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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, script(1), screen(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.


       /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)


       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.


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


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