[c-api]Transmutation of an extension object into a read-only buffer adding an integer in-place.

Fri Aug 10 05:25:36 EDT 2012

Il giorno venerdì 10 agosto 2012 11:22:13 UTC+2, Hans Mulder ha scritto:
> On 10/08/12 10:20:00, Giacomo Alzetta wrote:
>> > I'm trying to implement a c-extension which defines a new class(ModPolynomial on the python side, ModPoly on the C-side).
>> > At the moment I'm writing the in-place addition, but I get a *really* strange behaviour.
>> > 
>> > Here's the code for the in-place addition:
>> > 
>> > #define ModPoly_Check(v) (PyObject_TypeCheck(v, &ModPolyType))
>> > [...]
>> > static PyObject *
>> > ModPoly_InPlaceAdd(PyObject *self, PyObject *other)
>> > {
>> > 
>> > if (!ModPoly_Check(self)) {
>> > // This should never occur for in-place addition, am I correct?
>> > if (!ModPoly_Check(other)) {
>> > PyErr_SetString(PyExc_TypeError, "Neither argument is a ModPolynomial.");
>> > return NULL;
>> > }
>> > return ModPoly_InPlaceAdd(other, self);
>> > } else {
>> > if (!PyInt_Check(other) && !PyLong_Check(other)) {
>> > Py_INCREF(Py_NotImplemented);
>> > return Py_NotImplemented;
>> > }
>> > }
>> > 
>> > ModPoly *Tself = (ModPoly *)self;
>> > PyObject *tmp, *tmp2;
>> > tmp = PyNumber_Add(Tself->ob_item[0], other);
>> > tmp2 = PyNumber_Remainder(tmp, Tself->n_modulus);
>> > 
>> > Py_DECREF(tmp);
>> > tmp = Tself->ob_item[0];
>> > Tself->ob_item[0] = tmp2;
>> > Py_DECREF(tmp);
>> > 
>> > printf("%d\n", (int)ModPoly_Check(self));
>> > return self;
>> > 
>> > }
>>>> I have no experience writing extensions in C, but as I see it,
>> you're returning a new reference to self, so you'd need:
>>>> Py_INCREF(self);
>>>> If you don't, then a Py_DECREF inside the assignment operator
>> causes your polynomial to be garbage collected. Its heap slot
>> is later used for the unrelated buffer object you're seeing.
>>>> Hope this helps,
>>>> -- HansM

Yes, you're right. I didn't thought the combined operator would do a Py_DECREF if the iadd operation was implemented, but it obviosuly makes sense.