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.TH PTS 4 2016-03-15 "Linux" "Linux Programmer's Manual"
.SH NAME
ptmx, pts \- pseudoterminal master and slave
.SH DESCRIPTION
The file
.I /dev/ptmx
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 master and slave pair.
.PP
When a process opens
.IR /dev/ptmx ,
it gets a file
descriptor for a pseudoterminal master (PTM),
and a pseudoterminal slave (PTS) device is created in the
.I /dev/pts
directory.
Each file descriptor obtained by opening
.IR /dev/ptmx
is an independent PTM with its own associated PTS, whose path can
be found by passing the file descriptor to
.BR ptsname (3).
.PP
Before opening the pseudoterminal slave, you must pass the master's file
descriptor to
.BR grantpt (3)
and
.BR unlockpt (3).
.PP
Once both the pseudoterminal master and slave are open, the slave provides
processes with an interface that is identical to that of a real terminal.
.PP
Data written to the slave is presented on the master file descriptor as input.
Data written to the master is presented to the slave as input.
.PP
In practice, pseudoterminals are used for implementing terminal emulators
such as
.BR xterm (1),
in which data read from the pseudoterminal master 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 master 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, that should be mounted on
.IR /dev/pts .
.PP
Before this UNIX 98 scheme, master pseudoterminals were called
.IR /dev/ptyp0 ", ..."
and slave pseudoterminals
.IR /dev/ttyp0 ", ..."
and one needed lots of preallocated device nodes.
.SH SEE ALSO
.BR getpt (3),
.BR grantpt (3),
.BR ptsname (3),
.BR unlockpt (3),
.BR pty (7)