Re: [Valgrind-users] Catching changes to a memory location

[Alex B. <ker...@be...>]

2009/5/21 Dan Kegel <da...@ke...>:
> On Thu, May 21, 2009 at 7:22 AM, Alex Bennee <ker...@be...> wrote:
>> Is there any way I could mark an interesting location in memory with
>> the client mechanism and then get
>> Valgrind to show me a backtrace everytime the location was touched? If
>> not it would be a really useful tool :-)
>
> That would be cool.  That said, have you tried a watchpoint in gdb?
> http://stackoverflow.com/questions/58851/can-i-set-a-breakpoint-on-memory-access-in-gdb

Yep I had been trying that. The main problem was not being able to do
ith progamatically:

b 869 if awatch (void *) *&n->timer

Would of been useful. But conditinal breakpoints aren't really
designed for setting newer breakpoints.

In the end I did solve it with a client request:

	  n->timer = NULL;
          fprintf(stderr,"  2:checking n=%p n->timer=%p\n", n, n->timer);
          VALGRIND_MAKE_MEM_NOACCESS(&n->timer, sizeof(n->timer));

And almost imeadiatly Valgrind showed me an invalid write:

==31858== Invalid write of size 4
==31858==    at 0x413B1D: function (snmp_interface.c:2699)

Which lead to:
    n->vc_qam[us]++;

Which funnily enough was an incorrectly sized array just before n->timer

Time spent reading Valgrind's fine manual: 10 minutes, Time spent
hacking in the macro and running Valgrind 5 minutes, Time I *would
have* saved if I had done this earlier: 4 hours ;-)

-- 
Alex, homepage: http://www.bennee.com/~alex/
CV: http://www.bennee.com/~alex/cv.php