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From: dbj.rice%rand-relay@sri-unix.UUCP
Newsgroups: net.unix-wizards
Subject: Re: watch out when you modify /etc/ttys !!
Message-ID: <2079@sri-arpa.UUCP>
Date: Sun, 12-Jun-83 19:44:50 EDT
Article-I.D.: sri-arpa.2079
Posted: Sun Jun 12 19:44:50 1983
Date-Received: Thu, 16-Jun-83 01:52:57 EDT
Lines: 20

From:  Dave Johnson 

UCB Mail does a getlogin() to decide who to say the message is "From:", and
if this returns a NULL, it instead tries a getpwuid(getuid()).  Getlogin
uses ttyslot() to find the "slot" number of this tty in /etc/ttys, and then
uses this number also as the slot number in /etc/utmp (init maintains
/etc/utmp in order of tty "slot" number as returned by ttyslot()).  Thus, if
you change /etc/ttys, you change what value is returned by ttyslot(), and
thus, getlogin() starts looking at a different entry in /etc/utmp than was
used to record when this person logged on to this terminal.  Presumably,
Mail thought it was faster to read through /etc/ttys than to read all of
/etc/passwd to translate the uid...

It should be "safe" to add new entries to the bottom of /etc/ttys.  To
"delete" lines from /etc/ttys, you can just put a "0" as the first character
on the line and everybody will ignore that line.

                                        Dave Johnson
                                        Dept. of Math Science
                                        Rice University