[Python-Dev] Proposal: C API Macro to decref and set to NULL (original) (raw)

Jim Fulton jim at zope.com
Mon Jul 12 23:21:31 CEST 2004

Often, in C type implementations, you want to set a PyObject * slot to NULL. Often this goes like:

Py_XDECREF(self->foo); self->foo = NULL;

However, if your type participates in GC and self->foo is something that might get traversed, this can lead to traversing self->foo after it has been DECREFed, but before it's set to NULL.

The correct way to do this is something like:

tmp = self->foo; self->foo = NULL; Py_XDECREF(tmp);

I suggest that there should be a standard macro to automate this.

Py_CLEAR(self->foo)

This would be defined to be the same result as Py_XDECREF except that the argument will be set to NULL.

Thoughts?

If there are no objections I'll add the following definition to object.h, after the definition for Py_DECREF:

#define Py_CLEAR(op)
do {
if (op) {
PyObject *tmp = (op);
(op) = NULL;
Py_DECREF(tmp);
}
} while (0)

and update the docs and the tutorial on creating types in C.

Jim

-- Jim Fulton mailto:jim at zope.com Python Powered! CTO (540) 361-1714 http://www.python.org Zope Corporation http://www.zope.com http://www.zope.org

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