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.TH PTY 7 2017-09-15 "Linux" "Linux Programmer's Manual"
.SH NAME
pty \- pseudoterminal interfaces
.SH DESCRIPTION
A pseudoterminal (sometimes abbreviated "pty") device pair
is a pair of virtual character devices that
provide a bidirectional communication channel.
One end of the channel is called the
.IR "pseudoterminal device" ;
the other end is called the
.IR "terminal device" .
.PP
The terminal end of the device pair provides an interface
that behaves exactly like a classical terminal.
A process that expects to be connected to a terminal
can open the terminal end of a pseudoterminal device pair and
then be driven by a program that has opened the pseudoterminal end
of the device pair.
Anything that is written on the pseudoterminal end is provided to the process
on the terminal end as though it was input typed on a terminal.
For example, writing the interrupt character (usually control-C)
to the pseudoterminal end of the device pair would cause an interrupt signal
.RB ( SIGINT )
to be generated for the foreground process group
that is connected to the terminal end.
Conversely, anything that is written to the terminal end of the
device pair can be read by the process that is connected to
the pseudoterminal end.
.PP
Data flow between the two ends of the device pair is handled asynchronously,
much like data flow with a physical terminal.
Data written to the terminal end will be available at
the pseudoterminal end promptly,
but may not be available immediately.
Similarly, there may be a small processing delay between
a write to the pseudoterminal end,
and the effect being visible at the terminal end.
.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.
.PP
Since kernel 2.6.4, BSD-style pseudoterminals are considered deprecated:
support can be disabled when building the kernel by disabling the
.B 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.
.SS UNIX 98 pseudoterminals
An unused UNIX 98 pseudoterminal device is opened by calling
.BR posix_openpt (3).
(This function opens the pseudoterminal multiplexor device,
.IR /dev/ptmx ;
see
.BR pts (4).)
After performing any program-specific initializations,
changing the ownership and permissions of the terminal device using
.BR grantpt (3),
and unlocking the terminal using
.BR unlockpt (3)),
the corresponding terminal device can be opened by passing
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 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
(pseudoterminal) and
.I /dev/ttyXY
(corresponding terminal),
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 device until an open succeeds.
The corresponding terminal device (substitute "tty"
for "pty" in the name of the pseudoterminal device) can then be opened.
.SH FILES
.TP
.I /dev/ptmx
UNIX 98 pseudoterminal multiplexor device
.TP
.I /dev/pts/*
UNIX 98 terminal devices
.TP
.I /dev/pty[p\-za\-e][0\-9a\-f]
The BSD pseudoterminal devices
.TP
.I /dev/tty[p\-za\-e][0\-9a\-f]
The corresponding BSD terminal devices
.SH NOTES
Pseudoterminals are used by applications such as network login services
.RB ( ssh "(1), " rlogin "(1), " telnet (1)),
terminal emulators such as
.BR xterm (1),
.BR script (1),
.BR screen (1),
.BR tmux (1),
.BR unbuffer (1),
and
.BR expect (1).
.PP
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 pts (4),
.BR tty (4)