Path: utzoo!mnetor!uunet!husc6!mit-eddie!minya!jc
From: jc@minya.UUCP (John Chambers)
Newsgroups: comp.unix.wizards
Subject: /dev/tty doesn't exist?
Message-ID: <445@minya.UUCP>
Date: 2 Jan 88 13:50:01 GMT
Organization: home
Lines: 45

Well, here I am with something else I've screwed up, and can't make sense
of from the Unix manuals! (:-)

What's going wrong now is that I have a program, let's call it X, which
starts up a sub-process, and execs /bin/sh in it.  It then hangs around
waiting for the shell to die.  All very normal, you say?  Well, then,
explain why, to this shell and anything it starts, /dev/tty doesn't seem
to exist.

The first hint was from a script that did something like:
|	echo "This is an error message." >/dev/tty
so that it could write to the control terminal regardless of how output
had been redirected.  Instead of the error message, what appeared on the
control terminal was:
|	/dev/tty: cannot create
This seems a bit bizarre; a bit of looking turns up the fact that:
|	crw-rw-rw-   1 root     sys        2,  0 Jan  1 20:42 /dev/tty
I.e., /dev/tty does indeed exist, and is world-writable.  After a bit
of testing, I wrote a little test C program that says:
|	if ((fn = open("/dev/tty", 2)) < 0) 
|		fprintf(stderr,"Can't open(\"/dev/tty\",2) errno=%d.\n",errno);
Guess what it says?  OK, I'll tell you:
|	Can't open("/dev/tty",2) errno=6.
Replacing the '2' with '1' or '0' gets the same results.

Anyone have an idea what my program X might be doing to screw up /dev/tty?

Some hints:  This is a rather generic Unix Sys/V.2, so the problem should
be similar to most ATT Unices.  The most unusual thing about how X is run
is that it isn't being started by a login shell; it is started as a daemon
by init, and in some cases, decides to grab a terminal (whose name is on
its command line, and which doesn't have getty on it) and run a program
there.  This is a useful thing to do if, for instance, you are playing
around with process-control applications.   

Perhaps the question could be rephrased:  What is the official, approved
way for a daemon process like this to attach a terminal?  If the answer
is RTFM, can you say where?  The 1/4-page /dev/tty entry isn't much help.
[In fact, the Unix manuals aren't very helpful in general if you want to
write a free-running background program.  I've tried to help various users
who wanted to do this, and it's frustrating never quite knowing what I'm
talking about :-]

-- 
John Chambers <{adelie,ima,maynard,mit-eddie}!minya!{jc,root}> (617/484-6393)