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PTY(4)		       FreeBSD Kernel Interfaces Manual			PTY(4)

NAME
     pty -- pseudo terminal driver

SYNOPSIS
     device pty

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
     device and	a slave	device.	 The slave device provides to a	process	an
     interface 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.

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

     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	parameter and disabled by
		 specifying (by	reference) a zero 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 reflecting 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 output; it can be	used by	other
		 similar programs.

     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 parameter and disabled by
		 specifying (by	reference) a zero 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 operation 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 existence	of
		 this facility.	 As with TIOCPKT mode, command operations may
		 be detected with a select(2) for exceptional conditions.

     There is currently	two pty	systems	available: the original	BSD pty, and a
     SysVR4 pts-like implementation.  It is possible to	switch between the two
     implementations by	setting	the kern.pts.enable sysctl.  Setting it	to 0
     will use the BSD pty, to non-zero the pts implementation.	It defaults to
     0.	 It is possible	to set the maximum number of ptys which	can be allo-
     cated at the same time with the kern.pts.max sysctl.  It defaults to
     1000.  It is not recommended to use more than 1000	pseudo-terminals, as
     all software which	use utmp(5) will not be	able to	handle pseudo-termi-
     nals with number superior to 999.

     The pts implementation also supports the TIOCGPTN ioctl(2)	call, which
     takes a pointer to	an unsigned int	as a parameter and provides the	number
     of	the pty.

FILES
     The files used by the BSD pseudo terminals	implementation are:

     /dev/pty[p-sP-S][0-9a-v]  master pseudo terminals
     /dev/tty[p-sP-S][0-9a-v]  slave pseudo terminals

     The files used by the pts implementation are:

     /dev/ptmx	     control device, returns a file descriptor to a new	master
		     pseudo terminal when opened.
     /dev/pty[num]   master pseudo terminals
     /dev/pts/[num]  slave pseudo terminals

DIAGNOSTICS
     None.

SEE ALSO
     tty(4)

HISTORY
     The pty driver appeared in	4.2BSD.

FreeBSD	10.1		       November	30, 1993		  FreeBSD 10.1

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

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