and switch to another program. the test program
    displays the tab character and ‘alt’.
  • release all keys
  • now press alt-tab again to switch to the test program
  • release all keys
  • press a key, for example ‘d’ again. The test program prints 'd alt ’

This does not happen always, it happens at about 10% of all task
switches. I couldn’t find any reason why this is sometimes happening
and sometimes not.

Any ideas how this can be fixed ?

Here’s the test program:

#include
#include <sdl.h>

int main( )
{
SDL_Init(SDL_INIT_VIDEO);
SDL_SetVideoMode(800, 600, 8,0 );
while ( 1 ) {
SDL_Event event;
if ( SDL_PollEvent ( &event ) == 1) {
switch ( event.type ) {
case SDL_KEYDOWN:
{
printf("%c", event.key.keysym.sym );
if ( event.key.keysym.mod & KMOD_ALT )
printf(" alt");
printf("\n");
}
break;
case SDL_QUIT: exit(0);
}
}
}
}

2

Could this be a Windows bug? I know 95/98 have a lot of annoying keyboard
bugs…

Martin Bickel wrote:> Hi,

I’ve encountered a problem with SDL on Win32 (Windows 2000 SP2 and
using the 1.2.5 SDL.DLL from libsdl.org). When running an SDL app and
using alt-tab to switch to another task, then switching back by
pressing alt-tab again, SDL sometimes doesn’t notice that the alt-key
has been released.

I’ve written a simple test program. It displays the keys that are
pressed. A sample session is as follows:

  • run program
  • press ‘d’ for example , the programm prints ‘d’
  • press ‘alt-d’ for example , the programm prints ‘d alt’
  • now press alt-tab and switch to another program. the test program
    displays the tab character and ‘alt’.
  • release all keys
  • now press alt-tab again to switch to the test program
  • release all keys
  • press a key, for example ‘d’ again. The test program prints 'd alt ’

This does not happen always, it happens at about 10% of all task
switches. I couldn’t find any reason why this is sometimes happening
and sometimes not.

Any ideas how this can be fixed ?

Here’s the test program:

#include
#include <sdl.h>

int main( )
{
SDL_Init(SDL_INIT_VIDEO);
SDL_SetVideoMode(800, 600, 8,0 );
while ( 1 ) {
SDL_Event event;
if ( SDL_PollEvent ( &event ) == 1) {
switch ( event.type ) {
case SDL_KEYDOWN:
{
printf("%c", event.key.keysym.sym );
if ( event.key.keysym.mod & KMOD_ALT )
printf(" alt");
printf("\n");
}
break;
case SDL_QUIT: exit(0);
}
}
}
}

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3

yeah, it seems to me that this could be a windows bug, too, because i’ve
noticed similar things happening in pretty much every windows app

M@ the MadProgrammer_________________________________________________________________
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