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  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 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  implemented
       under Linux.

SEE ALSO

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

COLOPHON

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