PTS(4) Linux Programmer's Manual PTS(4)
ptmx and pts - pseudo-terminal master and slave
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 pseudo-terminal master and slave pair.
When a process opens /dev/ptmx, it gets a file descriptor for a pseudo-
terminal master (PTM), and a pseudo-terminal slave (PTS) device is cre-
ated in the /dev/pts directory. Each file descriptor obtained by open-
ing /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 pseudo-terminal slave, you must pass the master's
file descriptor to grantpt(3) and unlockpt(3).
Once both the pseudo-terminal 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, pseudo-terminals are used for implementing terminal emula-
tors such as xterm(1), in which data read from the pseudo-terminal 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 pseudo-terminal master is
sent across the network to a client program that is connected to a ter-
minal or terminal emulator.
Pseudo-terminals can also be used to send input to programs that nor-
mally refuse to read input from pipes (such as su(8), and passwd(8)).
The Linux support for the above (known as Unix98 pty naming) is done
using the devpts filesystem, that should be mounted on /dev/pts.
Before this Unix98 scheme, master ptys were called /dev/ptyp0, ... and
slave ptys /dev/ttyp0, ... and one needed lots of preallocated device
getpt(3), grantpt(3), ptsname(3), unlockpt(3)
Linux 2002-10-09 PTS(4)