Re: DLL declarations created by VC++ app wizard

From:

Norbert Unterberg <nunterberg@newsgroups.nospam>

Newsgroups:

microsoft.public.vc.language

Date:

Wed, 12 Dec 2007 22:37:27 +0100

Message-ID:

<#4RSycQPIHA.5184@TK2MSFTNGP05.phx.gbl>

SME schrieb:

Hi,

I have created a dll project using app wizard. It created the
dll_w32_export.h file as below:
----------------------------------------------
#ifdef DLL_W32_EXPORT_EXPORTS
#define DLL_W32_EXPORT_API __declspec(dllexport)
#else
#define DLL_W32_EXPORT_API __declspec(dllimport)
#endif

// This class is exported from the dll_w32_export.dll
class DLL_W32_EXPORT_API CDll_w32_export {
public:
CDll_w32_export(void);
// TODO: add your methods here.
};

extern DLL_W32_EXPORT_API int nDll_w32_export;

DLL_W32_EXPORT_API int fnDll_w32_export(void);
------------------------
My app code segement is:
int ret;
int OldErrMode;
HANDLE hDll;

// load dll
// set err mode to let app handle critical errors
OldErrMode = SetErrorMode (SEM_FAILCRITICALERRORS);
hDll = LoadLibrary ("dll_w32_export.dll");
SetErrorMode (OldErrMode);

if (!hDll)
{
ErrorNotify (IDS_LOADLIBFAILED);
return 1;
}

ret = fnDll_w32_export();
-------------------------------------
First I got link error:
error LNK2001: unresolved external symbol "__declspec(dllimport) int __cdecl
fnDll_w32_export(void)" (__imp_?fnDll_w32_export@@YAHXZ)

Then I added #define DLL_W32_EXPORT_EXPORTS

Now I get the error:
error LNK2001: unresolved external symbol "int __cdecl
fnDll_w32_export(void)" (?fnDll_w32_export@@YAHXZ)

What am I missing?

You are missing several things:

You need to #define DLL_W32_EXPORT_EXPORTS in your DLL project (when creating
the DLL), not in your application project. This makes the linker export the
class and the public symbols. It means that the classes functions and the
publich functions and variables are put into the DLL's export table and to the
import library file.

In your application, include the dll_w32_export.h without defining the
DLL_W32_EXPORT_EXPORTS.

Link the application against the dll_w32_export.lib file which is created when
linking the DLL. It is the DLL's import library that the linker generates when
linking the DLL.

Believe me, you do not want to dynamically load the DLL at runtime with
LoadLibrary. By linking the application against the DLL's import library you
tell windows to autimatically load the DLL when the application is started.

Norbert