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Date: Mon, 8 Apr 1996 01:26:43 +1000
From: Bruce Evans <bde@zeta.org.au>
Message-Id: <199604071526.BAA18884@godzilla.zeta.org.au>
To: louie@TransSys.COM, msmith@atrad.adelaide.edu.au
Subject: Re: The F_SETOWN problem..
Cc: hackers@freebsd.org, jkh@time.cdrom.com, roell@blah.a.isar.de,
        roell@xinside.com, terry@lambert.org
Sender: owner-hackers@freebsd.org
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>While not an X server application, you'll note that xntpd uses SIGIO
>so that it might read and, most importanty, timestamp traffic arriving
>on the network.  This is critical to making NTP accurately synchronize
>the clock.  ...

>What has this to do with ttys?  It's conceivable that you might have
>an external reference clock (GPS, WWVB, etc) which is sending you
>timestamps periodically.  It would be nice to be able to handle these
>in the same sort of way.

xntpd is the only other case where the FSETOWN behaviour has been
reported to cause problems under FreeBSD.  xntpd seems to be still
broken - I can't see any TIOCSCTTY ioctls or login_tty() calls in it.

Buffering in some (low level) tty device drivers also gets in the way
of periodic timestamps.  There's an average delay of 10ms.  The
TIOCTIMESTAMP ioctl can be used (for some tty device drivers) to get
an accurate timestamp for the last character that arrived.

Bruce