pty — pseudoterminal interfaces
A pseudoterminal (sometimes abbreviated "pty") is a pair
of virtual character devices that provide a bidirectional
communication channel. One end of the channel is called the
master; the other
end is called the slave. The slave end of the
pseudoterminal provides an interface that behaves exactly
like a classical terminal. A process that expects to be
connected to a terminal, can open the slave end of a
pseudoterminal and then be driven by a program that has
opened the master end. Anything that is written on the master
end is provided to the process on the slave end as though it
was input typed on a terminal. For example, writing the
interrupt character (usually control-C) to the master device
would cause an interrupt signal (SIGINT) to be generated for the foreground
process group that is connected to the slave. Conversely,
anything that is written to the slave end of the
pseudoterminal can be read by the process that is connected
to the master end. Psuedoterminals are used by applications
such as network login services (ssh(1), rlogin(1), telnet(1)), terminal emulators,
script(1), screen(1), and expect(1).
Historically, two pseudoterminal APIs have evolved: BSD and System V. SUSv1 standardized a pseudoterminal API based on the System V API, and this API should be employed in all new programs that use pseudoterminals.
Linux provides both BSD-style and (standardized) System V-style pseudoterminals. System V-style terminals are commonly called UNIX 98 pseudoterminals on Linux systems. Since kernel 2.6.4, BSD-style pseudoterminals are considered deprecated (they can be disabled when configuring the kernel); UNIX 98 pseudoterminals should be used in new applications.
An unused UNIX 98 pseudoterminal master is opened by
calling posix_openpt(3). (This
function opens the master clone device, /dev/ptmx; see pts(4).) After performing
any program-specific initializations, changing the
ownership and permissions of the slave device using
grantpt(3), and unlocking
the slave using unlockpt(3)), the
corresponding slave device can be opened by passing the
name returned by ptsname(3) in a call to
open(2).
The Linux kernel imposes a limit on the number of
available UNIX 98 pseudoterminals. In kernels up to and
including 2.6.3, this limit is configured at kernel
compilation time (CONFIG_UNIX98_PTYS), and the permitted
number of pseudoterminals can be up to 2048, with a default
setting of 256. Since kernel 2.6.4, the limit is
dynamically adjustable via /proc/sys/kernel/pty/max, and a
corresponding file, /proc/sys/kernel/pty/nr, indicates how
many pseudoterminals are currently in use. For further
details on these two files, see proc(5).
BSD-style pseudoterminals are provided as precreated
pairs, with names of the form /dev/ptyXY (master) and /dev/ttyXY (slave), where X is a letter
from the 16-character set [p-za-e], and Y is a letter from
the 16-character set [0-9a-f]. (The precise range of
letters in these two sets varies across UNIX
implementations.) For example, /dev/ptyp1 and /dev/ttyp1 constitute a BSD
pseudoterminal pair. A process finds an unused
pseudoterminal pair by trying to open(2) each
pseudoterminal master until an open succeeds. The
corresponding pseudoterminal slave (substitute "tty" for
"pty" in the name of the master) can then be opened.
/dev/ptmx (UNIX 98 master
clone device)
/dev/pts/* (UNIX 98 slave
devices)
/dev/pty[p-za-e][0-9a-f]
(BSD master devices)
/dev/tty[p-za-e][0-9a-f]
(BSD slave devices)
A description of the TIOCPKT
ioctl(2), which controls
packet mode operation, can be found in tty_ioctl(4).
The BSD ioctl(2) operations
TIOCSTOP, TIOCSTART, TIOCUCNTL, and TIOCREMOTE have not been implemented under
Linux.
This page is part of release 3.33 of the Linux man-pages project. A
description of the project, and information about reporting
bugs, can be found at http://man7.org/linux/man-pages/.
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Copyright (C) 2005 Michael Kerrisk <mtk.manpagesgmail.com> Permission is granted to make and distribute verbatim copies of this manual provided the copyright notice and this permission notice are preserved on all copies. Permission is granted to copy and distribute modified versions of this manual under the conditions for verbatim copying, provided that the entire resulting derived work is distributed under the terms of a permission notice identical to this one. Since the Linux kernel and libraries are constantly changing, this manual page may be incorrect or out-of-date. The author(s) assume no responsibility for errors or omissions, or for damages resulting from the use of the information contained herein. The author(s) may not have taken the same level of care in the production of this manual, which is licensed free of charge, as they might when working professionally. Formatted or processed versions of this manual, if unaccompanied by the source, must acknowledge the copyright and authors of this work. |