[Numpy-discussion] Problem with using PyArray_AsCArray

Dec. 9, 2011


      I am trying to pass a multi-dimensional ndarray to C as a multi-dimensional C array for the purposes of passing it to mathematica.  I am using PyArray_AsCArray but getting an error.

######################################################
Python Code:
import Image
from scipy.misc import fromimage
img = Image.open("../APLS/image709_enhanced_2.tif")
nimg = fromimage(img)
...
mathlink_PutIntegerArray(nimg)
######################################################################
C Wrapper Code:
static PyObject * mathlink_PutIntegerArray(mathlink_Link *self, PyObject *args)
{
	npy_intp dims[3]; /* PyArray_AsCArray is for ndim <= 3 */
	PyObject *o1;
	double *d1;
	long		result;

	i=PyArray_AsCArray(&o1, (void *)&d1, dims, PyArray_NDIM(o1), PyArray_DescrFromType(PyArray_TYPE(o1)));

...
}

Program received signal SIGSEGV, Segmentation fault.
0x00002aaaaefa3d2e in PyArray_AsCArray (op=0x7fffffffdf68, ptr=0x7fffffffdf60, 
    dims=0x7fffffffdf40, nd=3, typedescr=<value optimized out>)
    at numpy/core/src/multiarray/multiarraymodule.c:218
218                     ptr3[i][j] = ap->data + i*ap->strides[0] + j*ap->strides[1]
#################################################################################

I am able to read i,j,ap->data, ap->strides[0], ap->strides[1] so the error seems to be in the assignment to ptr3[i][j]
This happens on the first instance i=0 j=0
#################################################################################
PyArray_AsCArray code is below.
NPY_NO_EXPORT int
PyArray_AsCArray(PyObject **op, void *ptr, npy_intp *dims, int nd,
                 PyArray_Descr* typedescr)
{
    PyArrayObject *ap;
    npy_intp n, m, i, j;
    char **ptr2;
    char ***ptr3;

    if ((nd < 1) || (nd > 3)) {
        PyErr_SetString(PyExc_ValueError,
                        "C arrays of only 1-3 dimensions available");
        Py_XDECREF(typedescr);
        return -1;
    }
    if ((ap = (PyArrayObject*)PyArray_FromAny(*op, typedescr, nd, nd,
                                              CARRAY, NULL)) == NULL) {
        return -1;
    }
    switch(nd) {
    case 1:
        *((char **)ptr) = ap->data;
        break;
    case 2:
        n = ap->dimensions[0];
        ptr2 = (char **)_pya_malloc(n * sizeof(char *));
        if (!ptr2) {
            goto fail;
        }
        for (i = 0; i < n; i++) {
            ptr2[i] = ap->data + i*ap->strides[0];
        }
        *((char ***)ptr) = ptr2;
        break;
    case 3:
        n = ap->dimensions[0];
        m = ap->dimensions[1];
        ptr3 = (char ***)_pya_malloc(n*(m+1) * sizeof(char *));
        if (!ptr3) {
            goto fail;
        }
        for (i = 0; i < n; i++) {
            ptr3[i] = ptr3[n + (m-1)*i];
            for (j = 0; j < m; j++) {
                ptr3[i][j] = ap->data + i*ap->strides[0] + j*ap->strides[1];
            }
        }
        *((char ****)ptr) = ptr3;
    }
    memcpy(dims, ap->dimensions, nd*sizeof(npy_intp));
    *op = (PyObject *)ap;
    return 0;

 fail:
    PyErr_SetString(PyExc_MemoryError, "no memory");
    return -1;
}

[Numpy-discussion] Problem with using PyArray_AsCArray

Ng, Enrico