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Technical README  - 0.99
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23. Recovering the console after a crash

The below is a mail from Kevin Buhr <buhr@stat.wisc.edu> , that was posted on linux-msdos some time ago. Because it describes a way to recover from a totally locked console, the technique described below was partially intergrated in dosdebug. So, the below 'switchcon.c' is now part of dosdebug and you may use it via:

  console n

where n is the console you want to switch to.

But keep in mind, that dosdebug tries to kill your dosemu process the safest way it can, so first use dosdebug's kill command:


In the worst case you will get the following output on your remote terminal:

   ...oh dear, have to do kill SIGKILL
   dosemu process (pid 1234) is killed
   If you want to switch to an other console,
   then enter a number between 1..8, else just type enter:
   2      <========= this is what you enter
   dosdebug terminated
   NOTE: If you had a totally locked console,
         you may have to blindly type in 'kbd -a; texmode
         on the console you switched to.

23.1 The mail message

Date: Fri, 21 Apr 95 14:16 CDT
To: tegla@katalin.csoma.elte.hu
Cc: linux-msdos@vger.rutgers.edu
In-Reply-To: <Pine.LNX.3.91.950421163705.1348B-100000@katalin.csoma.elte.hu> (message from Nagy Peter on Fri, 21 Apr 1995 16:51:27 +0200 (MET DST))
Subject: Restoring text mode (was Re: talk)
From: buhr@stat.wisc.edu (Kevin Buhr)
Sender: owner-linux-msdos@vger.rutgers.edu
Precedence: bulk
Status: RO

| But when dosemu dies in graphics mode ( this happens every 30 minutes
| or so), it leaves the screen in graphics mode. You can do anything
| blindly (even start dosemu again) but the console screen is always left
| in graphics mode.

I know what you mean... this is a real pain in the ass.

Here's my solution.  A few useful scripts and programs are supplied
with the SVGA binaries.  "savetextmode" is a script that will write
register and font information to "/tmp/textregs" and "/tmp/fontdata".
Run this from the console as root while you're in text mode.  If
you've got a cron job that clears out your "/tmp" directory, you'll
probably want to copy these someplace safe.

The next time "dosemu" or something similar takes out your video, use
the "textmode" script (which reads the register and font from those
temporary files and also restores the palette), and everything should
be back to normal.  Of course, this assumes you're able to get enough
control of your computer to enter the "textmode" command as root at
the console ("restoretextmode" complains if executed from a terminal
other than the console).  One solution is to modify the source for
"restoretextmode" to operate correctly from off-console.

I'm lazy, so I use a little program called "switchcon" (source
attached) that takes a single integer argument and switches to that
virtual console.

So, if "dosemu" dies hard (so that "ctrl-alt-pagedown" doesn't work)
or exits without restoring text mode, I do this:

        (1)  Log in from another terminal
        (2)  Kill "dosemu", if necessary.  Killing with SIGTERM will
             usually restore text mode automatically, but if I have
             to SIGKILL it, I continue...
        (3)  Run "switchcon" as root to switch to another VC
        (4)  Sometimes I have to run "kbd_mode -a", too, if I'm stuck
             in raw mode

If Linux fails to automatically restore text mode, I log in (blindly)
as root on the console and run "textmode".  With the canned register
and font files in place, this inevitably brings me back to text mode

Kevin <buhr@stat.wisc.edu>

                        *       *       *


#include <stdio.h>
#include <stdlib.h>
#include <fcntl.h>
#include <linux/vt.h>
#include <sys/ioctl.h>

main( int argc, char **argv ) {
   int newvt;
   int vt;

   if(argc != 2 || !(newvt = atoi(argv[1]))) {
      fprintf(stderr, "syntax: switchcon number\n");

   vt = open( "/dev/tty1", O_RDONLY );
   if( vt == -1 ) {
   if( ioctl( vt, VT_ACTIVATE, newvt ) ) {
   if( ioctl( vt, VT_WAITACTIVE, newvt ) ) {



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