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 int x, const int y, int **Array) {
    int i = 0;
    int j = 0;

    for (i; i < x; i++) {
        for (j; j < y; j++) {
            Array[i][j] = 1;
        };

    };
}

cc -nostartfiles -shared -fPIC -o CamLibOS.os CamLibC.c

Now here is my python wrapper:

CamLibPy.py

import os,sys
import ctypes

dirname     = os.path.dirname(os.path.realpath(sys.argv[0]))
CamLibFile  = dirname + '/CamLibOS.os'

_CamLib = ctypes.CDLL(CamLibFile)
_CamLib.TableCam.argtypes = [ctypes.c_int, ctypes.c_int, ctypes.POINTER(ctypes.c_int)]

def TableCam (A) :
    global _CamLib

    x = len(A)
    y = len(A[0])

    print('x: ', x, ' y: ', y);

    arrayType = ((ctypes.c_int * x) * y)
    array = arrayType()

    _CamLib.TableCam(ctypes.c_int(x), ctypes.c_int(y), array)

    print(array)

And my python code where i use the function:

Test.py

import CamLibPy
from numpy import zeros

Anum = zeros((3,3))
print('Start: ', Anum)

CamLibPy.TableCam(Anum)

print('Ended: ', Anum)

In this test program i try to change all the zeros in the array to ones. but as soon as i try to run this is get the following output:

Start: [[ 0. 0. 0.] [ 0. 0. 0.] [ 0. 0. 0.]]

x: 3 y: 3

Traceback (most recent call last): File "/media/pi/USB DISK/Test/Test.py", line 7, in CamLibPy.TableCam(Anum) File "/media/pi/USB DISK/Test/CamLibPy.py", line 21, in TableCam _CamLib.TableCam(ctypes.c_int(x), ctypes.c_int(y), array) ctypes.ArgumentError: argument 3: : expected LP_c_long instance instead of c_long_Array_3_Array_3

it's saying it expected a c_long but i clearly used c_int to make the arrayType

Can somebody tell me what i did wrong?

Mark Tolonen · Accepted Answer · 2017-04-01 05:17:42Z

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]]

answered Apr 1, 2017 at 5:17

Mark Tolonen

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1 Comment

N Zuiderhoek Over a year ago

Thanks mate for the fast response. I tested the code om my raspberry and it works!! You have no idea how long i have been struggling with this code.

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