Microsoft KB Archive/102571: Difference between revisions
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Revision as of 21:57, 16 July 2020
INFO: Calling DdePostAdvise() from XTYP_ADVREQ |
Q102571
The information in this article applies to:
- Microsoft Windows Software Development Kit (SDK) 3.1
- Microsoft Win32 Application Programming Interface (API), used with:
- Microsoft Windows NT Server versions 3.5, 3.51
- Microsoft Windows NT Workstation versions 3.5, 3.51
- Microsoft Windows 95
SUMMARY
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:
If a server calls DdePostAdvise() with a topic/item/format name set that includes the set currently being handled in an XTYP_ADVREQ callback, a stack overflow may result.
MORE INFORMATION
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:
case WM_PAINT:
InvalidateRect (hWnd, NULL, TRUE); UpdateWindow (hWnd);
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:
DDEML and CBR_BLOCK
Additional query words: 3.10 3.50 4.00
Keywords :
Issue type : kbinfo
Technology : kbAudDeveloper kbSDKSearch kbWin32sSearch kbWin32API kbWinSDKSearch
Last Reviewed: December 16, 2000 |