gtk-text-buffer-select-range (buffer insertion selection)

buffer -- a gtk-text-buffer object
insertion -- a gtk-text-iter iterator where to put the "insert" mark
selection -- a gtk-text-iter iterator where to put the "selection_bound" mark

This function moves the "insert" and "selection_bound" marks simultaneously. If you move them in two steps with the gtk-text-buffer-move-mark function, you will temporarily select a region in between their old and new locations, which can be pretty inefficient since the temporarily selected region will force stuff to be recalculated. This function moves them as a unit, which can be optimized.