Re: [hackers] Re: [st][patch] Refresh keyboard mapping

From: Axel Svensson <>
Date: Thu, 24 Jan 2019 22:41:47 +0000

> > > Interesting but I can't reproduce any issue with that, could you
> > > provide a way to raise a problem this way?
> >
> > Expect it within 24 hours.
> Ok, I started the (final) countdown.

I have been able to reproduce this problem on a X VNC Server without
the XKEYBOARD extension.
I was not able to reproduce it on a local desktop using an X Server
with the XKEYBOARD extension.
Possibly, this problem only occurs on X Servers without this extension.
Use xdpyinfo to get a list of extensions on your current X Server.

To reproduce:
1. Connect to a VNC server (I use TightVNC on Debian).
2. Start st remotely inside the VNC session.
3. Find the key code for the Z key and remap it to ζ (greek zeta).
$xmodmap -pke|grep Z
keycode  63 = z Z
$xmodmap -e 'keycode 63 = Greek_zeta Greek_ZETA'
4. Go to the st window started in step 2.
5. Press the Z key.
6. A ζ character is produced.
6. A z character is produced.
7. Restore your setup.
$xmodmap -e 'keycode 63 = z Z'
While the above example might be a bit contrived, the problem happens
frequently in the wild. This is how:
1. The user presses a key on the client and a keyevent is sent to
vncclient by the local OS.
2. Irrespective of what platform the VNC Client is running on, per the
VNC standard it translates the local native keyevent to an X11 Keysym
in integer form, and transmits this to the connected VNC Server.
3. The VNC Server, which is also an X Server, translates the Keysym to
a corresponding key code according to its own (server-side) keymap. If
no such code exists, it will first remap an empty key code to the
received Keysym, which produces a MappingNotify event that is sent to
all connected X clients. It would appear that some applications handle
the MappingNotify event to synchronize their keymaps to that of the X
Server while others, like st, don't. At this point, the non-conforming
applications' keymap is out of sync with that of the X Server.
4. The (remote VNC and) X Server produces a KeyEvent using the
potentially newly mapped key code.
5. A receiving application handles the KeyEvent, typically by
translating it to the corresponding Keysym. In case the application's
and X Server's keymaps are out of sync, the application might produce
an erroneous Keysym.
Received on Thu Jan 24 2019 - 23:41:47 CET

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