Q108926: DdeCreateDataHandle() Memory Allocation Granularity DDEML

Article: Q108926
Product(s): Microsoft Windows Software Development Kit
Version(s): WINDOWS:3.0,3.1
Operating System(s): 
Keyword(s): kb16bitonly
Last Modified: 06-NOV-1999

-------------------------------------------------------------------------------
The information in this article applies to:

- Microsoft Windows Software Development Kit (SDK) versions 3.0, 3.1 
-------------------------------------------------------------------------------

SUMMARY
=======

The DdeCreateDataHandle function creates a global memory object and fills the
object with the data pointed to by the lpvSrcBuf parameter (second). A DDEML
application uses this function during transactions that involve passing data to
the partner application.

This application programming interface (API) allocates global memory in multiples
of 32 bytes to create data handles. If the application requests a data handle
for a buffer size that is not a multiple of 32 bytes, then there will be some
memory leak involved (less than 31 bytes).

MORE INFORMATION
================

For example, calling DdeCreateDataHandle() on a 3-byte buffer causes DDEML to
allocate a global memory block of 32 bytes and returns a data handle for these
32 bytes of memory. If DdeGetData() or DdeAccessData() is used to obtain the
size of the data handle, then the return value will be 32 bytes (or 28,
depending on the type of transaction; please see NOTE below). Similarly, if the
data buffer was 35 bytes long, DdeCreateDataHandle() allocates a global memory
block of 64 bytes and returns the handle to that memory block.

Internally, DDEML uses GlobalAlloc(GMEM_DDESHARE) to allocate global memory and
create data handles. GlobalAlloc() allocates memory from the global heap and
aligns the allocation size to a 32-byte boundary. Because the> function
DdeCreateDataHandle() uses GlobalAlloc() to create data handles, this overhead
is imminent.

NOTE: For transactions such as XTYP_REQUEST, DDEML uses the first 4 bytes of the
data buffer for maintaining header information. Calling DdeGetData(hData, NULL,
0, 0) to obtain the size of the memory object associated with the data handle
yields a return value of 28 bytes, instead of the allocated 32 bytes.
Transactions such as XTYP_EXECUTE, however, do not use the first 4 bytes, so the
return value is the full 32 bytes allocated.

Additional query words: 3.00 no32bit 3.10

======================================================================
Keywords          : kb16bitonly 
Technology        : kbAudDeveloper kbWin3xSearch kbSDKSearch kbWinSDKSearch kbWinSDK300 kbWinSDK310
Version           : WINDOWS:3.0,3.1

=============================================================================

THE INFORMATION PROVIDED IN THE MICROSOFT KNOWLEDGE BASE IS PROVIDED "AS IS" WITHOUT WARRANTY OF ANY KIND. MICROSOFT DISCLAIMS ALL WARRANTIES, EITHER EXPRESS OR IMPLIED, INCLUDING THE WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE. IN NO EVENT SHALL MICROSOFT CORPORATION OR ITS SUPPLIERS BE LIABLE FOR ANY DAMAGES WHATSOEVER INCLUDING DIRECT, INDIRECT, INCIDENTAL, CONSEQUENTIAL, LOSS OF BUSINESS PROFITS OR SPECIAL DAMAGES, EVEN IF MICROSOFT CORPORATION OR ITS SUPPLIERS HAVE BEEN ADVISED OF THE POSSIBILITY OF SUCH DAMAGES. SOME STATES DO NOT ALLOW THE EXCLUSION OR LIMITATION OF LIABILITY FOR CONSEQUENTIAL OR INCIDENTAL DAMAGES SO THE FOREGOING LIMITATION MAY NOT APPLY.

Copyright Microsoft Corporation 1986-2002.