Hi

I would like to be able to get an handle on PyGC_Head*_PyGC_generation0 from a CPython extension.
This is possible when building Python on a Posix host, but not on Windows because of a missing PyAPI_DATA wrapping of the said object in the objimpl.h header.

Having the possibility to inspect the PyGC_Head here can be very useful when customizing the way a garbage-collectable object created from CPython extension would be cleaned/inspected, etc.

Any chance this patch could be added to get the same capabilities on Windows and Posix platforms ?

Thanks for your feedback,
Eloi

This issue comes from a thread on python-ideas:
https://mail.python.org/archives/list/python-ideas@python.org/thread/7FC2NHAOMKBHONFTUKMAHO5F56UU5UUD/

I understand that it's specific to Python 2.7.

--

_PyGC_generation0 variable no longer exists in Python 3.7, it has been moved into _PyRuntime.gc.generation0:

Include/internal/mem.h:146:#define _PyGC_generation0 _PyRuntime.gc.generation0


In the master branch, _PyRuntime is exported with:

Include/internal/pycore_pystate.h: PyAPI_DATA(_PyRuntimeState) _PyRuntime;


In Python 3.7, _PyRuntime is also exported, but in a different header file:

Include/internal/pystate.h: PyAPI_DATA(_PyRuntimeState) _PyRuntime;

--

Python 2.7:

Modules/gcmodule.c:50:PyGC_Head *_PyGC_generation0 = GEN_HEAD(0);

Include/objimpl.h:276:extern PyGC_Head *_PyGC_generation0;

Victor,

extern PyGC_Head *_PyGC_generation0; 
is located at line 374 of Include/objimpl.h (ifndef Py_LIMITED_API).

I used it from a CPython extension we are developing as an handle to the collection of objects being garbage collected.

From what you explained, I should use the _PyRuntime exported symbol to get the handle this PyGC_Head. I'll do this.

I guess that cleaning the Include/objimpl.h is the only remaining action to do then.

Thanks for your feedback.

I think it is too late to add this feature in 2.7. See https://www.python.org/dev/peps/pep-0373/#release-schedule, the feature freeze was 9.5 years ago.

Yes, the only thing that remains to be done is cleaning objimpl.h.

The master branch of Python doesn't contain _PyGC_generation0 anymore.

> Yes, the only thing that remains to be done is cleaning objimpl.h.

We don't do cleanup changes in stable branches. Only in the development branch: master. In this case, it's already fixed. So I close the issue.

If it's specific to 2.7, why does the Version metadata say 3.7?

I originally submitted an issue asking for having the symbol exported.

But, as Victor mentioned, this part of the code was changed between 2.7 and 3.x.

I then asked if the part of the code defining the symbol (and not used anymore) in 3.7 could be removed/cleaned. This is why it was tagged back to 3.7.

As it was already removed in newer version (3.8?) and because the policy is not to clean dead-code, this issue is now closed.