Re: Delete a watchpoint by address to automate segfault tracking

["Laurent G." <vrygoud at gmail dot com>]

On Thu, Sep 27, 2012 at 3:39 PM, Tom Tromey <tromey@redhat.com> wrote:
>>>>>> "Laurent" == Laurent G <vrygoud@gmail.com> writes:
>
> Laurent> In the article, the author says "I couldn’t find an easy way to track
> Laurent> the memory watch number that was created during the first breakpoint,
> Laurent> I just built a gdb counter, and deleted the memory watch when leaving,
> Laurent> since I could predict gdb’s numbering" and implements watchpoints
> Laurent> destruction this way:
>
> I think $bpnum may hold this.
> See the 'Set Breaks' node of the manual.
> Or it can perhaps be done from Python.
>
> Laurent> How can I do a "delete *`print $d`" in GDB?
>
> A few ways.
>
> Simplest is to use "eval".
>
> Otherwise, Python.
>
> Otherwise, if you have an old gdb and can't upgrade, use set logging to
> write out a file with the command you want (using printf), then 'source'
> the file.
>
> Tom

Thanks for your reply that helped me a lot. In fact the true issue is that you
can clear a breakpoint by its address:
(gdb) clear *0x80a5f90
But for no reason (from a user point of view) you can't do the same thing for
a watchpoint. If it was possible I could have used:
(gdb) eval "delete  *0x%x", my_pointer

Even if the Python support is great I forgot to tell that the target
is an embedded
system so I didn't want to compile a Python environment. Anyway, that's maybe
not the cleanest way to do so but I did the write-out-a-file things to match the
address of the buffer with the number of the watchpoint associated with it.

I'm positing my script bellow in the case anyone encounter the same issue:
(gdb) set pagination off
(gdb) b open_filter.c:30
(gdb) b close_filter.c:100

(gdb) commands 1
(gdb) silent
(gdb) watch -l *(int*)*p_data
(gdb) set logging file watchpoints.gdb
(gdb) set logging on
(gdb) eval "echo %d 0x%x\n", $bpnum, p_data
(gdb) set logging off
(gdb) cont
(gdb) end

(gdb) commands 2
(gdb) silent
(gdb) set logging file match.gdb
(gdb) set logging overwrite on
(gdb) set logging on
(gdb) eval "echo 0x%x\n", p_data
(gdb) set logging off
(gdb) set logging overwrite off
(gdb) shell cat watchpoints.gdb | grep `cat match.gdb` | awk -F" "
'{print "delete "$1}' > delete_wp.gdb
(gdb) shell line=`cat match.gdb`; sed -i "/$line/d" watchpoints.gdb
(gdb) source delete_wp.gdb
(gdb) cont
(gdb) end

Regards,
Laurent