gtk-editable-select-region (editable start end)

editable -- a gtk-editable widget
start -- an integer with the start of region
end -- an integer with the end of region

Selects a region of text. The characters that are selected are those characters at positions from start up to, but not including end. If end is negative, then the the characters selected are those characters from start to the end of the text.

Note that positions are specified in characters, not bytes.