stretch$ gdb /usr/new/bin/emacs
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Reading symbols from /usr/new/bin/emacs...done.
(gdb) break xg_scroll_callback
Breakpoint 1 at 0x4be140: file xterm.c, line 5644.
(gdb) run -Q
Starting program: /usr/new/bin/emacs -Q
[Thread debugging using libthread_db enabled]
Using host libthread_db library "/lib/x86_64-linux-gnu/libthread_db.so.1".
[New Thread 0x7fffe8636700 (LWP 1991)]
[New Thread 0x7fffe6748700 (LWP 1992)]
[New Thread 0x7fffe5d3a700 (LWP 1993)]

Thread 1 "emacs" hit Breakpoint 1, xg_scroll_callback (range=0x2f006f0, 
    scroll=GTK_SCROLL_JUMP, value=7069105, 
    user_data=0x1493c30 <bss_sbrk_buffer+8990864>) at xterm.c:5644
5644	{
(gdb) print *range
$1 = {widget = {parent_instance = {g_type_instance = {g_class = 0x2e20400}, 
      ref_count = 3, qdata = 0x344fd10}, priv = 0x2f00600}, priv = 0x2f00520}
(gdb) next
5648	  GtkAdjustment *adj = GTK_ADJUSTMENT (gtk_range_get_adjustment (range));
(gdb) next
5649	  struct frame *f = g_object_get_data (G_OBJECT (range), XG_FRAME_DATA);
(gdb) next
5648	  GtkAdjustment *adj = GTK_ADJUSTMENT (gtk_range_get_adjustment (range));
(gdb) next
5649	  struct frame *f = g_object_get_data (G_OBJECT (range), XG_FRAME_DATA);
(gdb) print adj
$2 = (GtkAdjustment *) 0x2fc9c80
(gdb) print *adj
$3 = {parent_instance = {g_type_instance = {g_class = 0x2cde5e0}, 
    ref_count = 1, qdata = 0x0}, priv = 0x2fc9c10}
(gdb) next
5651	  if (xg_ignore_gtk_scrollbar) return false;
(gdb) next
5653	  switch (scroll)
(gdb) next
5657	      if (FRAME_DISPLAY_INFO (f)->grabbed != 0
(gdb) next
5660		  if (bar->horizontal)
(gdb) next
5663		      whole = (int)(gtk_adjustment_get_upper (adj) -
(gdb) print whole
$4 = 0
(gdb) next
5660		  if (bar->horizontal)
(gdb) print whole
$5 = 0
(gdb) next
5671		      whole = gtk_adjustment_get_upper (adj) -
(gdb) next
5672			gtk_adjustment_get_page_size (adj);
(gdb) next
5671		      whole = gtk_adjustment_get_upper (adj) -
(gdb) next
5672			gtk_adjustment_get_page_size (adj);
(gdb) next
5673		      portion = min ((int)value, whole);
(gdb) print whole
$6 = 0
(gdb) print gtk_adjustment_get_upper(adj)
$7 = 50109456
(gdb) print gtk_adjustment_get_page_size(adj)
$8 = 50109456
(gdb) cont
Continuing.