Python 2D array i C using ctypes

Question

i\'m trying to use a self written c lib to proces 2d array from python but with little success.

Here is my c code:

CamLibC.c

int TableCam(const i         


        

Answer 1

• c_long is the same as c_int, but that is not the problem.

There are a number of issues:

• The type of numpy.zeros is default float.
• In the Python TableCam, A is never modified.
• In the C TableCam, the 3rd parameter should be int* Array and the elements modified by computing i*y+j. That also agrees with argtypes, which was correct.
• The numpy array can be coerced to the right ctypes type.

Corrected code (on Windows, for my testing):

cam.c

#include <stdio.h>
__declspec(dllexport) void TableCam(const int x, const int y, int *Array)
{
    int i,j;
    for (i = 0; i < x; i++)
        for (j = 0; j < y; j++)
            Array[i*y+j] = 1;
}

camlib.py

import ctypes

_CamLib = ctypes.CDLL('cam')
_CamLib.argtypes = [ctypes.c_int, ctypes.c_int, ctypes.POINTER(ctypes.c_int)]
_CamLib.restype = None

def TableCam(A):
    x,y = A.shape
    array = A.ctypes.data_as(ctypes.POINTER(ctypes.c_int))
    _CamLib.TableCam(x,y,array)

test.py

import camlib
import numpy as np

Anum = np.zeros((3,3),dtype=np.int)
print('Start:')
print(Anum)
camlib.TableCam(Anum)
print('Ended:')
print(Anum)

Output

Start:
[[0 0 0]
 [0 0 0]
 [0 0 0]]
Ended:
[[1 1 1]
 [1 1 1]
 [1 1 1]]