[Numpy-discussion] C extension compiling question
Henry Gomersall
whg21@cam.ac...
Fri Oct 29 08:11:41 CDT 2010
I'm trying to get a really simple toy example for a numpy extension
working (you may notice its based on the example in the numpy docs and
the python extension docs). The code is given below.
The problem I am having is running the module segfaults at any attempt
to access &PyArray_Type (so, as presented, the segfault occurs at the
call to PyArg_ParseTuple). I've been assuming that importing numpy into
the calling python code implicitly provides the numpy libraries that are
necessary for linking. I can only assume I'm missing something. Do i
need some additional includes etc in my compile flags?
I'm currently using the distutils.core module and not the distutils
module that comes with numpy if that makes a difference.
I'm actually pretty keen to document what I've learnt as a 5 minute
how-to for writing basic numpy extensions, if there is demand for it. It
doesn't seem to exist as a coherent whole at the moment.
Thanks,
Henry
#include <Python.h>
#include <numpy/arrayobject.h>
PyMODINIT_FUNC
initspam(void);
static PyObject *
spam_system(PyObject *self, PyObject *args);
void
worker_function(double *input, double *output, npy_intp size);
static PyMethodDef SpamMethods[] = {
{"system", spam_system, METH_VARARGS,
"Execute a shell command."},
{NULL, NULL, 0, NULL} /* Sentinel */
};
PyMODINIT_FUNC
initspam(void)
{
(void) Py_InitModule("spam", SpamMethods);
}
static PyObject *
spam_system(PyObject *self, PyObject *args)
{
int n;
int ndims;
PyObject *inarg=NULL, *outarg=NULL;
PyObject *inarr=NULL, *outarr=NULL;
/* Both input and output need to be PyArray_Type type*/
if (!PyArg_ParseTuple(args, "O!O!", &PyArray_Type, &inarg,
&PyArray_Type, &outarg));
return NULL;
inarr = PyArray_FROM_OTF(inarg, NPY_DOUBLE, NPY_IN_ARRAY);
if (inarr == NULL) goto fail;
outarr = PyArray_FROM_OTF(outarg, NPY_DOUBLE, NPY_OUT_ARRAY);
if (outarr == NULL) goto fail;
/* Check the shape of the two arrays is the same */
ndims = ((PyArrayObject *)outarr)->nd;
if (((PyArrayObject *)inarr)->nd != ndims)
goto fail;
/* ...still checking...*/
for (n=1; n<ndims; n++)
{
if (PyArray_DIM(inarr,n) != PyArray_DIM(outarr,n))
goto fail;
}
/* Now actually do something with the arrays */
worker_function((double *)PyArray_DATA(inarr),
(double *)PyArray_DATA(outarr),
PyArray_Size(inarr));
Py_DECREF(inarr);
Py_DECREF(outarr);
return Py_None;
fail:
Py_XDECREF(inarr);
Py_XDECREF(outarr);
return NULL;
}
void
worker_function(double *input, double *output, npy_intp size)
{
npy_intp n;
for (n=0;n<size;n++)
{
output[n] = input[n] + 10;
}
return;
}
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