How to use a dll that returns array pointer in python?

Question

I'm trying to get out an array in a pythonic format [numpy or list] form a DLLwritten in c#/c++. This DLL gives out the pointer to the memory and not the evaluated array. How can I access the memory in a FAST way (I wrote the DLL to speedup the code)?

I have used the pythonnet module and i managed to get the memory pointer as a but I'm failing in extracting the memory pointer as an integer or binary and then to get the data of the output vector.

Here the python code:

import numpy as np
import clr
clr.AddReference(R'...\PyDLL.dll')  
from PyDLL import StringLibrary

my_instance = StringLibrary()

x = my_instance.arrayShift(np.array([1,2,3,4,5,6,7,8,9,0]).reshape(-1))

And then the C# code

namespace PyDLL
{
    public class StringLibrary
    {
        public static double[] arrayShift(double[] array)
        {
            int b = array.Length;
            double[] newArray = new double[b];

            for(int i=0; i<b; i++)
            {
                newArray[i] = array[b-i-1];
            }

            return newArray;
        }
    }
}

I expect that the output x in python is a list or a np array, while now it is a System.Double[] object.

the expected output of x is [0,9,8,7,6,5,4,3,2,1]

Thank you for any help

Alessio Salvaggio · Accepted Answer · 2019-10-11 07:15:53Z

After many tries and researches, I managed to solve the problem in this way:

Cpp part:

Open Visual Studio, then create a new project (Dynamic-Link Library). Name the project and add a new file in the project (Header file) and name it with the same project name. Insert the following code in the header:

// headerName.h - Contains declarations of math functions
#pragma once
#ifdef HEADERNAME_EXPORTS
#define HEADERNAME_API __declspec(dllexport)
#else
#define HEADERNAME_API __declspec(dllimport)
#endif

// Define the functions that will be called using the DLL
extern "C" HEADERNAME_API void funName1(...);
extern "C" HEADERNAME_API void funName2(...);
extern "C" HEADERNAME_API void funName3(...);
...;

In the folder 'Source Files' add a new file with extension .cpp (call it with the same name as the project), then write the function in this .cpp file. Here an example code:

#include "pch.h" // use stdafx.h in Visual Studio 2017 and earlier  COMPULSORY TO ADD
#include "HEADERNAME.h" // COMPULSORY TO ADD
#include lib#1
#include lib#2
#include lib#3
...
using namespace std;

void funName1(...)
{
    ...
}
void funName2(...)
{
    ...
}
void funName3(...)
{
    ...
}

To be able to pass the arrays between python and c++, in my solution, the use of pointers is needed and no return must be used. The output variable is passed as an input variable, initialized in python, using a pointer. In the DLL the memory cells of the output variables are re-wrote and, doing so, when the DLL completes the function, the output is computed and already usable by python without return. Here an example of how to write a function using pointers for input and output arrays:

void funName(double* inputArray_pointer, double* outputArray_pointer, int extraVariable)
{
    double inner_var = 100;
    for (int i = 0; i < extraVariable; i++)
    {
        inner_var = min(inner_var, *(inputArray_pointer+i));
        *(outputArray_pointer + i) = inner_var;
    }
}

Then rebuild the DLL.

Python part

To use correctly a DLL in the python code, the modules 'ctypes' and 'numpy' must be imported. Here follows an example of the python code (This will use the example DLL of point (9)):

import numpy as np
import ctypes

cpp_fun = ctypes.CDLL(R'pathToDll/DLLname.dll')  

# Define the types of the arguments of the DLL (pointer, int, double, ecc)
cpp_fun.funName.argtypes = [np.ctypeslib.ndpointer(),
                            np.ctypeslib.ndpointer(),
                            ctypes.c_int]

# Allocate or define the input variables using numpy or the standard scalar of python (int, float, ecc..)
inputArray = np.array([0,1,2,3,4,5,6,7,8,9,...,1000]).reshape(-1).astype('f8')
outputArray = np.zeros([inputArray.shape[0]])astype('f8')
extraVariable = inputArray.shape[0]
cpp_fun.funName(inputArray, outputArray, extraVariable)

Doing so, in the variable 'outputArray' we will find the result of the calculations performed inside the DLL as a np array structure.

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