IDirectDrawSurface5::GetPrivateData

This method copies the private data associated with the surface to a provided buffer.

HRESULT GetPrivateData(
  REFGUID guidTag,
  LPVOID lpBuffer,
  LPDWORD lpcbBufferSize
);

guidTag
Reference to (C++) or address of (C) the globally unique identifier that identifies the private data to be retrieved.
lpBuffer
Address of a previously allocated buffer that will be filled with the requested private data if the call succeeds. The application calling this method is responsible for allocating and releasing this buffer.
lpcbBufferSize
Size of the buffer at lpBuffer, in bytes. If this value is less than the actual size of the private data (such as zero), the method sets this parameter to the required buffer size, and the method returns DDERR_MOREDATA.