SNA Server Win 3.x APPC/CPIC Apps Hang Using Blocking Hook

• Microsoft SNA Server for Windows NT, versions 2.0, 2.1, 2.11 and 3.0.

SUMMARY

When using the SNA Server Windows 3.x client WinAPPCSetBlockingHook() or WinCPICSetBlockingHook() calls from a Windows 3.x application, the blocking hook function must call PeekMessage, or the Windows 3.x application will hang.

The SNA Server Windows 3.x CPIC and APPC API interfaces rely on the use of Windows messages to process an API call on behalf of the calling application. If the application registers it's own blocking hook function that blocks the reception of Windows messages, the calling application will hang.

MORE INFORMATION

The SNA Server Windows 3.x APPC and CPIC interfaces have a default mechanism by which blocking functions are implemented. This function gives the application the ability to execute it's own function at blocking time in place of the default function.

The default blocking function, documented in the SNA Server APPC and CPIC API references (under WinCPICSetBlockingHook and WinAPPCSetBlockingHook) is equivalent to:

   BOOL DefaultBlockingHook ( void ) {
      MSG msg;
      /*  get the next message if any */
      if ( PeekMessage ( &msg, NULL, 0, 0, PM_NOREMOVE) ) {
         if ( msg.message = WM_QUIT )
            return FALSE;   // let app process WM_QUIT
         PeekMessage ( &msg, NULL, 0, 0, PM_REMOVE) ;
         TranslateMessage (&msg) ;
         DispatchMessage (&msg) ;
     }
     /* TRUE if no WM_QUIT received */
     return TRUE;
   }

The NULL (window handle) is critical here in order for the underlying SNA client software to receive and process the Windows messages necessary to support the API calls being made.