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       ptmx, pts - pseudoterminal master and slave


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

       When  a  process  opens  /dev/ptmx,  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  /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 pseudoterminal slave, you must pass the  master's  file  descriptor  to
       grantpt(3) and unlockpt(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 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

       Pseudoterminals can also be used to send input to programs that normally  refuse  to  read
       input from pipes (such as su(1), and passwd(1)).


       /dev/ptmx, /dev/pts/*


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

       Before this UNIX 98 scheme, master pseudoterminals were called /dev/ptyp0, ...  and  slave
       pseudoterminals /dev/ttyp0, ...  and one needed lots of preallocated device nodes.


       getpt(3), grantpt(3), ptsname(3), unlockpt(3), pty(7)


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