hints on debugging memory corruption...

[Ian Lance Taylor]

Basile Starynkevitch <basile@starynkevitch.net> writes:

> In my MELT branch I have now some corrputed memory (maybe because I am
> inserting a pass at the wrong place in the pass tree). At some point, I call
> bb_debug, and it crashes because the field bb_next contains 0x101 (which is
> not a valid adress).
>
> So I need to understand who is writing the 0x101 in that field.
>
> How do you folks debug such issues. 
>
> An obvious strategy is to use the hardware watchpoint feature of GDB.
> However, one cannot nicely put a watchpoint on an address which is not
> mmap-ed yet.
>
> But I don't know how to ask gdb to be notified when a given adress is
> becoming valid in the address space. Putting a breakpoint on mmap is really
> not funny.

I usually put a breakpoint on the allocation routine and make it
conditional on returning the address I am interested in.  Once that
address is returned, I can put a hardware breakpoint on the field value
being changed.  This approach is only moderately successful in practice.

Ian