Question

I have to pass a HWND variable from the main program to a function exported by a DLL. Variable is called mainHwnd and DLL is defined in this way:

mydll.h

#ifdef MYDLL_EXPORTS
#define MYDLL_API extern "C" __declspec(dllexport)
#else
#define MYDLL_API extern "C" __declspec(dllimport)
#endif

MYDLL_API HWND mainHwnd;
MYDLL_API void testFunction(void);
MYDLL_API LRESULT CALLBACK mouseProc(int nCode, WPARAM wParam, LPARAM lParam);

mydll.cpp

#include "stdafx.h"
#include "mydll.h"
#include <string>

#define CLASSNAMELEN    5

MYDLL_API HWND mainHwnd = 0;

// This is an example of an exported function.
MYDLL_API void testFunction(void)
{
MessageBox(NULL, (LPCWSTR)L"Test", (LPCWSTR)L"Test", MB_OK);
}

MYDLL_API LRESULT CALLBACK mouseProc(int nCode, WPARAM wParam, LPARAM lParam)
{
// processes the message
if(nCode >= 0)
{
    if(wParam != NULL && (wParam == WM_RBUTTONDOWN || wParam == WM_RBUTTONUP))
    {
        std::wstring s;
        MessageBox(NULL, (LPCWSTR)L"Captured mouse right button", (LPCWSTR)L"Test", MB_OK);
        MOUSEHOOKSTRUCT *m = (MOUSEHOOKSTRUCT*) lParam;
        GetClassName(m->hwnd, (LPWSTR) s.c_str(), CLASSNAMELEN);
        if(s == L"Edit")
            SendMessage(mainHwnd, WM_APP, 0, (LPARAM) lParam);
    }
}

// calls next hook in chain
return CallNextHookEx(NULL, nCode, wParam, lParam);
}

main program

MYDLL_API HWND mainHwnd;
...
case WM_CREATE:
    {   
        // now it will load DLL and set up hook procedure for mouse events

        // declares local variables
        HOOKPROC hkprcMouseProc;
        HINSTANCE hinstDLL; 
        HHOOK hhookMouseProc;
        // loads DLL
        if((hinstDLL = LoadLibrary(TEXT("C:\\Users\\Francesco\\Dropbox\\poli\\bi\\not\\pds\\sp\\wk5\\lsp5\\Debug\\mydll.dll"))) == NULL)
        {
            MessageBox(hWnd, (LPCWSTR)L"Error loading DLL", (LPCWSTR)L"Error", MB_OK | MB_ICONERROR);
            break;
        }

        // saves main window handle for DLL functions
        mainHwnd = hWnd;
...

Compiling I get

error LNK2001: unresolved external symbol __imp__mainHwnd

while using dumpbin /exports mydll.dll I get that global variable name is mangled as:

mainHwnd = _mainHwnd

I went through a lot of pages on google with no results. Maybe there is a conceptual error..
Thanks

Answer 1

When you’re loading the DLL at run-time using LoadLibrary, you cannot access its symbols directly in your code. (This is because these symbols have to be fixed up when your code module is loaded, at which time LoadLibrary has not been called yet, so there is nothing to resolve them against. The linker can detect that this will be the case at compile time, so it friendlily refuses to create an .exe that would just fail immediately anyway).

For dynamically loaded DLLs, you need to use GetProcAddress to retrieve the address of the variable at run time. (Despite its name, it will work for either exported variables or exported functions, and you’d better be sure to treat the result as the right thing!)

But I suspect you don’t really want to be loading the DLL at run time. Why don’t you just link the main application with the generated import library?