Topic 2: Serial devices
STATE: APPROVAL
The following policy has been suggested and will become official unless
someone objects: (the rationale will probably be removed since it's too
long for the policy manual)
Serial Devices
====== =======
Debian uses the new standard of /dev/ttyS? devices for serial
ports. No package shall use the older /dev/cu* devices.
Rationale:
---------
This is a quote from Theodore Ts'o , who implemented
these devices in the first place:
/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.
-- Theodore Ts'o
----------------------------------------------------------------------------
-- _,, Christian Schwarz
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"DIE ENTE BLEIBT DRAUSSEN!"
