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       pty - pseudoterminal interfaces


       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 such as xterm(1), script(1), screen(1), tmux(1),
       unbuffer(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
       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

       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

   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.


              UNIX 98 master clone device

              UNIX 98 slave devices

              BSD master devices

              BSD slave devices


       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.


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


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