DdeCreateDataHandle() Memory Allocation Granularity DDEML

• Microsoft Windows Software Development Kit (SDK) for Windows versions 3.0 and 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.