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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.

       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  Linux  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

       The  Linux  kernel imposes a limit on the number of available UNIX 98 pseudoterminals.  Up
       to and including Linux  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 Linux 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


       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.


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