gtk-tree-sortable-set-sort-func (sortable column-id func)

sortable -- a gtk-tree-sortable object
column-id -- an integer with the sort column ID to set the function for
func -- a gtk-tree-iter-compare-func callback function

Sets the comparison function used when sorting to be func. If the current sort column ID of sortable is the same as column-id, then the model will sort using this function.