INFO: Portable DLL Example Using _declspec() and _export

• Microsoft Visual C++, 32-bit Edition, versions 1.0, 2.0, 2.1, 4.0, 5.0

SUMMARY

This article shows what you need to do to create portable code for both 16-bit and 32-bit application.

MORE INFORMATION

In Visual C++, 32-bit Edition, _declspec(dllexport) is used to export code and data and _declspec(dllimport) is used to import code and data. This is a change from Visual C++ for Windows where __export is used to export and import code and the .DEF file is used to export and import data.

It is not possible to maintain a single source for 16-bit and 32-bit code by simply replacing __export with _declspec() because _declspec() is a storage class modifier and __export is a type modifier.

The following sample application shows by example how you can create portable code for both 16-bit and 32-bit applications. The sample application exports code and data from a DLL. Both the EXE and DLL can be built by using this command:

   NMAKE /f declspec.mak [WIN16=1]

The code uses a single header file in both the EXE and DLL. You do not need _declspec(dllimport) for importing code, but it is required for data. Faster code is generated when using _declspec(dllimport) with functions.

For more information, please see the following article in the Microsoft Knowledge Base:

   ARTICLE-ID: Q107501
   TITLE     : __declspec(dllexport) Replaces __export in 32-bit Visual C++

Sample Code

   #
   # DECLSPEC.MAK
   #
   !IFNDEF WIN16
   # Win32 build commands:

   all: decldll.dll declexe.exe

   decldll.obj: decldll.c
      cl /c /DWIN32 /DDLL decldll.c

   decldll.dll: decldll.obj
      link /DLL /OUT:decldll.dll decldll.obj kernel32.lib user32.lib \
      gdi32.lib

   declexe.obj: declexe.c
      cl /c /DWIN32 declexe.c

   declexe.exe : declexe.obj decldll.dll
      link /OUT:declexe.exe declexe.obj decldll.lib kernel32.lib \
      user32.lib gdi32.lib

   !ELSE
   # Win16 build commands:

   all: decldll.dll declexe.exe

   decldll.obj: decldll.c
      cl /c /GD /ALw decldll.c

   decldll.dll: decldll.obj
      link decldll.obj, decldll.dll,,ldllcew.lib libw.lib, decldll.def
      implib decldll.lib decldll.dll

   declexe.obj: declexe.c
      cl /c /GA /AL /Gx- declexe.c

   declexe.exe : declexe.obj decldll.dll
      link declexe.obj, declexe.exe,,llibcew.lib libw.lib decldll.lib, \
      declexe.def

   !ENDIF

   -----------------------------------------------------------------------
   /*
    * DECLDLL.H
    */
   #if !defined (_WIN32)
   #   define EXPORT __export
   #   define EXPORT32
   #else
   #   define EXPORT
   #   if defined DLL
   #      define EXPORT32 __declspec(dllexport)
   #   else
   #      define EXPORT32 __declspec(dllimport)
   #   endif
   #endif

   EXPORT32 void EXPORT dll_func(void);
   extern EXPORT32 int EXPORT i;

   -----------------------------------------------------------------------
   /*
    * DECLDLL.C
    */
   #include <windows.h>
   #include "decldll.h"

   /* Both LibMain(), initialization function for 16-bit DLLs, and
      DllMain(), initialization function for 32-bit DLLs are optional. */

   EXPORT32 void EXPORT dll_func(void)
   {
      MessageBox(NULL, "We are in the DLL\n", "declspec() example", MB_OK);
   }

   /* In Win16, data must be exported using .DEF */
   EXPORT32 int i = 17;

   -----------------------------------------------------------------------
   ;
   ; DECLDLL.DEF - Used only in 16-bit project
   ;
   LIBRARY DECLDLL

   EXPORTS
           WEP @1
           _i
   -----------------------------------------------------------------------
   /*
    * DECLEXE.C
    */
   #include <windows.h>
   #include "decldll.h"

   int PASCAL WinMain(HINSTANCE hInstance, HINSTANCE hPrevInstance,
           LPSTR lpCmdLine, int cCmdShow)
   {
      MessageBox(NULL, "We are in the EXE\n", "declspec() sample", MB_OK);
      dll_func();
      MessageBox(NULL, "We are in the EXE\n", "declspec() sample", MB_OK);
      if (i==17)
         MessageBox(NULL, "Exported Data is OK", "declspec() sample",

      else
         MessageBox(NULL, "Exported Data is NOT OK\n",
                          "declspec() sample", MB_OK);
      return 1;
   }

   -----------------------------------------------------------------------
   ;
   ; DECLEXE.DEF - Used only in 16-bit version
   ;
   NAME Declexe

   EXETYPE WINDOWS

   STACKSIZE 4096
   -----------------------------------------------------------------------

Keywords          : CLngIss kbcode
Version           : WINNT:1.0,2.0,2.1,4.0,5.0;
Platform          : NT WINDOWS
Issue type        : kbinfo