Re: Instruction address catching

[Fredrik Tolf <fredrik@dolda2000.cjb.net>]

On Sat, 2002-10-05 at 02:52, Kevin Buettner wrote:
> On Oct 5,  2:21am, Fredrik Tolf wrote:
> 
> > On Sat, 2002-10-05 at 02:11, Kevin Buettner wrote:
> > > On Oct 5,  1:36am, Fredrik Tolf wrote:
> > > 
> > > > Is it possible, with the current version of gdb, to make it detect every
> > > > time an address is written to, and to make it stop only if the address
> > > > of the instruction that wrote to that address isn't equal to a specific
> > > > value.
> > > 
> > > Do it like this:
> > > 
> > >     (gdb) watch foo
> > >     Hardware watchpoint 1: foo
By the way: I don't seem to be able to set hardware watchpoints... I'm
running i 2.4.19 linux kernel on an AMD Thunderbird, could that have
something to do with it?
> > >     (gdb) condition 1 $pc != 0x80484c8
> > > 
> > > The address to compare $pc against should be the address of the
> > > instruction *after* the one that writes to ``foo''.  It's probably
> > > easier to figure this out by simply setting a watchpoint on foo,
> > > running 'til it's hit, and if that's not the one you're interested
> > > in, add a condition so that it won't stop there again.
> > > 
> > > > If it's possible, does it work in a multithreaded application?
> > > 
> > > It should.  If it doesn't, it's a bug.
> >
> > Are you sure? Wouldn't the pc it compares to be that of the selected
> > thread and not that of the thread that wrote?
> 
> The thread that triggers the watchpoint becomes the selected thread
> for the purposes of checking the condition.  (At least, I think that's
> the way it works.)  I've just tried it with a simple (threaded) example
> and it seems to work as expected...
OK... if you say so... I'll try it! Thanks a thousandfold!
> 
> Kevin
>