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The XKM_OUTPUT_DIR folder by default is defined as ${datadir}/X11/xkb/compiled
and it is usually defined as /var/lib/xkb or %{_localstatedir}/lib/xkb by
distributions. If X is executed as non-root it won't have permissions to write
into that folder. If we fallback directly to /tmp we might get name collisions:
```
> Error: Cannot open "/tmp/server-10.xkm" to write keyboard description
> Exiting
```
Where the file /tmp/server-10.xkm already exists but is owned by another user
that previously executed X and had the display number 10. This is specially
problematic when exeuting Xvfb.
Before falling back to /tmp/ check first the XDG_RUNTIME_DIR.
The X server uses this directory to store the compiled version of the
current keymap and/or any scratch keymaps used by clients. The X server
or some other tool might destroy or replace the files in this directory,
so it is not a safe place to store compiled keymaps for long periods of
time. The default keymap for any server is usually stored in:
X<num>-default.xkm
where <num> is the display number of the server in question, which makes
it possible for several servers *on the same host* to share the same
directory.
Unless the X server is modified, sharing this directory between servers on
different hosts could cause problems.