gtk-text-buffer-apply-tag (buffer tag start end)

buffer -- a gtk-text-buffer object
tag -- a gtk-text-tag object, or a string with the tag name
start -- a gtk-text-iter iterator with the start bound of the range to be tagged
end -- a gtk-text-iter iterator with the end bound of the range to be tagged

Emits the "apply-tag" signal on the text buffer. The default handler for the signal applies tag to the given range. The start and end iterators do not have to be in order.