Re: 64bit pointer

[Jan Kratochvil <jan.kratochvil@redhat.com>]

On Thu, 03 Sep 2009 14:21:41 +0200, freindlyuser@hushmail.com wrote:
> (gdb) x/s *($rdi+0x8)
> 0x4210a9b7:      <Address 0x4210a9b7 out of bounds>

This syntax is not much recommended, it means the same as:

(gdb) x/s *(int *) ($rdi+0x8)
0x4210a9b7:      <Address 0x4210a9b7 out of bounds>

On 64bit arch sizeof (int) == 4 but sizeof (void *) == 8 so you will not fetch
the whole address.


> (gdb) x/x $rdi+0x8
> 0x3a9b4210b7a4: 0x00003a9b4210a9b7
> (gdb) x/s 0x00003a9b4210a9b7
> 0x3a9b4210a9b7:  "The string it points to"

Therefore you want one indirection there.

> Should this be happening?

Yes.  Until GDB forbids dereferencing numeric arguments as `int *' which IMHO
is more confusing than convenient.  [Would a patch be approved?]


> Is there a work around where I can read the data in
> 0x00003a9b4210a9b7 without having to manually copy and paste (ie:
> in the commands that are executed on a breakpoint).

This way it should work:

(gdb) x/s *(void **) ($rdi+0x8)
OR
(gdb) p *(char **) ($rdi+0x8)


Regards,
Jan