Pass a 2d numpy array to c using ctypes

Question

What is the correct way to pass a numpy 2d - array to a c function using ctypes ? My current approach so far (leads to a segfault):

C code :

void te         


        

Answer 1

While the reply might be rather late, I hope it could help other people with the same problem.

As numpy arrays are internally saved as 1d arrays, one can simply rebuild 2d shape in C. Here is a small MWE:

// libtest2d.c
#include  // for malloc and free
#include   // for printf

// create a 2d array from the 1d one
double ** convert2d(unsigned long len1, unsigned long len2, double * arr) {
    double ** ret_arr;

    // allocate the additional memory for the additional pointers
    ret_arr = (double **)malloc(sizeof(double*)*len1);

    // set the pointers to the correct address within the array
    for (int i = 0; i < len1; i++) {
        ret_arr[i] = &arr[i*len2];
    }

    // return the 2d-array
    return ret_arr;
}

// print the 2d array
void print_2d_list(unsigned long len1,
    unsigned long len2,
    double * list) {

    // call the 1d-to-2d-conversion function
    double ** list2d = convert2d(len1, len2, list);

    // print the array just to show it works
    for (unsigned long index1 = 0; index1 < len1; index1++) {
        for (unsigned long index2 = 0; index2 < len2; index2++) {
            printf("%1.1f ", list2d[index1][index2]);
        }
        printf("\n");
    }

    // free the pointers (only)
    free(list2d);
}

and

# test2d.py

import ctypes as ct
import numpy as np

libtest2d = ct.cdll.LoadLibrary("./libtest2d.so")
libtest2d.print_2d_list.argtypes = (ct.c_ulong, ct.c_ulong,
        np.ctypeslib.ndpointer(dtype=np.float64,
            ndim=2,
            flags='C_CONTIGUOUS'
            )
        )
libtest2d.print_2d_list.restype = None

arr2d = np.meshgrid(np.linspace(0, 1, 6), np.linspace(0, 1, 11))[0]

libtest2d.print_2d_list(arr2d.shape[0], arr2d.shape[1], arr2d)

If you compile the code with gcc -shared -fPIC libtest2d.c -o libtest2d.so and then run python test2d.py it should print the array.

I hope the example is more or less self-explaining. The idea is, that the shape is also given to the C-Code which then creates a double ** pointer for which the space for the additional pointers is reserved. And these then are then set to point to the correct part of the original array.

PS: I am rather a beginner in C so please comment if there are reasons not to do this.