diff options
Diffstat (limited to 'doc/kppp/ttyS-cua.txt')
| -rw-r--r-- | doc/kppp/ttyS-cua.txt | 46 |
1 files changed, 46 insertions, 0 deletions
diff --git a/doc/kppp/ttyS-cua.txt b/doc/kppp/ttyS-cua.txt new file mode 100644 index 00000000..2369fd5e --- /dev/null +++ b/doc/kppp/ttyS-cua.txt @@ -0,0 +1,46 @@ +From: "Theodore Y. Ts'o" <tytso@mit.edu> +To: Tony Nugent <tonyn@sctnugen.ppp.gu.edu.au> +Cc: linux-net@vger.rutgers.edu, linux-ppp@vger.rutgers.edu +Subject: Re: /dev/cua? Vs /dev/ttyS? (was: Re: co-existence of pppd and mgetty ?) +Date: Mon, 13 May 1996 19:51:04 +0200 +Status: ROr + + 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 + + |