[Dwarf-Discuss] Question / Proposal: breakpoints at global C++ constructors and destructors

[Daniel Gutson]

On Sun, Oct 5, 2014 at 1:07 PM, Michael Eager <eager at eagercon.com> wrote:
> On 10/05/14 08:57, Daniel Gutson wrote:
>>
>> On Sun, Oct 5, 2014 at 12:47 PM, Michael Eager <eager at eagercon.com> wrote:
>>>
>>> On 10/05/14 07:27, Daniel Gutson wrote:
>>>>
>>>>
>>>> Hello,
>>>>
>>>>      this is my first post to this list.
>>>>
>>>> Context: I've been a gdb and gcc maintainer in the past, but I'm not a
>>>> DWARF expert, so my post is
>>>> from the user's POV.
>>>>
>>>> PROBLEM: ability to break in (all) the global objects' constructors
>>>> (that is, before main() is called),
>>>> and destructors (after main() ).
>>>>
>>>> As a matter of proof of concept, we've developed a gdb script that is
>>>> also nontrivial and imperfect.
>>>> FWIW, there were two ways: one implementation-agnostic by regex
>>>> finding the ctors of all objects and
>>>> adding breakpoints in all of them, then breaking at main, disabling
>>>> the former and continuing;
>>>> another way was implementation-specific, knowing the internals of the
>>>> __static_initialization_and_destruction_0
>>>> mechanism. Our proof of concept used the first approach but had a lot of
>>>> issues.
>>>>
>>>> So, it would be absolutely trivial if there was a DW_CTOR_CALL /
>>>> DW_DTOR_CALL with the address
>>>> of the call instruction, so the debugger can break on those calls, and
>>>> do a step-into to get into the ctor/dtor body.
>>>
>>>
>>>
>>> I don't think that a special DWARF TAG would be required for this.
>>> You could use DW_TAG_subprogram to describe the ctor/dtor routine,
>>> marked with with DW_AT_artificial.
>>
>>
>> It's not a solution, because the problem is not identifying the
>> function (the constructor
>> in this case), but the CALL to the constructor.
>> Why? Because the same class could have several global instances, and
>> non-global
>> (attributes, local) instances.
>> So what is needed is to identify the invoking places.
>
>
> If what you want to do is break at all constructors/destructors, I

No, not *all*, only for those instances which ar grlobal

> don't see why you need to break at the calling site.

because otherwise I would break in all non-global instances' construction too.

>
> Can you give an example?

OK:

struct Base
{
    Base(int) {}
};

struct Der: Base
{
    Der(int x) : Base(x) {}
};

Base b1; // break here
Der d1;   // break at d1's ctor body, not during Der::Base construction

int main()
{
   Base b2;  // don't break here
   Der d2;    // ditto
}



>
>
> --
> Michael Eager    eager at eagercon.com
> 1960 Park Blvd., Palo Alto, CA 94306  650-325-8077



-- 

Daniel F. Gutson
Chief Engineering Officer, SPD


San Lorenzo 47, 3rd Floor, Office 5

C?rdoba, Argentina


Phone: +54 351 4217888 / +54 351 4218211

Skype: dgutson