ConvertingDLLs.md

Converting DLLs

Overview

The code for shared-objects libraries in gcc is simpler than the code for Windows DLLs. For example, Windows has a DllMain() function and DLL_EXPORT declaration on functions. The code needed for a Windows DLL will not directly compile under gcc. So you have to do some conditional compilation to remove some of the extra stuff that Windows needs but gcc does not.

DLL_EXPORT

You need to undefine DLL_EXPORT for gcc builds. Include these lines before DLL_EXPORT is used. For example in header files that declare exported functions:

#ifdef WIN32

#undef DLL_EXPORT
#define DLL_EXPORT __declspec(dllexport)

#else

#undef DLL_EXPORT
#define DLL_EXPORT 

#endif

stdafx.h

You do not need many (if any) of the things in stdafx.h, so you can conditionally only compile them under Windows. For example:

#ifdef WIN32

// stdafx.h : include file for standard system include files,
// or project specific include files that are used frequently, but
// are changed infrequently
//
#include "targetver.h"

// Exclude rarely-used stuff from Windows headers
#define WIN32_LEAN_AND_MEAN

// Windows Header Files:
#include <windows.h>

#endif

dllmain.cpp

You do not need the DllMain() function in gcc, so you can conditionally compile it only under Windows. For example:

#ifdef WIN32

BOOL APIENTRY DllMain(
    HMODULE hModule,
    DWORD   ul_reason_for_call,
    LPVOID  lpReserved)
{
  switch (ul_reason_for_call) {
    case DLL_PROCESS_ATTACH:
    case DLL_THREAD_ATTACH:
    case DLL_THREAD_DETACH:
    case DLL_PROCESS_DETACH:
      break;
  }

  return TRUE;
}

#endif