pty(7) - man - phpman

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  master;
       the other end is called the slave.

       The slave end of the pseudoterminal provides an interface that behaves exactly like a classi‐
       cal 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 con‐
       trol-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 writ‐
       ten 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 pro‐
       grams 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 dis‐
       abled 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 pseudoter‐
       minals 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 ker‐
       nels  up  to  and  including 2.6.3, this limit is configured at kernel compilation time (CON‐‐
       FIG_UNIX98_PTYS), and the permitted number of pseudoterminals can be up to 2048, with  a  de‐
       fault  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
       /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(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 im‐
       plemented 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/.



Linux                                        2020-08-13                                       PTY(7)