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PTY(4)                     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
     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
     presented 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
     (defaults 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 disabled 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
                 specifying (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
                 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.

                 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
                                     activated ``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
                                     appropriate 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-
                 terminal, 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
                 disabled 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
                 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.

     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
                 remote line editing in a window manager, or whenever flow
                 controlled 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.

NetBSD 6.1.5                   February 1, 2011                   NetBSD 6.1.5