[Numpy-discussion] Cython: Passing numpy array into C via buffer?
Dag Sverre Seljebotn
Fri Sep 26 05:57:44 CDT 2008
Richard Shaw wrote:
> Forgive me if this is a stupid question, I've been looking around all
> the Cython documentation and I can't find out if this is possible.
> What I would like to do is generally is wrap a C function that takes a
> double array, and be able to pass in a numpy array, I was wondering if
> it's possible to do this using the buffer interface? I understand I
> could do it using this method (http://wiki.cython.org/WrappingNumpy),
> but the buffer interface seems much neater.
> Specifically, I have a C function that I would like to wrap with signature:
> #### cfile.h
> double do_something(double * array, int len);
> I was hoping with the buffer interface, I could do something like this:
> #### pyfile.pyx
> import numpy
> cimport numpy
> cdef extern from "cfile.h"
> double do_something(double * array, int len)
> cdef do_something_python(numpy.ndarray[numpy.float64, ndim = 1] numarr):
> cdef double x
> cdef double * data
> cdef int l
> data = ... # Not sure what to do here
> l = numarr.shape
> x = do_something(data, l)
I'm sorry, my previous answer was a really, really bad one. Here's a
data = <double*>numarr.data
:-) With the addition of the _t on float64 this should just work.
Please insert an extra check to make sure that your data is contiguous
though! Otherwise you need to make a copy to pass the data to an
external lib (one way is by using numpy.array with a contiguouos
argument to construct a copy, and then copy the data back again afterwards).
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