Question

I’d like to call my C function from Python, in order to manipulate some NumPy arrays. The function is like this:

void c_func(int *in_array, int n, int *out_array);

where the results are supplied in out_array, whose size I know in advance (not my function, actually). I try to do in the corresponding .pyx file the following, in order to able to pass the input to the function from a NumPy array, and store the result in a NumPy array:

def pyfunc(np.ndarray[np.int32_t, ndim=1] in_array):    
    n = len(in_array)
    out_array = np.zeros((512,), dtype = np.int32)
    mymodule.c_func(<int *> in_array.data, n, <int *> out_array.data)
    return out_array

But I get
"Python objects cannot be cast to pointers of primitive types" error for the output assignment. How do I accomplish this?

(If I require that the Python caller allocates the proper output array, then I can do

def pyfunc(np.ndarray[np.int32_t, ndim=1] in_array, np.ndarray[np.int32_t, ndim=1] out_array):  
    n = len(in_array)
    mymodule.cfunc(<int *> in_array.data, n, <int*> out_array.data)

But can I do this in a way that the caller doesn’t have to pre-allocate the appropriately sized output array?

Answer 1

You should add cdef np.ndarray before the out_array assignement:

def pyfunc(np.ndarray[np.int32_t, ndim=1] in_array):    
    cdef np.ndarray out_array = np.zeros((512,), dtype = np.int32)
    n = len(in_array)
    mymodule.c_func(<int *> in_array.data, n, <int *> out_array.data)
    return out_array