Quoted from here:

  if (to_end) 
  { 
    /* If we want to scroll to the end, including horizontal scrolling, 
     * then we just create a mark with right gravity at the end of the  
     * buffer. It will stay at the end unless explicitely moved with  
     * gtk_text_buffer_move_mark. 
     */ 
    gtk_text_buffer_create_mark (buffer, "end", &iter, FALSE); 

    /* Add scrolling timeout. */ 
    return g_timeout_add (50, (GSourceFunc) scroll_to_end, textview); 
  } 
  else 
  { 
    /* If we want to scroll to the bottom, but not scroll horizontally,  
     * then an end mark won't do the job. Just create a mark so we can  
     * use it with gtk_text_view_scroll_mark_onscreen, we'll position it 
     * explicitely when needed. Use left gravity so the mark stays where  
     * we put it after inserting new text. 
     */ 
    gtk_text_buffer_create_mark (buffer, "scroll", &iter, TRUE); 

    /* Add scrolling timeout. */ 
    return g_timeout_add (100, (GSourceFunc) scroll_to_bottom, textview); 
  } 

Though there are quite a few lines of comments, I still don't understand the logic in it,especially, what's the relation between an mark and the position of scroll bar?