EM_GETSEL

  Retrieves the current text selection points.

Parameters
  wParam is not used.
  lParam is a pointer to a user-supplied far pointer to a 2-word memory
   location. The 0-based starting point will be copies to the first word and
   the ending point will be copied to the second word.

Returns
  The current selection point. If there is no text selected, then the
   current cursor position within the edit buffer is returned.

Example
  WORD markers[2];
  WORD *pMarkers;
  pMarkers = markers;
  SendMessage(hEdit, EM_GETSEL, 0, (DWORD) (WORD far **) &szHandle);