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-.TH PTY 7 2005-10-10 "Linux 2.6.14" "Linux Programmer's Manual"
+.TH PTY 7 2017-09-15 "Linux" "Linux Programmer's Manual"
.SH NAME
-pty - pseudo-terminal interfaces
+pty \- pseudoterminal interfaces
.SH DESCRIPTION
-A pseudo-terminal is a pair of virtual character devices that
-provide a bidirectional communication channel.
-One end of the channel is called the
+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
+the other end is called the
.IR slave .
-The slave end of the pseudo-terminal provides an interface
+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 pseudo-terminal and
+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.
+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
+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
-pseudo-terminal can be read by the process that is connected to
+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.
-Pseudo-terminals are used by applications such as network login services
+Pseudoterminals are used by applications such as network login services
.RB ( ssh "(1), " rlogin "(1), " telnet (1)),
-terminal emulators,
+terminal emulators such as
+.BR xterm (1),
.BR script (1),
.BR screen (1),
+.BR tmux (1),
+.BR unbuffer (1),
and
.BR expect (1).
-
-Historically, two pseudo-terminal APIs have evolved: BSD and System V.
-SUSv1 standardized a pseudo-terminal API based on the System V API,
-and this API should be employed in all new programs that use
-pseudo-terminals.
-
-Linux provides both BSD-style and (standardised) System V-style
-pseudo-terminals.
-System V-style terminals are commonly called Unix 98 pseudo-terminals
+.PP
+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.
+.PP
+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.
+.PP
+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 pseudo-terminals are considered deprecated
-(they can be disabled when configuring the kernel);
-Unix 98 pseudo-terminals should be used in new applications.
-.SS "Unix 98 pseudo-terminals"
-An unused Unix 98 pseudo-terminal master is opened by calling
+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 .)
-After performing any program-specific initialisations,
-changing the ownership and permissions of the slave device using
+.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
+and unlocking the slave using
.BR unlockpt (3)),
the corresponding slave device can be opened by passing
-the name returned by
+the name returned by
.BR ptsname (3)
in a call to
.BR open (2).
-
+.PP
The Linux kernel imposes a limit on the number of available
-Unix 98 pseudo-terminals.
+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 pseudo-terminals can be up to 2048,
+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
+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 pseudo-terminals are currently in use.
+indicates how many pseudoterminals are currently in use.
For further details on these two files, see
.BR proc (5).
-.SS "BSD pseudo-terminals"
-BSD-style pseudo-terminals are provided as pre-created pairs, with
-names of the form
+.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
+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 pseudo-terminal pair.
-A process finds an unused pseudo-terminal pair by trying to
+constitute a BSD pseudoterminal pair.
+A process finds an unused pseudoterminal pair by trying to
.BR open (2)
-each pseudo-terminal master until an open succeeds.
-The corresponding pseudo-terminal slave (substitute "tty"
+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 "NOTES"
-A description of the
-.B TIOCPKT
-.BR ioctl (),
-which controls packet mode operation, can be found in
-.BR tty_ioctl (4).
-
-The BSD ioctl()s TIOCSTOP, TIOCSTART, TIOCUCNTL, TIOCREMOTE have
-not been implemented under Linux.
-.SH "FILES"
-.I /dev/ptmx
-(Unix 98 master clone device)
-.br
+.SH FILES
+.TP
+.I /dev/ptmx
+UNIX 98 master clone device
+.TP
.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)
-.I
-.SH "SEE ALSO"
+UNIX 98 slave devices
+.TP
+.I /dev/pty[p\-za\-e][0\-9a\-f]
+BSD master devices
+.TP
+.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 ioctl_tty (2).
+.PP
+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 ioctl_tty (2),
.BR select (2),
.BR setsid (2),
.BR forkpty (3),
.BR openpty (3),
.BR termios (3),
-.BR tty (4),
-.BR tty_ioctl (4)
+.BR pts (4),
+.BR tty (4)
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