Systemd sessionid like `c508`

[hai wu <haiwu.us@xxxxxxxxx>]

Where is the systemd sessionid like `c508` being generated? If kernel
auditd is disabled, then it seems systemd `loginctl list-sessions`
command would list user session ids all with `c` character prefix
instead.

I could not find the source code where these session ids got
generated. Are these session ids generated from systemd source code or
from Linux kernel source code?

systemd has this function `sd_pid_get_session` to get session id, it
seems that's parsing `/proc/self/cgroup`, instead of generating such
session ids..

Regards,
Hai