Date: Tue, 4 Aug 1998 05:24:21 +0000 (   )
From: "Gurunandan R. Bhat" <grbhat AT unigoa DOT ernet DOT in>
To: Martin Str|mberg <ams AT ludd DOT luth DOT se>
Cc: djgpp AT delorie DOT com
Subject: Re: fsdb crashes post emacs
In-Reply-To: <6pvlv3$hep$1@news.luth.se>
Message-Id: <Pine.LNX.3.91.980804051135.5934A-100000@aditya.unigoa.ernet.in>
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On 1 Aug 1998, Martin Str|mberg wrote:

> Hmm. I wonder what makefile.cws is? Why not use makefile?

I *think* makefile.cws builds fsdb only. The makefile in the directory 
src/debug/fsdb looks like it does other things too. 

> : xorl    %%al, %%al
> 
> How did you correct it? xorb or %%eax? And where (line number)?

Actually there is only one occurence of this staement in fullscr.c and 
a simple search will get you a line number. Unfortunately, the machine 
where I work is very far from the machine where I mail. I changed it thus:

xorb	%%al, %%al

> : Call frame traceback EIPs:
> :   0x00013748   _malloc+192
> :   0x00013229   _xmalloc+21
> :   0x000080a5   _initdisplay+133, line 2147 of fullscr.c
> :   0x00008c90   _initialize+364, line 2431 of fullscr.c
> :   0x0000aec8   _debugger+52, line 3456 of fullscr.c
> :   0x000020af   _main+503, line 139 of ed.c
> :   0x00012c5a   ___crt1_startup+454
> : ----------------------------------------------------------- 

> Now when you have debug info in fsdb, you could try to run it in
> gdb... Or add some "fprintf(stderr, ...)"s just before line 2147 of
> fullscr.c to see what it passes to xmalloc.

I have now built fsdb by linking in malloc by hand and with the -g option.
I now have some understanding of what is happening. As Eli correctly
reasoned (without the benefit of an unstripped malloc and -g!!) one member
of the linked list maintained by malloc/free points somewhere in the 
region of Mars.  In terms of malloc's variables,

op->ov_next = garbage 

so that when this member is handed out to the application dereferencing 
it causes the crash. I am currently using (learning) gdb to go through the 
code and will report progress if any.

Thank you for your interest
Gurunandan