pty(7) — Linux manual page


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

       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

       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

   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

              UNIX 98 master clone device

              UNIX 98 slave devices

              BSD master devices

              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 5.09 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

Linux                            2020-08-13                           PTY(7)

Pages that refer to this page: intro(2)ioctl_tty(2)forkpty(3)getpt(3)grantpt(3)login_tty(3)openpty(3)posix_openpt(3)ptsname(3)ptsname_r(3)unlockpt(3)ptmx(4)pts(4)tty(4)tty_ioctl(4)