Passing Numpy array to C with Ctypes differs between Linux and Windows

Question

I am trying to pass a Numpy array into C, but get different results in Windows and Linux.

In Python

import platform
import numpy as np
import ctypes

if platform.system() == 'Windows':
    c_fun = np.ctypeslib.load_library("/mypath/c_fun.dll", ".").c_fun
else:    # Linux
    c_fun = np.ctypeslib.load_library("/mypath/c_fun.so", ".").c_fun
c_fun.argtypes = [np.ctypeslib.ndpointer(dtype=np.int, ndim=2, flags="C_CONTIGUOUS"), ctypes.c_int, ctypes.c_int]

array = np.array([[0, 1, 0], [0, 1, 0], [0, 1, 0]])
rows, cols = array.shape
c_fun(array, rows, cols)

In C

void c_fun(int* array, int rows, int cols)
{
    for (int i = 0; i < rows * cols; i++)
        printf("%d ", array[i]);
}

When I run the program in Windows, the output is "0 1 0 0 1 0 0 1 0", it works well.

But in Linux, the output is "0 0 1 0 0 0 0 0 1", why?

user2357112 · Accepted Answer · 2019-11-08 03:11:43Z

2

First, don't use numpy.int. It's just int, not any sort of NumPy thing. I think it's there for backward compatibility.

NumPy converts Python ints to dtype numpy.int_ (note the underscore) by default, and numpy.int_ corresponds to C long, not C int. Your code thus only works when C int and long are the same size, which they are on Windows, but not Linux.

answered Nov 8, 2019 at 3:11

user2357112

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