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PTY(7)				    Linux Programmer's Manual				   PTY(7)

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  mas-
       ter;  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.  Pseudoter-
       minals are used by applications such as network login services  (ssh(1),  rlogin(1),  tel-
       net(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 func-
       tion 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 pseudoter-
       minal  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 3.55 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)
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