ptmx
DESCRIPTION
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
terminal.
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(1), and passwd(1)).
FILES
/dev/ptmx, /dev/pts/*
NOTES
The Linux support for the above (known as Unix98 pty naming) is done
using the devpts file system, 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
nodes.
SEE ALSO
getpt(3), grantpt(3), ptsname(3), unlockpt(3), pty(7)
COLOPHON
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