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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 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.  Psuedoterminals 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

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