File I/O in a C++ DLL?

I am trying to build a dll that reads a text file to populate a 2d array, then change that array as needed. I'm using a VB GUI to access it. The overall program is a micromouse simulator in which the user is able to customize the wall placement in a 5x5 maze, as well as mouse start position and goal placement, and allow the search algorithm (dll) to solve it. Here's the code inside my dll:

/*testDLL.cpp*/

#include "testDLL.h"
#include <stdio.h>

FILE *maze;

char mazearray[12][12];

void _stdcall wallfunction(int x, int y){

    maze = fopen ("C:\Users\Public\Documents\5x5mazedefault.txt", "r");
    fread (mazearray, sizeof(mazearray), 1, maze);
    fclose(maze);
    if (mazearray[x][y] == 'X'){
            mazearray[x][y] = ' ';
    }
    else if (mazearray[x][y] == ' '){
        mazearray[x][y] = 'X';
    }
}

I want to be able to put in two input variables as the index of the matrix and add or subtract a wall from that location. Whenever I try to call the function from VB, it sends me a message: PInvoke restriction cannot return variants. The function returns nothing, so I don't understand...

Here's the declaration statement inside my VB program:

Private Declare Function wallfunction Lib "C:\Path\Path\testDLL.dll" (ByVal x As Integer, ByVal y As Integer)

I'm aware I'm not going to be able to c开发者_C百科all the fread function everytime the user wants to change a wall; I'm just trying to get this working once first. Any thoughts?

Change Function to Sub in your Declare statement in VB. This is because your C++ function returns void.