xmodmap broken

[Markus Kuhn]

I used to be able to get the adiaeresis and Adiaeresisa keysyms (German
umlauts) onto the AltGr-a and AltGr-A keys of a UK keyboard with

  xmodmap -e 'keysym a = a NoSymbol adiaeresis NoSymbol'

exactly as "man xmodmap" suggests.

However, on modern systems (e.g., openSUSE 10.2 with KDE, Fedora Core 6,
etc.) I discovered only after a lot of experimentation, and no help from
any documentation that I could find, that this line has to be changed to

  xmodmap -e 'keysym a = a NoSymbol a NoSymbol adiaeresis Adiaeresis'

In other words, the AltGr level has mysteriously moved from the 3rd and
4th to the 5th and 6th keysym position in .Xmodmap files, and has also
lost the automatic case mapping provided by NoSymbol.

This obviously contradicts what "man xmodmap" claims:

       keycode NUMBER = KEYSYMNAME ...
               The  list  of  keysyms  is  assigned to the indicated keycode
               (which may be specified in decimal, hex or octal and  can  be
               determined  by running the xev program).  Up to eight keysyms
               may be attached to a key, however the last four are not  used
                                                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^
               in  any  major  X server implementation.  The first keysym is
               used when no modifier key is pressed in conjunction with this
               key,  the  second  with Shift, the third when the Mode_switch
               key is used with this  key  and  the  fourth  when  both  the
               Mode_switch and Shift keys are used.

What is going on here? Why does this happen under KDE, but not under fvwm/

Is this a bug in the keyboard code or a bug in the xmodmap manpage?

Where does one start reading documentation these days in order to
understand X11 keyboard handling end-to-end if the man pages are out of
date?

Details:

$ xmodmap -pm
xmodmap:  up to 4 keys per modifier, (keycodes in parentheses):

shift       Shift_L (0x32),  Shift_R (0x3e)
lock
control     Control_L (0x25),  Control_R (0x6d)
mod1        Alt_L (0x40),  Alt_L (0x7d),  Meta_L (0x9c)
mod2        Num_Lock (0x4d)
mod3
mod4        Super_L (0x73),  Super_R (0x74),  Super_L (0x7f),  Hyper_L (0x80)
mod5        Mode_switch (0x5d),  ISO_Level3_Shift (0x7c)
$ sed /etc/X11/xorg.conf -ne '/^Section "InputDevice"/,/^EndSection/p'
Section "InputDevice"
  Driver       "kbd"
  Identifier   "Keyboard[0]"
  Option       "Protocol" "Standard"
  Option       "XkbLayout" "gb"
  Option       "XkbModel" "pc105"
  Option       "XkbRules" "xfree86"
EndSection
[...]

Possibly related:

  https://bugzilla.novell.com/show_bug.cgi?id=281965

Markus

-- 
Markus Kuhn, Computer Laboratory, University of Cambridge
http://www.cl.cam.ac.uk/~mgk25/ || CB3 0FD, Great Britain