PTY(4)                  NetBSD 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 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
     (defaults to 992).

     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 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 a
                                     copy of the new termios(4) structure.

                                     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 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.

     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 5.1                      October 7, 2006                     NetBSD 5.1

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