[Python-Dev] A little GC confusion (original) (raw)

Martin v. Loewis martin@v.loewis.de
23 Feb 2002 11:51:57 +0100

"David Abrahams" <david.abrahams@rcn.com> writes:

Nice try, but no cigar I'm afraid: copying the tpisgc slot from PyTypeType into my metatype before PyTypeReady() doesn't prevent the crash.

Does anyone really understand what happens here?

After studying your code in a debugger, it turns out that the code now crashes for a different reason: The "AA" class created in make_class is traversed in subtract_refs. To do so, its type's traverse function is invoked, i.e. classtype_meta_object.tp_traverse. This is a null pointer, hence the crash.

If you want objects (in your case, classes) to participate in GC, the of the objects (in your case, the metaclass) needs to implement the GC API. IOW, don't set Py_TPFLAGS_HAVE_GC in a type unless you also set tp_clear and tp_traverse in the same type, see

http://www.python.org/doc/current/api/supporting-cycle-detection.html

for details. This likely has been the problem all the time; if I remove tp_is_gc, but implement tp_traverse, your test case (import AA,pdb) does not crash anymore.

BTW, gcc rejects the code you've posted, as you cannot use PyType_Type.tp_basicsize in an initializer of a global object (it's not a constant).

HTH, Martin