a #GtkTextIter
if non-%NULL, location to store an integer indicating where in the grapheme the user clicked. It will either be zero, or the number of characters in the grapheme. 0 represents the trailing edge of the grapheme.
x position, in buffer coordinates
y position, in buffer coordinates
%TRUE if the position is over text
2.6
Retrieves the iterator pointing to the character at buffer coordinates @x and @y. Buffer coordinates are coordinates for the entire buffer, not just the currently-displayed portion. If you have coordinates from an event, you have to convert those to buffer coordinates with gtk_text_view_window_to_buffer_coords().
Note that this is different from gtk_text_view_get_iter_at_location(), which returns cursor locations, i.e. positions between characters.