pty(7) — Linux manual page

NAME | DESCRIPTION | FILES | NOTES | SEE ALSO | COLOPHON

PTY(7)                  Linux Programmer's Manual                 PTY(7)

NAME         top

       pty - pseudoterminal interfaces

DESCRIPTION         top

       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 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 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.

FILES         top

       /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         top

       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         top

       ioctl_tty(2), select(2), setsid(2), forkpty(3), openpty(3),
       termios(3), pts(4), tty(4)

COLOPHON         top

       This page is part of release 5.12 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                          2020-08-13                         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)tty(4)