Re: /dev/cua? Vs /dev/ttyS? (was: Re: co-existance of pppd and mgetty ?)

Theodore Y. Ts'o (
Mon, 13 May 1996 13:51:04 -0400

Date: Mon, 13 May 1996 07:57:09 +1000
From: Tony Nugent <>

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/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