INFO: Calling DdePostAdvise() from XTYP_ADVREQ

The documentation for DdePostAdvise() in the Windows 3.1 Software Development Kit "Programmer's Reference, Volume 2: Functions" states the following in the Comments section:

This is merely a warning against calling DdePostAdvise() from within a DDE callback function's XTYP_ADVREQ transaction, because it may result in a stack overflow.

Like window procedures, DDE callbacks must be coded with care to avoid infinite recursion (eventually resulting in a stack overflow). Because DdePostAdvise() causes DDEML to send an XTYP_ADVREQ transaction to the calling application's DDE callback function, calling DdePostAdvise() on the same topic/item/format name set as the one currently being handled results in an infinite loop.

An analogous piece of code that has become a classic problem in Windows programming involves calling UpdateWindow() in a WM_PAINT case:



Calling UpdateWindow() as in the code above causes a WM_PAINT message to be sent to a window procedure, and thus results in the same type of infinite recursion that occurs when calling DdePostAdvise() from an XTYP_ADVREQ transaction.

An example of a situation that would lend itself to this scenario would be one where data needs to be updated as a result of a previous data change. There are two ways to work around the stack overflow problem in this case:

• Post a user-defined message and handle the data change asynchronously. For example,

        // in DdeCallback:
        case XTYP_ADVREQ:
                  if ((!DdeCmpStringHandles (hsz1, ghszTopic)) &&
                      (!DdeCmpStringHandles (hsz2, ghszItem)) &&
                      (fmt == CF_SOMEFORMAT))
                    {
                          HDDEDATA  hData;
  
                          hData = DdeCreateDataHandle ();
                          PostMessage (hWnd, WM_DATACHANGED,hData,);
                          return (hData);
                    }
                    break;
  
         // in MainWndProc():
         case WM_DATACHANGED:
                 DdePostAdvise (idInst, ghszTopic, ghszItem);
                   :
  						

• Return CBR_BLOCK from the XTYP_ADVREQ and let DDEML suspend further transactions on that conversation, while the server prepares data asynchronously.

More information on how returning CBR_BLOCK allows an application to process data "asynchronously" may be derived from Section 5.8.6 of the Windows 3.1 Software Development Kit (SDK) "Programmer's Reference, Volume 1: Overview," or by querying on the following words in the Microsoft Knowledge Base: