man4/pts.4

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.TH PTS 4 2016-03-15 "Linux" "Linux Programmer's Manual"
.SH NAME
ptmx, pts \- pseudoterminal devices
.SH DESCRIPTION
The file
.I /dev/ptmx
(the pseudoterminal multiplexor device)
is a character file with major number 5 and
minor number 2, usually with mode 0666 and ownership root:root.
It is used to create a pseudoterminal device pair.
.PP
When a process opens
.IR /dev/ptmx ,
it gets a file
descriptor for a new pseudoterminal device
and a corresponding terminal device is created in the
.I /dev/pts
directory.
Each file descriptor obtained by opening
.IR /dev/ptmx
is an independent pseudoterminal device with its own associated terminal device,
whose path can
be found by passing the file descriptor to
.BR ptsname (3).
.PP
Before opening the corresponding terminal device,
you must pass the pseudoterminal device's file
descriptor to
.BR grantpt (3)
and
.BR unlockpt (3).
.PP
Once both the pseudoterminal device and the terminal device are open,
the terminal device provides
processes with an interface that is identical to that of a real terminal.
.PP
Data written to the terminal device is presented on
the pseudoterminal device as input.
Data written to the pseudoterminal device is presented
to the terminal device as input.
.PP
In practice, pseudoterminals are used for implementing terminal emulators
such as
.BR xterm (1),
in which data read from the pseudoterminal device is interpreted by the
application in the same way
a real terminal would interpret the data, and for implementing remote-login
programs such as
.BR sshd (8),
in which data read from the pseudoterminal device is sent across the network
to a client program that is connected to a terminal or terminal emulator.
.PP
Pseudoterminals can also be used to send input to programs that normally
refuse to read input from pipes (such as
.BR su (1),
and
.BR passwd (1)).
.SH FILES
.IR /dev/ptmx ,
.I /dev/pts/*
.SH NOTES
The Linux support for the above (known as UNIX 98 pseudoterminal naming)
is done using the
.I devpts
filesystem, which should be mounted on
.IR /dev/pts .
.SH SEE ALSO
.BR getpt (3),
.BR grantpt (3),
.BR ptsname (3),
.BR unlockpt (3),
.BR pty (7)