LB_GETSELITEMS

  This message is sent to a multiple selection listbox to retrieve the
indices of the items which have been selected.

Parameters
  wParam is the maximum number of two-bytes indices to place in the buffer.
  lParam is the far pointer to a buffer which will be used to hold a list of
   the indices of the items which were selected.

Returns
  The number of items placed into the buffer is returned. LB_ERR is returned
if the listbox is not a multiple selection listbox.