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
is an independent PTM with its own associated PTS,
whose path can be found by passing the file descriptor to ptsname
Before opening the pseudoterminal slave, you must pass the master's file
descriptor to grantpt
(3) and unlockpt
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
(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
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
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
, ... and slave pseudoterminals /dev/ttyp0
, ... and
one needed lots of preallocated device nodes.