|PTS(4)||Linux Programmer's Manual||PTS(4)|
When a process opens /dev/ptmx, it gets a file descriptor for a pseudoterminal master and a pseudoterminal slave device is created in the /dev/pts directory. Each file descriptor obtained by opening /dev/ptmx is an independent pseudoterminal master with its own associated slave, whose path can be found by passing the file descriptor to ptsname(3).
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 as xterm(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(8), 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.