Question

I started studying DLL’s with implicit linking. I don’t really fully understand how it works. Please correct me where I’m wrong.
I failed to compile the next code(3 modules):

MyLib.h

#ifdef MYLIBAPI

#else


#define MYLIBAPI extern "C" __declspec(dllimport)

#endif


MYLIBAPI int g_nResult;

MYLIBAPI int Add(int nLeft, int nRight);

As far as I understand this is the header of the DLL. #define MYLIBAPI extern "C" __declspec(dllimport) means that here we are going to declare some functions/variables that will be described in devoted .cpp file and will be contained in a DLL.

MyLibFile1.cpp

#include <windows.h>

#define MYLIBAPI extern "C" __declspec(dllexport)

#include "MyLib.h"

int g_nResult; 
int Add(int nLeft, int nRight) {
   g_nResult = nLeft + nRight;
   return(g_nResult);
}

So, this is obviously the file where our functions are implemented. This is the part of the DLL, right?

MyExeFile1.cpp

#include <windows.h>
#include <strsafe.h>
#include <stdlib.h>

#include "MyLib.h"

int WINAPI _tWinMain(HINSTANCE, HINSTANCE, LPTSTR, int) {

   int nLeft = 10, nRight = 25;

   TCHAR sz[100];
   StringCchPrintf(sz, _countof(sz), TEXT("%d + %d = %d"),
      nLeft, nRight, Add(nLeft, nRight));
   MessageBox(NULL, sz, TEXT("Calculation"), MB_OK);

   StringCchPrintf(sz, _countof(sz),
      TEXT("The result from the last Add is: %d"), g_nResult);
   MessageBox(NULL, sz, TEXT("Last Result"), MB_OK);
   return(0);
}

So, this is the executable file where we use the functions from the library.
The whole thing doesn’t work. I tried to put this all into one directory and compile at once. I tried first to compile a DLL from the first two modules(successfully) and then compile the executable (changing the path to the header file). However it resulted in 2 errors both times:

error LNK2019: unresolved external symbol _WinMain@16 referenced in function ___tmainCRTStartup

\Visual Studio 2008\Projects\MyExeFile1\Debug\MyExeFile1.exe : fatal error LNK1120: 1 unresolved externals

What’ s the correct way to do that – what should I change in the code and how should I compile the code (I use VS2008)?
Thanks.

Answer 1

#include <tchar.h> to solve the linker error.

Your header file should look like this:

#ifdef BUILDING_DLL
#  define MYLIBAPI extern "C" __declspec(dllexport)
#else
#  define MYLIBAPI extern "C" __declspec(dllimport)
#endif

MYLIBAPI int __stdcall Add(int nLeft, int nRight);

Right-click your DLL project in Solution Explorer, Properties, C/C++, Preprocessor, Preprocessor Definitions, add “BUILDING_DLL”. Repeat for the Release configuration.

You can verify that your DLL properly exports the functions with Dumpbin.exe /exports.

The __declspec(dllimport) declarator is not strictly necessary, it does however make it more efficient. The __stdcall attribute is not necessary either, it does however make your DLL usuable from any language that supports calling DLL exports.