MyDLL – Example Windows Dynamic-Link Library

This project demonstrates how to build a basic Windows DLL in C/C++, including:

• Exported and non-exported functions
• Use of __declspec(dllexport) and __declspec(dllimport)
• Handling DLL load/unload events using DllMain
• Providing a header file for consumers of the DLL

This repository contains two main files:

• mydll.cpp – Implementation of the DLL
• mydll.h – Public header for import/export macros

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Features

✔ Exported function (Share)

This function is marked with __declspec(dllexport) and can be called from any external program that loads the DLL:

DECLDIR void Share();

When called, it prints:

I am an exported function, can be called outside the DLL

✔ Internal function (Keep)

This function is not exported, meaning it can only be used inside the DLL:

void Keep();

It prints:

I am not exported, can be called only within the DLL

✔ DLL Entry Point (DllMain)

The DLL defines DllMain, which is executed automatically by Windows whenever:

• The DLL is loaded
• A process or thread attaches/detaches
• The DLL is unloaded

In DLL_PROCESS_ATTACH, both Share() and Keep() are called automatically.

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File Breakdown

mydll.h

#ifdef DLL_EXPORT
#define DECLDIR __declspec(dllexport)
#else
#define DECLDIR __declspec(dllimport)
#endif

extern "C"
{
    DECLDIR void Share();
    void Keep();
}

This header:

• Exports Share() when building the DLL
• Imports Share() when used by external programs
• Uses extern "C" to prevent C++ name mangling

mydll.cpp

DECLDIR void Share() { ... }
void Keep() { ... }

BOOL APIENTRY DllMain(HANDLE hModule, DWORD reason, LPVOID reserved)
{
    switch(reason)
    {
        case DLL_PROCESS_ATTACH:
            Share();
            Keep();
            break;
        ...
    }
}

This file includes:

• The implementation of Share() and Keep()
• The DllMain function that runs on DLL load/unload

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How to Build the DLL

Using Visual Studio

1. Create a new Win32 Project

2. Choose DLL

3. Add mydll.cpp and mydll.h

4. Ensure Preprocessor Definitions contains:

   DLL_EXPORT
   

5. Build → Build Solution

6. Visual Studio will generate:

   • mydll.dll
   • mydll.lib

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How to Use the DLL from C++

Example Usage Code

#include <Windows.h>
#include <iostream>

typedef void (*ShareFunc)();

int main()
{
    HMODULE dll = LoadLibrary("mydll.dll");
    if (!dll)
    {
        std::cout << "Failed to load DLL\n";
        return 1;
    }

    ShareFunc Share = (ShareFunc)GetProcAddress(dll, "Share");
    if (Share)
        Share();

    FreeLibrary(dll);
    return 0;
}

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How to Use the DLL from C#

using System;
using System.Runtime.InteropServices;

class Program
{
    [DllImport("mydll.dll")]
    public static extern void Share();

    static void Main()
    {
        Share();
    }
}

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Notes

• Share() is the only exported function
• Keep() cannot be used externally
• DllMain will execute automatically upon DLL load
• extern "C" ensures function names remain unmangled

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