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 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 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 provides
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 emulators
such as
xterm(1) , in which data read from the pseudo-terminal 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 pseudo-terminal master is sent across the network
to a client program that is connected to a terminal or terminal emulator.
Pseudo-terminals can also be used to send input to programs that normally
refuse to read input from pipes (such as
su(8) , and
passwd(8) ).
FILES
/dev/ptmx , /dev/pts/*
NOTES
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 nodes.
