Re: [Numpy-discussion] Cython: Passing numpy array into C via buffer?
Richard Shaw wrote: Hello, 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. Questions like this should be posted to the Cython list (otherwise us Cython developers might easily ignore it, we do not follow everything that goes on in numpy-discussion). 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): numpy.float64_t, currently at least. cdef double x cdef double * data cdef int l data = ... # Not sure what to do here l = numarr.shape[0] Yes, I understand this. I definitely plan to make this easier in a coming Cython release, i.e. something like cimport cython ... data = cython.buffer.bufptr(array) I just haven't got there yet. For now, there's an ugly hack you can do (untested though): your_header.h: #define GET_BUFPTR_HACK(x) __pyx_bstruct_#x.buf yourfile.pyx: cdef extern from your_header.h: void* GET_BUFPTR_HACK(x) ... data = GET_BUFPTR_HACK(buf) (The reason this is so horribly ugly is the #x in the macro, meaning this depends very much of the particular mangling scheme used. This will likely break in a future version of Cython, but then cython.buffer.butptr should be available.) -- Dag Sverre ___ Numpy-discussion mailing list Numpy-discussion@scipy.org http://projects.scipy.org/mailman/listinfo/numpy-discussion
Re: [Numpy-discussion] Cython: Passing numpy array into C via buffer?
Richard Shaw wrote: Hello, 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[0] x = do_something(data, l) I'm sorry, my previous answer was a really, really bad one. Here's a better one: 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). -- Dag Sverre ___ Numpy-discussion mailing list Numpy-discussion@scipy.org http://projects.scipy.org/mailman/listinfo/numpy-discussion
Re: [Numpy-discussion] Cython: Passing numpy array into C via buffer?
Dag Sverre Seljebotn wrote: I'm sorry, my previous answer was a really, really bad one. Here's a better one: data = double*numarr.data :-) With the addition of the _t on float64 this should just work. That sounds great. I thought there must be something like that, it's glad to see that Cython doesn't disappoint. I'll get straight to trying it out. 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). That make sense. I'll be sure to do check that one. Thanks for your very swift response, it's really helpful. I'll be sure to post to the Cython-dev list in future if that's more appropriate. Cheers, Richard ___ Numpy-discussion mailing list Numpy-discussion@scipy.org http://projects.scipy.org/mailman/listinfo/numpy-discussion
