How to use C/C++ plugins (C/C++ lib) in Unity3D

Create C and C++ plugins for Unity3D

"LowLevelPlugin.h" 

#pragma once 
#if UNITY_METRO 
#define EXPORT_API __declspec(dllexport) __stdcall 
#elif UNITY_WIN 
#define EXPORT_API __declspec(dllexport) 
#else 
#define EXPORT_API 
#endif


Open C source file and paste following code:
#include <stdlib.h> 
#include <math.h> 
#include "LowLevelPlugin.h" 
extern "C" int ** EXPORT_API fillArray(int size) { 
    int i = 0, j = 0; 
    int ** array = (int**) calloc(size, sizeof(int*)); 
    for(i = 0; i < size; i++) { 
        array[i] = (int*) calloc(size, sizeof(int)); 
        for(j = 0; j < size; j++) { 
            array[i][j] = i * size + j; 
        } 
    } 
    return array; 
}

Using library in Unity3d:
[DllImport ("LowLevelPlugin")]
private static extern int [,] fillArray(int size);



Reference:
[1] https://medium.com/@rafalwilinski/tutorial-create-c-c-plugins-for-unity3d-dbde7f67454