PTY(7) Linux Programmer's 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. 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).
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 (they can be disabled when
configuring the 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. 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 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 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 release 4.12 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 2017-05-03 PTY(7)
Pages that refer to this page: intro(2), ioctl_tty(2), getpt(3), grantpt(3), openpty(3), posix_openpt(3), ptsname(3), unlockpt(3), pts(4)