pty

PTY(4)                    BSD 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 presented 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
                 specifying (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-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 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.

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

DIAGNOSTICS
     None.

SEE ALSO
     tty(4)

HISTORY
     The pty driver appeared in 4.2BSD.

BSD                             November 30, 1993                            BSD