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47 lines
2.1 KiB
47 lines
2.1 KiB
15 years ago
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From: "Theodore Y. Ts'o" <tytso@mit.edu>
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To: Tony Nugent <tonyn@sctnugen.ppp.gu.edu.au>
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Cc: linux-net@vger.rutgers.edu, linux-ppp@vger.rutgers.edu
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Subject: Re: /dev/cua? Vs /dev/ttyS? (was: Re: co-existence of pppd and mgetty ?)
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Date: Mon, 13 May 1996 19:51:04 +0200
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Status: ROr
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Date: Mon, 13 May 1996 07:57:09 +1000
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From: Tony Nugent <tonyn@sctnugen.ppp.gu.edu.au>
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Can someone kindly explain the difference between the /dev/cua? and
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/dev/ttyS? devices?
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/dev/ttySxx devices are fully POSIX-compliant TTY devices. If you are
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only going to be using one set of tty devices, you should be using
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/dev/ttySxx.
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/dev/cuaXX devices are different from /dev/ttySXX in two ways --- first
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of all, they will allow you to open the device even if CLOCAL is not set
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and the O_NONBLOCK flag was not given to the open device. This allows
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programs that don't use the POSIX-mondated interface for opening
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/dev/ttySxx devices to be able to use /dev/cuaXX to make outgoing phone
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calls on their modem (cu stands for "callout", and is taken from SunOS).
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The second way in which /dev/cuaXX differs from /dev/ttySXX is that if
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they are used, they will trigger a simplistic kernel-based locking
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scheme: If /dev/ttySXX is opened by one or more processes, then an
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attempt to open /dev/cuaXX will return EAGAIN. If /dev/cuaXX is opened
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by one or more processes, then an attempt to open /dev/ttySXX will
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result the open blocking until /dev/cuaXX is closed, and the carrier
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detect line goes high.
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While this will allow for simple lockouts between a user using a modem
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for callout and a getty listening on the line for logins, it doesn't
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work if you need to arbitrate between multiple programs wanting to do
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dialout --- for example, users wanting to do dialout and UUCP.
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I originally implemented the cuaXX/ttySXX lockout mechanism back before
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FSSTND established a standard convention for the use of tty lock files.
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Now that it's there, people should use the tty lock files and not try
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using /dev/cuaXX. The only reason why /dev/cuaXX hasn't disappeared yet
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is for backwards compatibility reasons.
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- Ted
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