gtk-text-buffer-selection-bounds (buffer)

buffer -- a gtk-text-buffer object

start -- a gtk-text-iter iterator with the selection start, or nil
end -- a gtk-text-iter iterator with the selection end, or nil

Returns the start and end iterators if some text is selected. If the selection has length 0, then the start and end iterators are filled in with the same value. The start and end iterators will be in ascending order.