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

       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

              UNIX 98 master clone device

              UNIX 98 slave devices

              BSD master devices

              BSD slave devices

       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.

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

       This page is part of release 5.13 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)