PTY(4)                    OpenBSD Programmer's 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 de-
     vice 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 are configured; the default count is 32.

     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 non-zero 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 in-
                 clusive-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 tel-
                                     netd(8) to detect tty(4) state changes by
                                     the slave, and negotiate the appropriate
                                     TELNET protocol equivalents with the re-
                                     mote peer.

     TIOCUCNTL   Enable/disable a mode that allows a small number of simple
                 user ioctl 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 en-
                 abled 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 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 us-
                 er control command consists of a special ioctl 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 re-
                 ceived as a single byte on the next read from the master
                 side.  The ioctl 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 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 re-
                 mote line editing in a window manager, or whenever flow con-
                 trolled input is required.

FILES
     /dev/pty[p-r][0-9a-f]   master pseudo terminals
     /dev/tty[p-r][0-9a-f]   slave pseudo terminals

SEE ALSO
     tty(4)

HISTORY
     The pty driver appeared in 4.2BSD.

OpenBSD 3.0                    November 30, 1993                             2
