A C pointer passed between C and Python (using capsule) is empty

Question

I am creating a python extension for some C functions that I have developed.

That code makes use of complex structures that we don't need to analyze from the python code so we are using pointers capsules to pass the structures from one funtion to another.

This is a simplification of a python extension that I am creating in C (here I use only a double instead of our structures):

#include "Python.h"

static PyObject *example2 (PyObject *dummy, PyObject *args)
{
    PyObject *pymodel=NULL;

    if (!PyArg_ParseTuple(args, "O", &pymodel))
    return NULL;

    double *numero;
    if (!(numero = (double*) PyCapsule_GetPointer(pymodel, "DOUBLE"))) {
        return NULL;
    }

    printf("Pointer %x and value %f\n",numero,*numero);

    return Py_BuildValue("");
}

static PyObject* example (PyObject *dummy, PyObject *args)
{
    double *numero;
    double valnum = 6.0;
    numero = &valnum;

    printf("Pointer %x and value %f\n",numero,*numero);
    PyObject * ret = PyCapsule_New(numero, "DOUBLE", NULL);

    return ret;
}

/* Module method table */
static PyMethodDef LIBIRWLSMethods[] = {
    {"example", example, METH_VARARGS, "descript of example"},
    {"example2", example2, METH_VARARGS, "prediction using a trained model"},
    { NULL, NULL, 0, NULL}
};

/* module initialization */
PyMODINIT_FUNC initLIBIRWLS(void)
{
    (void) Py_InitModule("LIBIRWLS", LIBIRWLSMethods);
}

After installing this extension with a setup.py file we call the functions from python:

import LIBIRWLS

myCapsule = LIBIRWLS.example()
LIBIRWLS.example2(myCalsule)

This is the standard output after running the python code:

Pointer b9c27b58 and value 6.000000
Pointer b9c27b58 and value 0.000000

The pointer address has been passed correctly but It has lost the value and we don't know the reason.

score 4 · Accepted Answer · edited May 23 '17 at 10:27

You are trying to pass around a pointer to a value on the stack. See this question for why that is a bad idea. To pass around a pointer, you need to allocate memory from the heap:

static PyObject *example2 (PyObject *dummy, PyObject *args)
{
    PyObject *pymodel = NULL;
    if (!PyArg_ParseTuple(args, "O", &pymodel))
        return NULL;

    double *numero;
    if (!(numero = (double*) PyCapsule_GetPointer(pymodel, "DOUBLE")))
        return NULL;

    printf("Pointer %p and value %f\n", numero, *numero);

    return Py_BuildValue("");
}

static PyObject* example (PyObject *dummy, PyObject *args)
{
    double *numero = (double *) PyMem_Malloc(sizeof(double *));
    if (NULL == numero)
        return NULL;
    *numero = 6.0;

    printf("Pointer %p and value %f\n", numero, *numero);

    PyObject * ret = PyCapsule_New(numero, "DOUBLE", NULL);
    return ret;
}

Output

Pointer 0x7f9103d173f0 and value 6.000000
Pointer 0x7f9103d173f0 and value 6.000000

It worked. This is my first python extension (I am creating wrappers for my phd code) and I am completely lost. — Rob, Oct 21 '16 at 18:33
I used to write cmex extensions for Matlab, so this looks familiar to me. — , Oct 21 '16 at 18:40

A C pointer passed between C and Python (using capsule) is empty

1 Answers1