The file /dev/ptmx is a character file with major number 5 and minor
number 2, usually of mode 0666 and owner.group of 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 pseu-
doterminal 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 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 pro-
vides processes with an interface that is identical to that of a real
Data written to the slave is presented on the master descriptor as
input. Data written to the master is presented to the slave as input.
In practice, pseudoterminals are used for implementing terminal emula-
tors such as xterm(1), in which data read from the pseudoterminal mas-
ter 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 ter-
minal or terminal emulator.
Pseudoterminals can also be used to send input to programs that nor-
mally refuse to read input from pipes (such as su(1), and passwd(1)).
The Linux support for the above (known as UNIX 98 pseudoterminal nam-
ing) is done using the devpts filesystem, that should be mounted on
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)
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