gtk-editable-selection-bounds (editable)

editable -- a gtk-editable widget

selected-p -- true if an area is selected, false otherwise
start -- an integer with the starting position, or nil
end -- an integer with the end position, or nil

Retrieves the selection bound of the editable. start will be filled with the start of the selection and end with end. If no text was selected both will be identical and nil will be returned.

Note that positions are specified in characters, not bytes.