ptmx, pts - pseudoterminal master and slave
The file
/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.
When a process opens
/dev/ptmx, it gets a file descriptor for a
pseudoterminal master (PTM), and a pseudoterminal slave (PTS) device is
created in the
/dev/pts directory. Each file descriptor obtained by
opening
/dev/ptmx is an independent PTM with its own associated PTS,
whose path can be found by passing the file descriptor to
ptsname(3).
Before opening the pseudoterminal slave, you must pass the master's file
descriptor to
grantpt(3) and
unlockpt(3).
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.
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.
In practice, pseudoterminals are used for implementing terminal emulators such
as
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
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.
Pseudoterminals can also be used to send input to programs that normally refuse
to read input from pipes (such as
su(1), and
passwd(1)).
/dev/ptmx,
/dev/pts/*
The Linux support for the above (known as UNIX 98 pseudoterminal naming) is done
using the
devpts filesystem, that should be mounted on
/dev/pts.
Before this UNIX 98 scheme, master pseudoterminals were called
/dev/ptyp0, ... and slave pseudoterminals
/dev/ttyp0, ... and
one needed lots of preallocated device nodes.
getpt(3),
grantpt(3),
ptsname(3),
unlockpt(3),
pty(7)