I'm trying to write a C-Extension for python. What I'd like to write is a ModPolynomial class which represents a polynomial on (Z/nZ)[x]/x^r-1[even though you may answer to my question without knowing anything of such polynomials].
I've written some code, which seems to work. Basically I just store three PyObject* in my ModPoly structure. Now I'd like to add the storage for the coefficients of the polynomial.
Since I want the coefficients to be read-only, I'd like to add a getter/setter pair of functions through PyGetSetDef. But when I access the getter from python(e.g print pol.coefficients) I receive a Segmentation Fault.
The original code, without the "coefficients" can be found here. The code with the coefficients is here.
I hope someone of you can tell me where I'm doing wrong here. By the way, also comments on the code are welcome. This is my first extension and I know that I'm probably doing things quite badly.
As ecatmur says in the comments PyVarObject store a certain number of "slots" at the end of the struct. So I've decided to avoid them.
The relevant code is:
typedef struct {
PyObject_HEAD
/* Type specific fields */
Py_ssize_t ob_size;
PyObject **ob_item;
Py_ssize_t allocated;
PyObject *r_modulus;
PyObject *n_modulus;
PyObject *degree;
} ModPoly;
static PyObject *
ModPoly_new(PyTypeObject *type, PyObject *args, PyObject *kwds)
{
ModPoly *self;
self = (ModPoly *)type->tp_alloc(type, 0);
if (self != NULL) {
[...]
self->ob_size = 0;
self->ob_item = NULL;
self->allocated = 0;
}
return (PyObject *)self;
}
static int
ModPoly_init(ModPoly *self, PyObject *args, PyObject *kwds)
{
PyObject *r_modulus=NULL, *n_modulus=NULL, *coefs=NULL, *tmp;
PyObject **tmp_ar;
static char *kwlist[] = {"r_modulus", "n_modulus", "coefficients", NULL};
if (!PyArg_ParseTupleAndKeywords(args, kwds, "OO|O", kwlist,
&r_modulus, &n_modulus, &coefs))
return -1;
[...]
// The polynomial defaults to "x", so the coefficients should be [0, 1].
tmp_ar = (PyObject **)malloc(2 * sizeof(PyObject*));
if (tmp_ar == NULL) {
Py_DECREF(self->r_modulus);
Py_DECREF(self->n_modulus);
Py_DECREF(self->degree);
return -1;
}
tmp_ar[0] = PyInt_FromLong(0);
if (tmp_ar[0] != NULL) {
tmp_ar[1] = PyInt_FromLong(1);
}
if (tmp_ar[0] == NULL || tmp_ar[0] == NULL) {
Py_DECREF(self->r_modulus);
Py_DECREF(self->n_modulus);
Py_DECREF(self->degree);
Py_XDECREF(tmp_ar[0]);
Py_XDECREF(tmp_ar[1]);
free(tmp_ar);
return -1;
}
self->ob_size = 2;
self->allocated = 2;
return 0;
}
[...]
static PyObject *
ModPoly_getcoefs(ModPoly *self, void *closure)
{
printf("here"); // "here" is never printed
PyTupleObject *res=(PyTupleObject*)PyTuple_New(self->ob_size);
Py_ssize_t i;
PyObject *tmp;
if (res == NULL)
return NULL;
for (i=0; i < self->ob_size; i++) {
tmp = self->ob_item[i];
Py_INCREF(tmp);
PyTuple_SET_ITEM(res, i, tmp);
}
return (PyObject *)res;
}
static PyObject *
ModPoly_setcoefs(ModPoly *self, PyObject *value, void* closure)
{
PyErr_SetString(PyExc_AttributeError,
"Cannot set the coefficients of a polynomial.");
return NULL;
}
[...]
static PyGetSetDef ModPoly_getsetters[] = {
{"coefficients",
(getter)ModPoly_getcoefs, (setter)ModPoly_setcoefs,
"The polynomial coefficients.", NULL},
{NULL, 0, 0, NULL, NULL}
};
static PyTypeObject ModPolyType = {
PyObject_HEAD_INIT(NULL)
0, /* ob_size */
[...]
ModPoly_members, /* tp_members */
ModPoly_getsetters, /* tp_getset */
0, /* tp_base */
[...]
};
[...]
edit
I tried to reimplement the getter instruction by instruction, and I understood what I wasn't doing. In the ModPoly_init function I create the tmp_ar where I store the coefficients, but I do not assign it to self->ob_item.
-facepalm-
You only seem to be assigning to ModPoly.ob_item in ModPoly_new() (setting it to NULL).
ModPoly_getcoefs() then dereferences the null pointer, which would give you your segfault. It looks like you intended to assign to ob_item in ModPoly_init(), but don't actually get around to doing so.