PRB: CWinApp-Based DLL Gets Assertion Failure on Win32s

The information in this article applies to:

• The Microsoft Foundation Classes (MFC), included with: Microsoft Visual C++ 32-bit Edition, version 1.0

SYMPTOMS

There is no problem starting a second instance of a Win32-based application that uses a CWinApp-based DLL on Windows NT. However, under Win32s, the same Win32-based application generates an assertion failure in the file APPCORE.CPP, line 193.

CAUSE

The inability to run two applications that use the same CWinApp-based DLL is because of differences in the way Windows NT and Win32s manage memory and the fact that the C run-time initializes global data and objects whenever a process loads a DLL.

Under Windows NT, each application gets its own copy of the DLL data, including the C run-time and MFC data. Under Win32s, all applications that use the DLL share the DLL data. When another process loads the DLL, the global objects are reinitialized. The assertion is generated because another copy of the CWinApp data already exists.

RESOLUTION

Replace the DLL entry point to work around the limitation. The sample code to do this is shown in the "MORE INFORMATION" section. The new entry point checks to see whether Win32s is loaded. If the application is running under Win32s, the entry point makes sure that the C run-time entry point is called only for the first process attach and the last process detach. This problem has been fixed in Microsoft Visual C++ 32-bit edition, versions 2.0 and 2.1.

MORE INFORMATION

To use this code as your new entry point, you need to change the entry point of your DLL to "NewDLLEntry()".

Sample Code

   BOOL WINAPI _DllMainCRTStartup( HINSTANCE, DWORD, LPVOID );

   extern "C" BOOL WINAPI
   NewDLLEntry( HINSTANCE hInstDLL, DWORD dwReason, LPVOID lpReserved )
   {
      BOOL bRet = TRUE;
      static DWORD dwVer = 0;
      static int nAttached = 0;

      if( !dwVer )
         dwVer = GetVersion();

      // Check for Win32s and version < 4.0 (Windows "Chicago").
      if( ((HIWORD(dwVer) & 0x8000) != 0x8000) || ((dwVer & 0xFF) >= '4') )
         bRet = _DllMainCRTStartup( hInstDLL, dwReason, lpReserved );
      else
      {
         switch (dwReason)
         {
         case DLL_PROCESS_ATTACH:
            if( nAttached++ == 0 )  // Call CRT entry if first process.
               bRet = _DllMainCRTStartup( hInstDLL, dwReason, lpReserved );
            break;

         case DLL_PROCESS_DETACH:
            if( --nAttached == 0 )  // Call CRT entry if last process.
               bRet = _DllMainCRTStartup( hInstDLL, dwReason, lpReserved );
            break;
         }
      }
      return bRet;
   }