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pty(7) Miscellaneous Information Manual pty(7)
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.
Data flow between master and slave is handled asynchronously, much
like data flow with a physical terminal. Data written to the
slave will be available at the master promptly, but may not be
available immediately. Similarly, there may be a small processing
delay between a write to the master, and the effect being visible
at the slave.
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 Linux 2.6.4, BSD-style pseudoterminals are considered
deprecated: support can be disabled when building the kernel by
disabling the CONFIG_LEGACY_PTYS option. (Starting with Linux
2.6.30, that option is disabled by default in the mainline
kernel.) UNIX 98 pseudoterminals should be used in new
applications.
UNIX 98 pseudoterminals
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. Up to and including Linux 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 Linux 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 pseudoterminals
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
Pseudoterminals are used by applications such as network login
services (ssh(1), rlogin(1), telnet(1)), terminal emulators such
as xterm(1), script(1), screen(1), tmux(1), unbuffer(1), and
expect(1).
A description of the TIOCPKT ioctl(2), which controls packet mode
operation, can be found in ioctl_tty(2).
The BSD ioctl(2) operations TIOCSTOP, TIOCSTART, TIOCUCNTL, and
TIOCREMOTE have not been implemented under Linux.
ioctl_tty(2), select(2), setsid(2), forkpty(3), openpty(3),
termios(3), pts(4), tty(4)
This page is part of the man-pages (Linux kernel and C library
user-space interface documentation) project. Information about
the project can be found at
⟨https://www.kernel.org/doc/man-pages/⟩. If you have a bug report
for this manual page, see
⟨https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/tree/CONTRIBUTING⟩.
This page was obtained from the tarball man-pages-6.10.tar.gz
fetched from
⟨https://mirrors.edge.kernel.org/pub/linux/docs/man-pages/⟩ on
2025-02-02. If you discover any rendering problems in this HTML
version of the page, or you believe there is a better or more up-
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man-pages@man7.org
Linux man-pages 6.10 2024-05-02 pty(7)
Pages that refer to this page: screen(1), intro(2), ioctl_tty(2), getpt(3), grantpt(3), openpty(3), posix_openpt(3), ptsname(3), unlockpt(3), pts(4), tty(4)
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HTML rendering created 2025-02-02 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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