Re: GDB cannot access memory after Emacs abort

Stephen Berman
2007-11-13T22:34:40Z

On Mon, 12 Nov 2007 09:46:43 -0800 Jim Blandy <jimb@codesourcery.com> wrote:

> Stephen Berman <Stephen.Berman at gmx.net> writes:
>> On Sun, 11 Nov 2007 14:22:37 -0500 Daniel Jacobowitz <drow@false.org> wrote:
>>
>>> On Sat, Nov 10, 2007 at 10:38:14PM -0800, Michael Snyder wrote:
>>>> > > At this point my desktop (I tried in KDE, GNOME and twm, same behavior
>>>> > > in all) is totally locked up, but I can switch to a virtual tty and
>>>> > > there kill emacs with SIGKILL (kill -9); SIGTERM (kill -15) does not do
>>>> > > the job.
>>>>
>>>> Making sure that I understand -- you ran emacs under gdb,
>>>> you set a breakpoint at abort, you hit the breakpoint --
>>>> and your desktop is locked up?
>>>>
>>>> That seems unusual -- do you have any idea of the cause?
>>>
>>> This is pretty common when debugging X programs, IIRC. I believe
>>> there's some ways in which an application can "own" a display while
>>> something is in progress.
>>
>> That's interesting; do you have any pointers to further information
>> about this? Yet, as I mentioned in my other followups, this has never
>> happened to me before when running Emacs under gdb, even when it's in an
>> infinite loop. It sounds like you, too, don't suspect a bug in GDB that
>> prevented getting a backtrace.
>
> Actually, these are legit X Windows behavior; they're called 'server
> grabs'. They're supposed to be rare (for obvious reasons), but if
> Emacs died while it had the server grabbed, you'd certainly not be
> able to interact with the debugger in another window.

Note, however, that this only happens when running emacs under gdb; if I
start emacs directly from the shell, induce the abort, then the Emacs
window vanishes, but the desktop remains responsive and no other
problems are apparent.

> Am I correct in understanding that:
> - your X session locks up, and all your windows are unresponsive, not
> just GDB's and Emacs

Yes, all open X apps are unresponsive to keyboard or mouse input, but
the apps continue to operate normally, e.g., the newsticker scrolls, the
displays in gkrellm (time, CPU activity, free memory, etc.) continue to
be updated.

> - you kill Emacs via some other means, which unfreezes your X session

Yes, by switching to a virtual tty.

> - now you can interact with GDB again, but GDB can't get the backtrace.

Yes.

> GDB produces backtraces by reading memory from the process. So if my
> sequence above is correct, once you have killed the Emacs process in
> another window, then it's expected that GDB won't be able to get its
> backtrace.

So again the (or a) question is, why does this only happen when running
emacs under gdb?

Steve Berman