Microsoft KB Archive/108933

Top 10 DDEML Support Issues

Q108933

The information in this article applies to:

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

SUMMARY

The following are the top 10 commonly asked DDEML issues:

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:

Q102570 DOCERR: DdeCreateStringHandle() lpszString param
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:

Q107387 PRB: Inadequate Buffer Length Causes Strange Problems in DDEML.
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:

Q102574 XTYP EXECUTE and its Return Value Limitations
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:

Q83413 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:

Q83912 Freeing Memory for Transactions in a DDEML Client App
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:

Q94954 PRB: DdeInitialize Fails in Implicitly-Loaded DLL's LibMain()
Q. How do I implement Paste Link in my client/server application? Q94954 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.
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.
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:

Q102571 Calling DdePostAdvise() from XTYP ADVREQ

-and-

Q92540 Is DdePostAdvise Synchronous?
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:

Q108926 DdeCreateDataHandle Memory Allocation Granularity DDEML
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); 
```

Additional query words: 3.10 3.50 4.00 95

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