How to return array from C++ function to Python using ctypes

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I am using ctypes to implement a C++ function in Python. The C++ function should return a pointer to an array. Unfortunately I haven’t figured out, how to access the array in Python. I tried numpy.frombuffer, but that was not successful. It just returned an array of arbitrary numbers. Obviously I didn’t used it correctly. Here is a simple example with an array of size 10:

Content of function.cpp:

extern "C" int* function(){
int* information = new int[10];
for(int k=0;k<10;k++){
    information[k] = k;
}
return information;
}

Content of wrapper.py:

import ctypes
import numpy as np

output = ctypes.CDLL('./library.so').function()

ArrayType = ctypes.c_double*10
array_pointer = ctypes.cast(output, ctypes.POINTER(ArrayType))
print np.frombuffer(array_pointer.contents)

To compile the C++ file i am using:

g++ -c -fPIC function.cpp -o function.o
g++ -shared -Wl,-soname,library.so -o library.so function.o

Do you have any suggestions what I have to do to access the array values in Python?

Your python code will work after some minor modifications:

import ctypes

f = ctypes.CDLL('./library.so').function
f.restype = ctypes.POINTER(ctypes.c_int * 10)
print [i for i in f().contents] # output: [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]

Basically there are two changes:

  1. remove numpy-related code and ctypes.cast call since we don’t need them.

  2. specify the return type to ctypes.POINTER(ctypes.c_int * 10).

    By default foreign functions are assumed to return the C int type, hence we need change it to the desired pointer type.

BTW, returning a newed array from C code to Python code seems inappropriate. Who and when will free the memory? It’s better to create arrays in Python code and pass them to C code. This way it’s clear that the Python code owns the arrays and takes the responsibility of creating and reclaiming their spaces.

function.cpp returns an int array, while wrapper.py tries to interpret them as doubles. Change ArrayType to ctypes.c_int * 10 and it should work.


It’s probably easier to just use np.ctypeslib instead of frombuffer yourself. This should look something like

import ctypes
from numpy.ctypeslib import ndpointer

lib = ctypes.CDLL('./library.so')
lib.function.restype = ndpointer(dtype=ctypes.c_int, shape=(10,))

res = lib.function()


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