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PTY(7) Linux Programmer's Manual PTY(7)
NAME
pty - pseudoterminal interfaces
DESCRIPTION
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 mas-
ter; 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 char-
acter (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 kernel 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 func-
tion 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 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 pseudoter-
minal 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.
FILES
/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
NOTES
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.
SEE ALSO
ioctl_tty(2), select(2), setsid(2), forkpty(3), openpty(3), termios(3), pts(4), tty(4)
COLOPHON
This page is part of release 5.10 of the Linux man-pages project. A description of the
project, information about reporting bugs, and the latest version of this page, can be
found at https://www.kernel.org/doc/man-pages/.
Linux 2020-08-13 PTY(7)
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