DdeCreateDataHandle() Memory Allocation Granularity DDEML
ID: Q108926
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The information in this article applies to:
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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
Version : WINDOWS:3.0,3.1
Platform : WINDOWS
Issue type :