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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. 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.
select(2), setsid(2), forkpty(3), openpty(3), termios(3), pts(4), tty(4),
tty_ioctl(4)
This page is part of release 3.32 of the Linux man-pages project. A
description of the project, and information about reporting bugs, can be found
at http://www.kernel.org/doc/man-pages/.
Linux 2005-10-10 PTY(7)
HTML rendering created 2010-12-03 by Michael Kerrisk, author of The Linux Programming Interface