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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.

     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
     allocated 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-
     terminals 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.