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

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  mas‐
       ter;  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.  Pseudoter‐
       minals are used by applications such as network login services  (ssh(1),  rlogin(1),  tel‐
       net(1)), terminal emulators, script(1), screen(1), and expect(1).

       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 (they  can  be  disabled
       when configuring the 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 func‐
       tion 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 pseudoter‐
       minal 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
       /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
       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.

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

COLOPHON
       This page is part of release 4.04 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 http://www.kernel.org/doc/man-pages/.

Linux                                       2015-07-23                                     PTY(7)

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