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From: dhesi@bsu-cs.UUCP
Newsgroups: comp.unix.questions
Subject: Re: Identify file name given file descriptor
Message-ID: <692@bsu-cs.UUCP>
Date: Fri, 22-May-87 13:22:43 EDT
Article-I.D.: bsu-cs.692
Posted: Fri May 22 13:22:43 1987
Date-Received: Sat, 23-May-87 10:39:43 EDT
References: <710@twitch.UUCP> <877@killer.UUCP> <1037@hropus.UUCP>
Reply-To: dhesi@bsu-cs.UUCP (Rahul Dhesi)
Organization: CS Dept, Ball St U, Muncie, Indiana
Lines: 25
Keywords: microport, ttyname, cu

Following up on the discussion of how to find a name given a file
descriptor, I want to point out that the ttyname() library function
already does that to a limited extent.  It accepts a file descriptor
and returns the name of the terminal device corresponding to it.
Presumably it only searches /dev.

And the name it finds that may not be the one that is needed.  Using cu
under Microport System V/AT when multiple links exist to the same
serial device, I find that cu creates the lock file
/usr/spool/uucp/LCK..tty0 when it is invoked, because /dev/tty0 is the
default device for cu.  But when it exits, it does not use the same
name, because it apparently calls ttyname() and uses whatever name is
returned.  In my case it tries to then remove the lock file
/usr/spool/uucp/LCK..ttycg and fails to find it and complains.

This was clearly an oversight on part of the author, who assumed that
there would be only one link to a serial device.  The right thing to do
would be to always use ttyname(), so no matter how many links exist,
the same lock file will be used for the same physical device.

I found similar strange things happening under 4.3BSD if there was more
than one link to a terminal in /dev, though I don't remember which
program (tip or cu or uucico) was affected.
-- 
Rahul Dhesi         UUCP:  {ihnp4,seismo}!{iuvax,pur-ee}!bsu-cs!dhesi