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PTY(7)			   Linux Programmer's Manual			PTY(7)

       pty - pseudoterminal interfaces

       A  pseudoterminal  (sometimes  abbreviated  "pty") is a pair of virtual
       character devices that provide a bidirectional  communication  channel.
       One  end	 of  the channel is called the master; the other end is called
       the slave.  The slave end of the pseudoterminal provides	 an  interface
       that behaves exactly like a classical terminal.	A process that expects
       to be connected to a terminal, can open the slave end of a pseudotermi-
       nal  and	 then  be  driven by a program that has opened the master end.
       Anything that is written on the master end is provided to  the  process
       on the slave end as though it was input typed on a terminal.  For exam-
       ple, writing the interrupt character (usually control-C) to the	master
       device would cause an interrupt signal (SIGINT) to be generated for the
       foreground process group that is connected to the  slave.   Conversely,
       anything	 that is written to the slave end of the pseudoterminal can be
       read by the process that is connected to the master end.	  Pseudotermi-
       nals  are  used by applications such as network login services (ssh(1),
       rlogin(1), telnet(1)), terminal emulators,  script(1),  screen(1),  and

       Data flow between master and slave is handled asynchronously, much like
       data flow with a physical terminal.  Data written to the slave will  be
       available at the master promptly, but may not be available immediately.
       Similarly, there may be a small processing delay between a write to the
       master, and the effect being visible at the slave.

       Historically,  two  pseudoterminal APIs have evolved: BSD and System V.
       SUSv1 standardized a pseudoterminal API based on the System V API,  and
       this  API  should be employed in all new programs that use pseudotermi-

       Linux provides both BSD-style and (standardized) System	V-style	 pseu-
       doterminals.   System  V-style  terminals  are  commonly called UNIX 98
       pseudoterminals on Linux systems.  Since kernel 2.6.4, BSD-style	 pseu-
       doterminals  are	 considered deprecated (they can be disabled when con-
       figuring the kernel); UNIX 98 pseudoterminals should  be	 used  in  new

   UNIX 98 pseudoterminals
       An   unused   UNIX  98  pseudoterminal  master  is  opened  by  calling
       posix_openpt(3).	  (This	 function  opens  the  master  clone   device,
       /dev/ptmx; see pts(4).)	After performing any program-specific initial-
       izations, changing the ownership and permissions of  the	 slave	device
       using  grantpt(3), and unlocking the slave using unlockpt(3)), the cor-
       responding slave device can be opened by passing the name  returned  by
       ptsname(3) in a call to open(2).

       The  Linux  kernel  imposes  a limit on the number of available UNIX 98
       pseudoterminals.	 In kernels up to and including 2.6.3, this  limit  is
       configured  at  kernel  compilation  time (CONFIG_UNIX98_PTYS), and the
       permitted number of pseudoterminals can be up to 2048, with  a  default
       setting	 of  256.   Since  kernel  2.6.4,  the	limit  is  dynamically
       adjustable via  /proc/sys/kernel/pty/max,  and  a  corresponding	 file,
       /proc/sys/kernel/pty/nr,	 indicates  how	 many pseudoterminals are cur-
       rently in use.  For further details on these two files, see proc(5).

   BSD pseudoterminals
       BSD-style pseudoterminals are provided as precreated pairs, with	 names
       of  the	form  /dev/ptyXY (master) and /dev/ttyXY (slave), where X is a
       letter from the 16-character set [p-za-e], and Y is a letter  from  the
       16-character  set [0-9a-f].  (The precise range of letters in these two
       sets varies across UNIX implementations.)  For example, /dev/ptyp1  and
       /dev/ttyp1  constitute  a  BSD pseudoterminal pair.  A process finds an
       unused pseudoterminal pair by trying  to	 open(2)  each	pseudoterminal
       master  until an open succeeds.	The corresponding pseudoterminal slave
       (substitute "tty" for "pty" in the name of  the	master)	 can  then  be

       /dev/ptmx (UNIX 98 master clone device)
       /dev/pts/* (UNIX 98 slave devices)
       /dev/pty[p-za-e][0-9a-f] (BSD master devices)
       /dev/tty[p-za-e][0-9a-f] (BSD slave devices)

       A description of the TIOCPKT ioctl(2), which controls packet mode oper-
       ation, can be found in tty_ioctl(4).

       The BSD ioctl(2) operations TIOCSTOP, TIOCSTART, TIOCUCNTL, and TIOCRE-
       MOTE have not been implemented under Linux.

       select(2),   setsid(2),	forkpty(3),  openpty(3),  termios(3),  pts(4),
       tty(4), tty_ioctl(4)

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Linux				  2015-07-23				PTY(7)