ptmx, pts − pseudoterminal master and slave

The file /dev/ptmx is a character file with major number 5
and minor number 2, usually with mode 0666 and ownership
root:root.  It is used to create a pseudoterminal master and
slave pair.

     When a process opens it gets a file descriptor for a
pseudoterminal master (PTM), and a pseudoterminal slave
(PTS) device is created in the /dev/pts directory.  Each
file descriptor obtained by opening is an independent PTM
with its own associated PTS, whose path can be found by
passing the file descriptor to

     Before opening the pseudoterminal slave, you must pass
the master’s file descriptor to and

     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 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 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.

     Pseudoterminals can also be used to send input to
programs that normally refuse to read input from pipes (such
as and


The Linux support for the above (known as UNIX 98
pseudoterminal naming) is done using the devpts filesystem,
that should be mounted on

     Before this UNIX 98 scheme, master pseudoterminals were
called and slave pseudoterminals and one needed lots of
preallocated device nodes.

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