From mboxrd@z Thu Jan  1 00:00:00 1970
From: jnc@mercury.lcs.mit.edu (Noel Chiappa)
Date: Fri,  9 Jan 2015 13:06:07 -0500 (EST)
Subject: [TUHS] single user mode in v5 & v6
Message-ID: <20150109180607.E198918C0CE@mercury.lcs.mit.edu>

    > From: Noel Chiappa

    > For some reason, the code for /etc/init .. bashes the command line so
    > it just has '-' in it, so it looks just like a shell.

BTW, that may be accidental, not a deliberate choice - e.g. someone copied
another line of code which exec'd a shell, and didn't change the second arg.

    > I fixed my copy so it says "/etc/init", or something like that. ... I
    > can upload the 'fixed' code tomorrow.

The change is pretty minor: in this piece of code:

	case reboot:
		termall();
		execl(init, minus, 0);
		reset();

just change the execl() line to say:

		execl(init, init, 0);


    >> I'm not sure if unix of the v6 or v5 era was designed to go from multi
    >> user to single user mode and then back again.

    > I seem to recall there's some issue, something like in some cases
    > there's an extra shell left running attached to the console

So the bug is that in going from single-user to multi-user, by using "kill -1
1" in single-user with the switch register set for multi-user, it doesn't
kill the running single-user shell on the console. The workaround to that bug
which I use is to set the CSWR and then ^D the running shell.

In general, the code in init.c isn't quite as clean/clear as would be optimal
(which is part of why I haven't tried to fix the above bug), but _in general_
it does support going back and forth.


    > From: Ronald Natalie

    > our init checked the switch register to determine whether to bring up
    > single or multiuser

I think that's standard from Bell, actually.

    > I believe our system shutdown if you kill -1-1 (HUP to init).

The 'stock' behaviour is that when that happens, it checks the switch
register, and there are three options (the code is a little hard to follow,
but I'm pretty sure this is right):

- if it's set for single-user, it shuts down all the other children, and
brings up a console shell; when done, it does the next
- if it's set for 'reboot', it just shuts down all children, and restarts
the init process (presumably so one can switch to a new version of the init
without restarting the whole machine);
- if it's not set for either, it re-reads /etc/ttys, and for any lines which
have switched state in that file, it starts/kills the process listening to
that line (this allows one to add/drop lines dynamically).


    > From: Clem Cole

    > it's probably worth digging up the v6 version of fsck.

That's on that MIT V6 tape, too. Speaking of which, time to write the code to
grok the tape...

	Noel