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
processes 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 are configured;
the default count is 32.

The following ioctl 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
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 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 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 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
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 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 remote line
editing in a window manager, or whenever flow controlled input
is required.

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