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PTY(4)			 BSD Kernel Interfaces Manual			PTY(4)

NAME
     pty -- pseudo terminal driver

SYNOPSIS
     pseudo-device pty [count]

DESCRIPTION
     The pty driver provides support for a device-pair termed a	pseudo
     terminal.	A pseudo terminal is a pair of character devices, a master de-
     vice and a	slave device.  The slave device	provides to a process an in-
     terface identical to that described in tty(4).  However, whereas all
     other devices which provide the interface described in tty(4) have	a
     hardware device of	some sort behind them, the slave device	has, instead,
     another process manipulating it through the master	half of	the pseudo
     terminal.	That is, anything written on the master	device is given	to the
     slave device as input and anything	written	on the slave device is pre-
     sented as input on	the master device.

     In	configuring, if	an optional count is given in the specification, that
     number of pseudo terminal pairs is	initially configured; the default
     count is 16.  Additional pseudo terminal pairs are	allocated on as-needed
     basis, maximum number of them is controlled via kern.maxptys sysctl (de-
     faults to 992).

     The following ioctl(2) calls apply	only to	pseudo terminals:

     TIOCEXT	 Enable/disable	"external processing".	This affects delivery
		 of TIOCPKT_IOCTL packets.  External processing	is enabled by
		 specifying (by	reference) a nonzero int parameter and dis-
		 abled by specifying (by reference) a zero int parameter.

		 TIOCEXT is reset to its default (disabled) when the slave
		 closes	the pty.

     TIOCSTOP	 Stops output to a terminal (e.g. like typing `^S').  Takes no
		 parameter.

     TIOCSTART	 Restarts output (stopped by TIOCSTOP or by typing `^S').
		 Takes no parameter.

     TIOCPKT	 Enable/disable	packet mode.  Packet mode is enabled by	speci-
		 fying (by reference) a	nonzero	int parameter and disabled by
		 specifying (by	reference) a zero int parameter.  When applied
		 to the	master side of a pseudo	terminal, each subsequent
		 read(2) from the terminal will	return data written on the
		 slave part of the pseudo terminal preceded by a zero byte
		 (symbolically defined as TIOCPKT_DATA), or a single byte re-
		 flecting control status information.  In the latter case, the
		 byte is an inclusive-or of zero or more of the	bits:

		 TIOCPKT_FLUSHREAD   whenever the read queue for the terminal
				     is	flushed.

		 TIOCPKT_FLUSHWRITE  whenever the write	queue for the terminal
				     is	flushed.

		 TIOCPKT_STOP	     whenever output to	the terminal is
				     stopped a la `^S'.

		 TIOCPKT_START	     whenever output to	the terminal is
				     restarted.

		 TIOCPKT_DOSTOP	     whenever t_stopc is `^S' and t_startc is
				     `^Q'.

		 TIOCPKT_NOSTOP	     whenever the start	and stop characters
				     are not `^S/^Q'.

				     While this	mode is	in use,	the presence
				     of	control	status information to be read
				     from the master side may be detected by a
				     select(2) for exceptional conditions.

				     This mode is used by rlogin(1) and
				     rlogind(8)	to implement a remote-echoed,
				     locally `^S/^Q' flow-controlled remote
				     login with	proper back-flushing of	out-
				     put; it can be used by other similar pro-
				     grams.

		 TIOCPKT_IOCTL	     When this bit is set, the slave has
				     changed the termios(4) structure (TTY
				     state), and the remainder of the data
				     read from the master side of the pty is
				     the new termios(4)	structure.  The	master
				     side of the pty can also use tcgetattr(3)
				     to	read the new termios(4)	structure.

				     The master	will not read packets with the
				     bit TIOCPKT_IOCTL set until it has	acti-
				     vated "external processing" using
				     TIOCEXT.

				     This is used by telnetd(8)	to implement
				     TELNET "line mode"	- it allows the
				     telnetd(8)	to detect tty(4) state changes
				     by	the slave, and negotiate the appropri-
				     ate TELNET	protocol equivalents with the
				     remote peer.

     TIOCUCNTL	 Enable/disable	a mode that allows a small number of simple
		 user ioctl(2) commands	to be passed through the pseudo-termi-
		 nal, using a protocol similar to that of TIOCPKT.  The
		 TIOCUCNTL and TIOCPKT modes are mutually exclusive.  This
		 mode is enabled from the master side of a pseudo terminal by
		 specifying (by	reference) a nonzero int parameter and dis-
		 abled by specifying (by reference) a zero int parameter.
		 Each subsequent read(2) from the master side will return data
		 written on the	slave part of the pseudo terminal preceded by
		 a zero	byte, or a single byte reflecting a user control oper-
		 ation on the slave side.  A user control command consists of
		 a special ioctl(2) operation with no data; the	command	is
		 given as UIOCCMD(n), where n is a number in the range 1-255.
		 The operation value n will be received	as a single byte on
		 the next read(2) from the master side.	 The ioctl(2)
		 UIOCCMD(0) is a no-op that may	be used	to probe for the exis-
		 tence of this facility.  As with TIOCPKT mode,	command	opera-
		 tions may be detected with a select(2)	for exceptional	condi-
		 tions.

     TIOCREMOTE	 A mode	for the	master half of a pseudo	terminal, independent
		 of TIOCPKT.  This mode	causes input to	the pseudo terminal to
		 be flow controlled and	not input edited (regardless of	the
		 terminal mode).  Each write to	the control terminal produces
		 a record boundary for the process reading the terminal.  In
		 normal	usage, a write of data is like the data	typed as a
		 line on the terminal; a write of 0 bytes is like typing an
		 end-of-file character.	 TIOCREMOTE can	be used	when doing re-
		 mote line editing in a	window manager,	or whenever flow con-
		 trolled input is required.

FILES
     /dev/pty[p-zP-T][0-9a-zA-Z]   master pseudo terminals
     /dev/tty[p-zP-T][0-9a-zA-Z]   slave pseudo	terminals

DIAGNOSTICS
     None.

SEE ALSO
     ioctl(2), read(2),	select(2), write(2), openpty(3), tty(4)

HISTORY
     The pty driver appeared in	4.2BSD.

BSD			       February	1, 2011				   BSD

NAME | SYNOPSIS | DESCRIPTION | FILES | DIAGNOSTICS | SEE ALSO | HISTORY

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