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-.TH PTY 7 2005-10-10 "Linux" "Linux Programmer's Manual"
-.SH NAME
-pty \- pseudoterminal interfaces
-.SH 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
-.IR master ;
-the other end is called the
-.IR 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
-.RB ( 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
-.RB ( ssh "(1), " rlogin "(1), " telnet (1)),
-terminal emulators,
-.BR script (1),
-.BR screen (1),
-and
-.BR 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.
-.SS UNIX 98 pseudoterminals
-An unused UNIX 98 pseudoterminal master is opened by calling
-.BR posix_openpt (3).
-(This function opens the master clone device,
-.IR /dev/ptmx ;
-see
-.BR pts (4).)
-After performing any program-specific initializations,
-changing the ownership and permissions of the slave device using
-.BR grantpt (3),
-and unlocking the slave using
-.BR unlockpt (3)),
-the corresponding slave device can be opened by passing
-the name returned by
-.BR ptsname (3)
-in a call to
-.BR 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
-.RB ( 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
-.IR /proc/sys/kernel/pty/max ,
-and a corresponding file,
-.IR /proc/sys/kernel/pty/nr ,
-indicates how many pseudoterminals are currently in use.
-For further details on these two files, see
-.BR proc (5).
-.SS BSD pseudoterminals
-BSD-style pseudoterminals are provided as precreated pairs, with
-names of the form
-.I /dev/ptyXY
-(master) and
-.I /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,
-.I /dev/ptyp1
-and
-.I /dev/ttyp1
-constitute a BSD pseudoterminal pair.
-A process finds an unused pseudoterminal pair by trying to
-.BR 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.
-.SH FILES
-.I /dev/ptmx
-(UNIX 98 master clone device)
-.br
-.I /dev/pts/*
-(UNIX 98 slave devices)
-.br
-.I /dev/pty[p-za-e][0-9a-f]
-(BSD master devices)
-.br
-.I /dev/tty[p-za-e][0-9a-f]
-(BSD slave devices)
-.SH NOTES
-A description of the
-.B TIOCPKT
-.BR ioctl (2),
-which controls packet mode operation, can be found in
-.BR tty_ioctl (4).
-
-The BSD
-.BR ioctl (2)
-operations
-.BR TIOCSTOP ,
-.BR TIOCSTART ,
-.BR TIOCUCNTL ,
-and
-.BR TIOCREMOTE
-have not been implemented under Linux.
-.SH SEE ALSO
-.BR select (2),
-.BR setsid (2),
-.BR forkpty (3),
-.BR openpty (3),
-.BR termios (3),
-.BR pts (4),
-.BR tty (4),
-.BR tty_ioctl (4)
-.SH COLOPHON
-This page is part of release 3.79 of the Linux
-.I man-pages
-project.
-A description of the project,
-information about reporting bugs,
-and the latest version of this page,
-can be found at
-\%http://www.kernel.org/doc/man\-pages/.
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