Top 10 DDEML Support Issues

• Microsoft Windows Software Development Kit (SDK) version 3.1
• Microsoft Win32 SDK, versions 3.5, 3.51, and 4.0

The following are the top 10 commonly asked DDEML issues:

 1. Q. The documentation for DdeCreateStringHandle() states that it applies
       to strings of any length. Why am I finding that strings are limited
       to 255 characters?

    A. Strings in DDEML are internally implemented as atoms, and as such,
       inherit the 255 character limit. This limit is maintained in 32-bit
       DDEML for backward compatibility.

       For more information, please see the following articles in the
       Microsoft Knowledge Base:

          ARTICLE-ID: Q102570
          TITLE     : DOCERR: DdeCreateStringHandle() lpszString param

 2. Q. Why is my call to DdeClientTransaction (..XTYP_EXECUTE,,) failing?
       I have specified a length of lstrlen (szBuffer).

    A. The documentation doesn't mention it, however, the buffer length
       specified should account for the null-terminating character, and
       therefore should be lstrlen(szBuffer) +1. You can verify this easily
       with DDESPY; watch garbage characters get appended to the command-
       string if an inadequate length is specified in the
       DdeClientTransaction() call.

       For additional information, please see the following article(s) in
       the Microsoft Knowledge Base:

          ARTICLE-ID: Q107387
          TITLE     : PRB: Inadequate Buffer Length Causes Strange Problems
                     in DDEML.

 3. Q. Is there a way for a client application to get to the result of an
       execute command it had sent a server application to execute?
       DdeClientTransaction()'s return value does not seem to help, nor
       does its lpuResult parameter contain anything relevant.

    A. You will have to establish your own protocol to do this. Either
       issue a separate XTYP_REQUEST for some item name (for example,
       ExecResult) after the XTYP_EXECUTE was sent, or set up an advise
       loop that immediately notifies the client of the execute result as
       soon as the command has executed.

       For additional information, please see the following article(s) in
       the Microsoft Knowledge Base:

          ARTICLE-ID: Q102574
          TITLE     : XTYP EXECUTE and its Return Value Limitations

 4. Q. When should a server application free data handles? How about a
       client application?

    A. Servers should free data handles when:

        - Created as HDATA_APPOWNED

        - The data handle is not passed to a client application

       For additional information, please see the following article(s) in
       the Microsoft Knowledge Base:

          ARTICLE-ID: Q83413
          TITLE     : Freeing Memory in a DDEML Server Application

       Clients should free data handles when:

        - Received synchronously as a return value from a call to
          DdeClientTransaction (..XTYP_REQUEST,,).

       For additional information, please see the following article(s) in
       the Microsoft Knowledge Base:

          ARTICLE-ID: Q83912
          TITLE     : Freeing Memory for Transactions in a DDEML
                      Client App

 5. Q. Why does my DdeInitialize() call fail when called from my dynamic-
       link library's (DLL's) LibMain() function? Calling it elsewhere in
       my DLL seems to work.

    A. DdeInitialize() causes windows to be created for DDEML's own use.
       This then causes messages to be sent to the calling application's
       message queue, which does not exist yet during LibMain() or
       DllMain() in an implicitly-loaded DLL. Moving the call to
       DdeInitialize() anywhere else in the DLL, or explicitly loading the
       DLL via LoadLibrary(), should fix the problem.

       For additional information, please see the following article(s) in
       the Microsoft Knowledge Base:

          ARTICLE-ID: Q94954
          TITLE     : PRB: DdeInitialize Fails in Implicitly-Loaded DLL's
                      LibMain()

 6. Q. How do I implement Paste Link in my client/server application?

    A. Client and Server applications that support the Paste Link command
       should register the "Link" clipboard format as shown:

          cfLink = RegisterClipboardFormat ("Link");

       When a user chooses Copy from a server application's Edit menu, the
       server copies to the clipboard a string in this format

          application\0topic\0item\0\0

       with a single null character separating the names, and two null
       characters terminating the string.

       When the user chooses Paste Link from a client's Edit menu, the
       client opens the clipboard and starts parsing the application,
       topic, and item names from the clipboard data. Using these names,
       it then attempts to establish a conversation on the
       application!topic, and sets up a hot advise loop on the item name.

       Refer to the Windows SDK version 3.1 SDK "Guide to Programming,"
       manual, Section 22.4.3.2, page 529.

 7. Q. I'm having a problem with the DdeCreateDataHandle() function; it
       does not seem to work as documented.

    A. The cbInitData param should be documented as specifying the amount
       of memory, in bytes, to copy from the source buffer pointed to by
       lpvSrcBuff.

       The offSrcBuf should have been documented as offTargetBuff, to point
       to the beginning of the global memory object (NOT the source buffer)
       where data is supposed to be copied.

 8. Q. I get a stack overflow when I call DdePostAdvise().

    A. You might be calling DdePostAdvise() from your DdeCallback, while
       processing the XTYP_ADVREQ transaction. This is the equivalent of
       calling InvalidateRect() from a window procedure's WM_PAINT case,
       which results in a similar stack overflow, because InvalidateRect()
       generates a WM_PAINT message.

       For additional information, please see the following articles in
       the Microsoft Knowledge Base:

          ARTICLE-ID: Q102571
          TITLE     : Calling DdePostAdvise() from XTYP ADVREQ

          -and-

          ARTICLE-ID: Q92540
          TITLE     : Is DdePostAdvise Synchronous?

 9. Q. 16-bit Windows only: When I call DdeCreateDataHandle() to allocate
       memory for a buffer 5-bytes long, why does
       DdeGetData(hData,NULL,0,0) return 32 bytes (or 28 bytes, depending
       on the transaction type) as the size of the memory block allocated?

    A. Internally, DDEML calls GlobalAlloc (GMEM_DDESHARE) to allocate
       global memory and create data handles. When GlobalAlloc() allocates
       memory, it aligns the allocation size to a 32 byte boundary, thus
       all memory allocations are done in multiples of 32 bytes.

       For more information, please see the following article in the
       Microsoft Knowledge Base:

          ARTICLE-ID: Q108926
          TITLE     : DdeCreateDataHandle Memory Allocation Granularity
                      DDEML

10. Q. How do I set up a conversation with Word/Excel and get either of
       them to execute commands?

    A. Call DdeConnect(), specifying "WinWord" and "System" or "Excel" and
       "System" as service and topic names respectively, and then send the
       command string to execute (in WordBasic command or Excel macro
       format respectively), enclosed in open and close brackets [].

       For example, to send WinWord a command to open a file, your call to
       DdeClientTransaction() should resemble the following:

          char szExecute[100];
          lstrcpy (szExecute,"[FileOpen(\"C:\\README.DOC\")]");
          DdeClientTransaction(lpszExecString, lstrlen (lpszExecString) +1,
                               hConvServer, NULL,
                               CF_TEXT, XTYP_EXECUTE,
                               1000, NULL);