[Python-Dev] Re: C new-style classes and GC

[Jim Fulton]

Jim Fulton wrote:
> Lately I've been re-learning how to write new types in C.  Things
> changed drastically (for the better) in 2.2. I've been updating the
> documentation on writing new types as I go:
> 
> http://www.python.org/dev/doc/devel/ext/defining-new-types.html
> 
> (I'm also updating modulator.)
> 
> I'm starting to try to figure out how to integrate support for GC.
> The current documentation in the section "Supporting the Cycle
> Collector" doesn't reflect new-style types and is, thus, out of date.
> 
> Frankly, I'm taking the approach that there is only One Way to create
> types in C, the new way, based on new-style types as now documented
> in the manual.
> 
> I'll also note that most new-style types don't need and thus don't
> implement custom allocators. They leave the tp_alloc and tp_free slots
> empty.
> 
> So given that we have a new style type, to add support for GC, we need
> to:
> 
> - Set the Py_TPFLAGS_HAVE_GC type flag,
> 
> - Provide implementations of tp_traverse and tp_clear, as described in
>   the section "Supporting the Cycle Collector" section of the docs.
> 
> - Call PyObject_GC_UnTrack at the beginning of the deallocator,
>   before decrefing any members.
> 
> I think that that is *all* we have to do.

It looks like the answer is "no". :)

I tried to write a type using this formula and segfaulted.
Looking at other types, I found that if I want to support GC and
am using the default allocator, which I get for free, I have to fill
the tp_free slot with PyObject_GC_Del (_PyObject_GC_Del if I want to
support Python 2.2 and 2.3).

I *think* this is all I have to do.

Jim


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