| DirectX SDK | 
The IDirectMusicBuffer::GetUsedBytes method retrieves the number of bytes of data in the buffer.
HRESULT GetUsedBytes( LPDWORD pcb );
If the method succeeds, the return value is S_OK.
If it fails, the method can return E_POINTER.
  Windows NT/2000: Requires Windows 2000.
  Windows 95/98: Requires Windows 95 or later. Available as a redistributable for Windows 95.
  Header: Declared in dmusicc.h.