Messages in this thread |  | | Subject | Re: 2.0.1 hangs during boot | Date | Fri, 05 Jul 1996 15:37:26 +0100 | From | Manuel Eduardo Correia <> |
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> I'd suggest using /dev/cua? instead. The ttyS? is intended as dialin and > may well hang while waiting for a carrier before completing the opening > of the serial port.
I do not think this is the case... As can be seen from the ttyS-cua.txt file from the mgetty distribution :
Date: Mon, 13 May 1996 07:57:09 +1000 From: Tony Nugent <tonyn@sctnugen.ppp.gu.edu.au>
Can someone kindly explain the difference between the /dev/cua? and /dev/ttyS? devices?
/dev/ttySxx devices are fully POSIX-compliant TTY devices. If you are only going to be using one set of tty devices, you should be using /dev/ttySxx.
/dev/cuaXX devices are different from /dev/ttySXX in two ways --- first of all, they will allow you to open the device even if CLOCAL is not set and the O_NONBLOCK flag was not given to the open device. This allows programs that don't use the POSIX-mondated interface for opening /dev/ttySxx devices to be able to use /dev/cuaXX to make outgoing phone calls on their modem (cu stands for "callout", and is taken from SunOS).
The second way in which /dev/cuaXX differs from /dev/ttySXX is that if they are used, they will trigger a simplistic kernel-based locking scheme: If /dev/ttySXX is opened by one or more processes, then an attempt to open /dev/cuaXX will return EAGAIN. If /dev/cuaXX is opened by one or more processes, then an attempt to open /dev/ttySXX will result the open blocking until /dev/cuaXX is closed, and the carrier detect line goes high.
While this will allow for simple lockouts between a user using a modem for callout and a getty listening on the line for logins, it doesn't work if you need to arbitrate between multiple programs wanting to do dialout --- for example, users wanting to do dialout and UUCP.
I originally implemented the cuaXX/ttySXX lockout mechanism back before FSSTND established a standard convention for the use of tty lock files. Now that it's there, people should use the tty lock files and not try using /dev/cuaXX. The only reason why /dev/cuaXX hasn't disappeared yet is for backwards compatibility reasons.
- Ted
Regards,
=============================================================================== Manuel Eduardo C. D. Correia =============================================================================== LIACC, Rua do Campo Alegre, 823, 4150 Porto, PORTUGAL Tel: (351-2) 600 1672, Ext: 114, Fax: (351-2) 600 3654, Internet: mcc@ncc.up.pt ===============================================================================
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