Pass integer array by reference using ctypes

Question

I want to run a C++ function from python that returns an int* array by reference and converts it into a python list.

Here is the example C++ code:

#include "stdafx.h"
#include <iostream>

#define DLLEXPORT extern "C" __declspec(dllexport)

DLLEXPORT int getResponse(int*& hash_list)
{

    std::cout << hash_list << std::endl;

    int* hashes = new int[3];
    hashes[0] = 8;
    hashes[1] = 9;
    hashes[2] = 10;
    hash_list = hashes;

    std::cout << hash_list << std::endl;
    std::cout << *hash_list << std::endl;
    std::cout << *(hash_list + 1) << std::endl;
    std::cout << *(hash_list + 2) << std::endl;

    return 0;
}

DLLEXPORT void testDLL()
{
    std::cout << "DLL can be read" << std::endl;
}

Here is my best attempt in python:

from ctypes import cdll, c_int, POINTER, ARRAY, byref

LIB = cdll.LoadLibrary("<path to DLL>")

LIB.testDLL()

func = LIB.getResponse
itype = c_int
func.argtypes = [POINTER(ARRAY(itype,3))]
func.restype = c_int

chashes = (itype * 3)(*[0,1,2])
print(chashes)

func(byref(chashes))

print(chashes)
print(list(chashes))

This is the output:

DLL can be read
<ctypes.c_long_Array_3 object at 0x000001B00FB7FEC8>
0000000100000000
000001B00DB0AC70
8
9
10
<ctypes.c_long_Array_3 object at 0x000001B00FB7FEC8>
[229682288, 432, 2]

This approach seems to have some success, however I think the initial array passed to the C++ function has invalid entries. And then the returned values are mangled anyway.

Is this possible with ctypes? Any suggestions would be appreciated.

I also tried using a c_void_p instead of the ARRAY. In that case it also seems to work, but I don't know how to turn the resulting pointer into a python list.

Cffi is a newer library that can do the same job

joeforker
– joeforker

2019-03-13 16:01:02 +00:00
Commented Mar 13, 2019 at 16:01 — joeforker
– joeforker, Commented Mar 13, 2019 at 16:01

Mark Tolonen · Accepted Answer · 2019-03-13 18:17:15Z

1

ctypes accesses C interfaces. C doesn't have pointer references, but when treated as extern "C" a pointer reference is marshaled the same as a pointer pointer. Just don't pass None from Python since C++ will see that as a null reference. You'd be better off actually declaring the function to take int** so you can deal with the possibility that None could be passed from Python.

Use:

func.argtypes = [POINTER(POINTER(c_int))]

Call it with:

p = POINTER(c_int)()
func(byref(p))
print(p[:3])

edited Mar 13, 2019 at 18:17

answered Mar 13, 2019 at 17:59

Mark Tolonen

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