C / C++ / MFC

#pragma data_seg(".SHARDAT")
static int connections = 0;
#pragma data_seg()

Server side (Dll)

1. Create a Win32 DLL (I called mine DllSharedDataServer)
   
2. Create a header file for the DLL (DllSharedDataServer.h)
   
3. Add the following code to define the shared data. Note that I would normally just define this in the DLL's main CPP file, but I'm guessing from your question that your DLL has multiple CPP files, all of which need access to this variable.
   

   #pragma once
   
   #pragma data_seg(".SHARDAT")
   static int connections = 0;
   #pragma data_seg()

   
   
4. Add the following function to the DllSharedDataServer.h header file so that clients can query as to the number of connected users.
   

   int GetNumberOfConnections();

   
   
5. Add an include directive to the DllSharedDataServer.cpp file
   

   #include "DllSharedDataServer.h"

   
   
6. Update the DllMain function to increment the number of connected users when a client attaches and decrement the counter when a client detaches:
   

   BOOL APIENTRY DllMain( HANDLE hModule, 
                          DWORD  ul_reason_for_call, 
                          LPVOID lpReserved
   					 )
   {
     switch (ul_reason_for_call)
     {
       case DLL_PROCESS_ATTACH:
         connections++;
         break;
       case DLL_PROCESS_DETACH:
         connections--;
         break;
       default:
         break;
     }
     return TRUE;
   }

   
   
7. Add the GetNumberOfConnections function to return the counter's value
   

   int GetNumberOfConnections()
   {
     return connections;
   }

   
   
8. Add a DEF file to the DLL and update it as follows so that the GetNumberOfConnections function is exported and the shared data segment is properly defined:
   

   LIBRARY	DllSharedDataServer
   
   SECTIONS
   .SHARDAT READ WRITE SHARED
   
   EXPORTS
     GetNumberOfConnections

   
   
9. Build the DLL
   

Client side

1. Create an MFC dialog-based app to test with (DllSharedDataClient). In cases like this where the two projects are meant to work together I usually specify the Add to solution option so that it's easier to work on both projects at the same time.
   
2. Include the DllSharedDataServer.h file in the DllSharedDataClientDlg.cpp file.
   
3. In the dialog's OnOK handler place the following call to the GetNumberOfConnections function
   

   void CDllSharedDataClient1Dlg::OnBnClickedOk()
   {
     int i = GetNumberOfConnections();
     CString str;
     str.Format("There are currently %ld connections to the DLL", i);
     AfxMessageBox(str);
   }

   
   
4. Add the DllSharedDataServer.lib to the DllSharedDataClient project
   
5. Build the client
   
6. Finally, copy the DLL to a folder where it can be found when running the client.
   

   Testing the DLL

   
   Now simply run a few instances of the client and you will see that each time you click the OK button the correct number of connections will be displayed; thereby proving that your data is being shared among multiple connections to the DLL.
   
   
   Cheers,
   Tom Archer
   Author, Inside C#
   Author, Visual C++.NET Bible
   
   A total abstainer is one who abstains from everything but abstention, and especially from inactivity in the af