INFO: Do Not Forward DDEML Messages from a Hook Procedure

• Microsoft Windows Software Development Kit (SDK) version 3.1
• Microsoft Win32 Application Programming Interface (API) included with: - Microsoft Windows NT versions 3.5, 3.51 - Microsoft Windows 95

SUMMARY

If an application for Windows uses the Dynamic Data Exchange Management Library (DDEML) in addition to a message hook [for example, by calling SetWindowsHook() or SetWindowsHookEx()], it is possible that your hook procedure will receive messages that are intended for the DDEML libraries.

For the DDEML libraries to work properly, you must make sure that your hook function does not forward on any messages that are intended for the DDEML libraries.

MORE INFORMATION

If your hook procedure receives a code of type MSGF_DDEMGR, you should return FALSE instead of calling the CallNextHookEx() function.

Use the following code to handle this situation:

   // This needs to be defined if you haven't already.
   #ifndef MSGF_DDEMGR
   #define MSGF_DDEMGR             0x8001
   #endif

   if (MSGF_DDEMGR == code)
      return FALSE;
   else
   {
   ...
   }

Note, however, how the message filter function is called from within DDEML:

   while (TimeOutHasntExpired) {
       GetMesage (&msg, (HWND)NULL, 0, 0);
       if ( !CallMsgFilter (&msg, MSGF_DDEMGR))
            DispatchMessage (&msg);
    }

Therefore, a callback function that processes the message may not just return TRUE, but should also translate and dispatch messages appropriately.

The Windows 3.1 SDK's DDEMLCL sample demonstrates how to do this correctly in its MessageFilterProc() found in DDEMLCL.C:

   if (nCode == MSGF_DDEMGR) {

   /*
   * If a keyboard message is for MDI, let MDI client take care of it.
   * Otherwise, check to see if it is a normal accelerator key.
   * Otherwise, just handle the message as usual.
   */

       if ( !TranslateMDISysAccel (hWndMDIClient, lpmsg) &&
             !TranslateAccelerator (hWndFrame, hAccel, lpmsg))  {
             TranslateMessage (lpmsg);
             DispatchMessage (lpmsg);
         }
        return 1;
    }