Passing numpy array to c++ double(**) pointer array inside a c++ dll using python ctypes

Question

How can I pass numpy array to c++ double(**) pointer array inside a c++ dll using python ctypes?

My c++ function definition looks like this:

int do_calc(unsigned short** input_array)
{
//doing something with array
}

I tried calling from python in 2 ways:

1.

libhandle = ctypes.CDLL(library_path)
libhandle.do_calc.argtypes = [ctypes.c_void_p]
libhandle.do_calc.restype = ctypes.c_int
//np_array: python numpy array
error_code = libhandle.do_calc(np_array.ctypes.data_as(ctypes.c_void_p))

libhandle = ctypes.CDLL(library_path)
libhandle.do_calc.argtypes = [ctypes.c_void_p]
libhandle.do_calc.restype = ctypes.c_int
//np_array: python numpy array
int_pointer = ctypes.POINTER(ctypes.c_ushort)
error_code = libhandle.do_calc(np_array.ctypes.data_as(int_pointer))

But I am getting this access error in both:

OSError: exception: access violation reading 0x000001C89A480000

You can't do that; NumPy arrays are "flat" since that's more efficient. — molbdnilo
– molbdnilo, Commented Dec 6, 2022 at 10:09

Mark Tolonen · Accepted Answer · 2022-12-06 16:45:35Z

As commented, numpy doesn't create double pointers as they are inefficient. Assuming you can alter your C++ definition, this works:

test.cpp

#ifdef _WIN32
#   define API __declspec(dllexport)
#else
#   define API
#endif

extern "C" {

API void do_calc(unsigned short* input_array, size_t rows, size_t cols)
{
    unsigned short x = 0;
    for(size_t r = 0; r < rows; ++r)
        for(size_t c = 0; c < cols; ++c)
            input_array[r * cols + c] = ++x;
}

}

test.py

import numpy as np
import ctypes as ct

dll = ct.CDLL('./test')
# explicitly accept a 2-D numpy array of ushorts
dll.do_calc.argtypes = np.ctypeslib.ndpointer(dtype=ct.c_ushort, ndim=2), ct.c_size_t, ct.c_size_t
dll.do_calc.restype = None

rows, cols = 3, 4
arr = np.zeros((rows, cols), dtype=ct.c_ushort)
dll.do_calc(arr, *arr.shape)
print(arr)

Output:

[[ 1  2  3  4]
 [ 5  6  7  8]
 [ 9 10 11 12]]

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