Re: Help please! To track down GC trying to free an already freed object.

[Alan Mackenzie <acm@muc.de>]

Hello, Eli.

On Wed, Apr 03, 2019 at 07:39:35 +0300, Eli Zaretskii wrote:
> > Date: Tue, 2 Apr 2019 20:46:53 +0000
> > From: Alan Mackenzie <acm@muc.de>
> > Cc: Daniel Colascione <dancol@dancol.org>, emacs-devel@gnu.org

> > > GDB supports reverse execution as well, on some platforms.

> > On my GNU/Linux system, I tried to run 'reverse-next', and got the error
> > message:

> >     Target multi-thread does not support this command.

> I think you are supposed to record the execution, and then say

>   (gdb) target record-core

> or

>   (gdb) target record-btrace

> before the reverse execution is available.

Yes.  I thought there was something missing.  ;-)  There's no mention of
such recording in the GDB manual's "Reverse Execution" page, nor any
cross reference to "Process Record and Replay" there.

I'll try again and see if I can get it working.

> But I was always able to debug GC problems by using last_marked array.

The problem I think I'm up against is that the symbol-with-pos object is
not being marked at a particular garbage_collect_1, and thus gets freed
prematurely.

I intend to get the hex values of the Lisp_Objects which constitute the
list in which the symbol-with-pos is embedded and search for these in
last_marked.  Putting a conditional breakpoint on Fcons slows down Emacs
somewhat.  ;-)

-- 
Alan Mackenzie (Nuremberg, Germany).