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     NAME
          pty - pseudoterminal interfaces

     DESCRIPTION
          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 pro-
          gram that has opened the master end.  Anything that is writ-
          ten 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 asyn-
          chronously, 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 com-
          monly 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

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          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 pseudoter-
          minals 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 try-
          ing to open(2) each pseudoterminal master until an open suc-
          ceeds.  The corresponding pseudoterminal slave (substitute
          "tty" for "pty" in the name of the master) can then be
          opened.

     FILES
          /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
          Pseudoterminals are used by applications such as network
          login services (ssh(1), rlogin(1), telnet 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).

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          The BSD ioctl(2) operations TIOCSTOP, TIOCSTART, TIOCUCNTL,
          and TIOCREMOTE have not been implemented under Linux.

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

     COLOPHON
          This page is part of release 5.10 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/.

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