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.  Pseudoterminals are used by applications such as network
       login services (ssh(1), rlogin(1), telnet(1)), terminal emulators, script(1), screen(1), and expect(1).

       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 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

       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 3.54 of the Linux man-pages project.  A  description  of  the  project,  and
       information about reporting bugs, can be found at http://www.kernel.org/doc/man-pages/.

Linux                                              2005-10-10                                             PTY(7)