Re: [rfc] Allow watchpoints on inaccessible memory

[Vladimir Prus <vladimir at codesourcery dot com>]

Daniel Jacobowitz <drow <at> false.org> writes:

> 
> Here's something I've been meaning to try for ages.  Suppose you have
> a global variable pointing to some dynamically allocated storage, and
> you want to find writes into that storage.  When the pointer isn't
> initialized you can't even set the watchpoint:
......
> -         v = evaluate_expression (bpt->owner->exp);
> -         value_contents (v);
> -         value_release_to_mark (mark);
> +         gdb_evaluate_expression (bpt->owner->exp, &v);
> +         if (v != NULL)
> +           {
> +             gdb_value_fetch_lazy (v);
> +             value_release_to_mark (mark);
> +           }

In fact, this patch is likely to fix a nasty issue with watchpoints in shared 
libraries. Say you have a watchpoint set on global variable 'g' in a shared 
library, and the shared library is unloaded. Next time you try to "next", 
or "continue", insert_bp_location will try to evaluate watchpoint expression 
again, and throw -- which will terminate next or continue operation, leaving 
program at the same place -- which is extremely confusing.

- Volodya