Access violation in Python garbage collector (visit_decref) - how to debug?

[Geoff Bache]

Yes, this is hard, that's why I'm here :)

I've enabled the equivalent tools to valgrind in Visual Studio, and tried
setting PYTHONMALLOC=debug, but neither of those seem to be showing
anything either. I don't really know what else to try in this direction.

/Geoff

On Sat, Oct 5, 2019 at 7:22 AM dieter <dieter at handshake.de> wrote:

> Geoff Bache <geoff.bache at gmail.com> writes:
> > ...
> > We are running Python embedded in our C++ product and are now
> experiencing
> > crashes (access violation reading 0xffffffffff on Windows) in the Python
> > garbage collector.
>
> Errors like this are very difficult to analyse. The main reason:
> the memory corruption is likely far away from the point when
> it is finally detected (by an access violation in your case).
>
> Python can be built in a special way to add marks to
> its allocated memory blocks and verify their validity.
> This increases the chance to detect a memory corruption earlier
> and thereby facilitates the detection of the root cause.
>
> There are tools for the analysis of memory management problems
> (e.g. "valgrind", though this may be for Linux). In my
> experience, even with those tools, the analysis is very difficult.
>
> I have several times successfully analysed memory corruption
> problems. In those cases, I have been lucky that the corruption
> was reproducible and affected typically the same address.
> Thus, I could put a (hardware) memory breakpoint at this address
> stopping the program as soon as this address was written and
> then analyse the state in the debugger. This way, I could detect
> precisely which code caused the corruption. However,
> this was quite a long time ago; nowadays, modern operating systems
> employ address randomization thus reducing significantly that
> the corruption affects the same address (which may mean that
> you need to deactivate address randomization to get a better chance
> for this kind of analysis.
>
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>