Which line in the /proc/tty/drivers reflect w command?

Question

In the web page there is a sentence to describe tty drivers.

registered tty drivers

The /proc/tty/drivers file shows all of the currently registered tty drivers.

Here is some info about w and /proc/tty/drivers.

debian8@hwy:~$ w
 23:53:38 up 2 min,  1 user,  load average: 0.47, 0.46, 0.20
USER     TTY      FROM             LOGIN@   IDLE   JCPU   PCPU WHAT
debian8  :0       :0               23:51   ?xdm?   4.62s  0.05s /usr/bin/lxsess
debian8@hwy:~$ cat /proc/tty/drivers
/dev/tty             /dev/tty        5       0 system:/dev/tty
/dev/console         /dev/console    5       1 system:console
/dev/ptmx            /dev/ptmx       5       2 system
/dev/vc/0            /dev/vc/0       4       0 system:vtmaster
serial               /dev/ttyS       4 64-95 serial
pty_slave            /dev/pts      136 0-1048575 pty:slave
pty_master           /dev/ptm      128 0-1048575 pty:master
unknown              /dev/tty        4 1-63 console

Which line in the /proc/tty/drivers reflect w command?

Answer 1

The w utility reads the utmp database wtmp to display its TTY info, as far as I know. This is written to by xdm, apparently by using something called sessreg (an X11 utility that "manage utmp/wtmp entries for non-init clients").

The sessreg manual says to put a line like

sessreg -a -l $DISPLAY -x /etc/X11/xdm/Xservers $USER

in the X11 Xstartup script. On my OpenBSD machine, the line actually reads (in /etc/X11/xdm/Xstartup)

exec /usr/X11R6/bin/sessreg  -a -w /var/log/wtmp -u /var/run/utmp       -x /etc/X11/xdm/Xservers -l $DISPLAY -h "" $USER

The -l flag has the following description in the sessreg manual:

This describes the "line" name of the entry. For terminal sessions, this is the final pathname segment of the terminal device filename (e.g. ttyd0). For X sessions, it should probably be the local display name given to the users session (e.g. :0). If none is specified, the terminal name will be determined with ttyname(3) and stripped of leading components.

As an experiment:

$ doas sessreg -a -l hello kk

(kk is me, and doas is the OpenBSD "equivalent" of sudo)

$ w
 7:31PM  up 10:55, 2 users, load averages: 1.20, 1.16, 1.10
USER    TTY FROM              LOGIN@  IDLE WHAT
kk       p0 192.168.216.1     8:47AM     0 tmux: client (/tmp/tmux-1000/default)
kk       he -                 7:31PM     0 -

$ doas sessreg -d -l hello kk

$ w
 7:32PM  up 10:56, 1 user, load averages: 1.22, 1.17, 1.11
USER    TTY FROM              LOGIN@  IDLE WHAT
kk       p0 192.168.216.1     8:47AM     0 tmux: client (/tmp/tmux-1000/default)

$ last | head -n 3
kk        hello                             Wed Jan 11 19:31 - 19:31  (00:00)
kk        ttyC0                             Wed Jan 11 18:54 - 19:21  (00:27)
kk        ttyp0    192.168.216.1            Wed Jan 11 08:47   still logged in

In conclusion: :0 is just a text string put there by the sessreg utility. It could be anything, but it's most likely the value of $DISPLAY.