[vtkusers] Debugging a Segfault

[Peter F Bradshaw]

Hi;

On Fri, 31 Mar 2006, Martin Baumann wrote:

> Hi,
>
> I am using KDbg. I guess that I did what you meant:

I'm unfamiliar with both KDbg and Kdbg but I imagine that it is a gdb
front end.

> I set a stop point on the line that causes the error. I opened the
> window that show the local variables.
> Next to _plane_widget stands 0x832aee0. So not 0.

Ok. I suspect that the VTK library you are linking to has been compiled
without symbols. If this is the case the segfault is actually occuring
within vtkPlaneWidget::Delete().

One of the more common causes of a segfault in the dtor is that an
object that the class wants to manage is deleted from underneath it.
That is, a pointer to an object managed by vtkPlaneWidget is obtained
but Register() is not called on it and, subsequently, has Delete()
called on it. This will cause problems within the managing object's
Delete() (if not before). It might be worth while to scan the code that
uses any object pointers obtained from _plane_widget to ensure that
something like this is not occuring.

>
> To make sure _plane_widget is referenced only once I did the following:
>   std::cout << "LW: " << _line_widget->GetReferenceCount() << std::endl;
>   _line_widget ->Delete();
>   std::cout << "PW: " << _plane_widget->GetReferenceCount() << std::endl;
> _plane_widget->Delete(); // This causes the SIGFAULT
>
> This lead to the following output:
>   LW: 1
>   PW: 1
>
> Hmmm...
>
> Thanks, M.B.
>
>
> Peter F Bradshaw schrieb:
>
> >
> >Step one is to determine the value of _plane_widget. My guess is that it
> >is 0 (i.e. 0x0). The value of _plane_widget may be examined by
> >"printing" it - however that's done in Kdbg. Using gdb you would type:
> >
> >p _plane_widget
> >
> >in the context of the segfault.
> >
> >The reason that _plane_widget may be zero is because it was initally
> >asigned that value when it was declared and there has been no
> >
> >_plane_widget = vtkPlaneWidget::New();
> >
> >However, as you point out, it may be that you are deleting an object
> >that no longer exists. That is it has been deleted elsewhere. You can
> >examine the reference count of _plane_widget at any time by something
> >like:
> >
> >  cout << _plane_widget->GetReferenceCount() << endl;
> >
> >If the value of GetReferenceCount() is 1 just before Delete() then the
> >Delete() will really delete the object.
> >
> >Vtk reference counting is normally handled in vtkObjectBase (assuming
> >no overloading).
> >
>

Cheers

-- 
Peter F Bradshaw, http://www.exadios.com, ICQ 75431157 (exadios).
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