Re: malloc() debugging

[Daniel Jacobowitz <drow@mvista.com>]

On Wed, Oct 30, 2002 at 09:17:17PM +0100, Fredrik Tolf wrote:
> ----- Original Message ----- 
> From: "Daniel Jacobowitz" <drow@mvista.com>
> To: "Fredrik Tolf" <fredrik@dolda2000.cjb.net>
> Cc: <gdb@sources.redhat.com>
> Sent: Wednesday, October 30, 2002 9:11 PM
> Subject: Re: malloc() debugging
> 
> 
> > On Wed, Oct 30, 2002 at 09:07:46PM +0100, Fredrik Tolf wrote:
> > > I'm trying to make a multithreaded program, but I've run into some
> > > problems. It appears as I am somehow modifying malloc's data
> > > structures (those in front of the allocated chunk, I'm using glibc),
> > > and thus the program crashes sometimes when the malloc related are
> > > called, most usually in free. The strangest thing, as I see it, is
> > > that (at least last time it crashed from this), the first word of
> > > that structure (prev_size) wasn't modified, but only the second word
> > > (size). The most probable reason I can see for an error of this kind
> > > would otherwise be a buffer overflow from the chunk before. Anyhow,
> > > the reason that I'm mailing to this list is that I'm wondering if
> > > it's possible to configure gdb to break whenever one of these
> > > structures is modified by an instruction outside the address range of
> > > the malloc related functions. If noone knows how this would be done,
> > > could you notify of any other good tool to find these conditions? Can
> > > valgrind detect it?
> > 
> > Nope, GDB can't do this.  You may want to try either Electric Fence or
> > dmalloc; dmalloc does work in multithreaded programs (although it needs
> > a little coaxing).
> > 
> Shouldn't gdb be enabled to do this in that case? Speaking of which,
> can't gdb create watchpoints to these structuress every time malloc
> is called with conditions of $pc outside the malloc functions and
> delete them when free is called? I'm not very experienced with gdb,
> but shouldn't it be possible, or, if not, be made possible?

No.  Every real system (non-simulator) with watchpoints at all has a
finite number of them.  Usually no more than a handful.

You can use GDB in combination with dmalloc; dmalloc triggers an
exception when you corrupt the data structures and GDB tells you where
you are.

-- 
Daniel Jacobowitz
MontaVista Software                         Debian GNU/Linux Developer