gtk-text-iter-order (iter-1 iter-2)

first -- a gtk-text-iter instance
second -- another gtk-text-iter instance

Swaps the value of first and second if second comes before first in the text buffer.

That is, ensures that first and second are in sequence. Most text buffer functions that take a range call this automatically on your behalf, so there is no real reason to call it yourself in those cases. There are some exceptions, such as the function gtk-text-iter-in-range, that expect a pre-sorted range.