man/man4/pty.4

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.\"     @(#)pty.4	8.2 (Berkeley) 11/30/93
.\"
.Dd November 30, 1993
.Dt PTY 4
.Os BSD 4.2
.Sh NAME
.Nm pty
.Nd pseudo terminal driver
.Sh SYNOPSIS
.Cd "pseudo-device pty" Op Ar count
.Sh DESCRIPTION
The
.Nm pty
driver provides support for a device-pair termed a
.Em pseudo terminal .
A pseudo terminal is a pair of character devices, a
.Em primary
device and a
.Em replica
device.
The replica device provides to a process an interface identical
to that described in
.Xr tty 4 .
However, whereas all other devices which provide the
interface described in
.Xr tty 4
have a hardware device of some sort behind them, the replica
device has, instead, another process manipulating
it through the primary half of the pseudo terminal.
That is, anything written on the primary device is
given to the replica device as input and anything written
on the replica device is presented as input on the primary
device.
.Pp
In configuring, if an optional
.Ar count
is given in
the specification, that number of pseudo terminal pairs are configured;
the default count is 32.
.Pp
The following
.Xr ioctl 2
calls apply only to pseudo terminals:
.Bl -tag -width TIOCREMOTE
.It Dv TIOCSTOP
Stops output to a terminal (e.g.\& like typing
.Ql ^S ) .
Takes
no parameter.
.It Dv TIOCSTART
Restarts output (stopped by
.Dv TIOCSTOP
or by typing
.Ql ^S ) .
Takes no parameter.
.It Dv TIOCPKT
Enable/disable
.Em 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 primary side of a pseudo terminal, each subsequent
.Xr read 2
from the terminal will return data written on the replica part of
the pseudo terminal preceded by a zero byte (symbolically
defined as
.Dv 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:
.Bl -tag -width TIOCPKT_FLUSHWRITE
.It Dv TIOCPKT_FLUSHREAD
whenever the read queue for the terminal is flushed.
.It Dv TIOCPKT_FLUSHWRITE
whenever the write queue for the terminal is flushed.
.It Dv TIOCPKT_STOP
whenever output to the terminal is stopped a la
.Ql ^S .
.It Dv TIOCPKT_START
whenever output to the terminal is restarted.
.It Dv TIOCPKT_DOSTOP
whenever
.Em t_stopc
is
.Ql ^S
and
.Em t_startc
is
.Ql ^Q .
.It Dv TIOCPKT_NOSTOP
whenever the start and stop characters are not
.Ql ^S/^Q .
.El
.Pp
While this mode is in use, the presence of control status information
to be read from the primary side may be detected by a
.Xr select 2
for exceptional conditions.
.Pp
This mode is used by
.Xr rlogin 1
and
.Xr rlogind 8
to implement a remote-echoed, locally
.Ql ^S/^Q
flow-controlled
remote login with proper back-flushing of output; it can be
used by other similar programs.
.It Dv TIOCUCNTL
Enable/disable a mode that allows a small number of simple user
.Xr ioctl 2
commands to be passed through the pseudo-terminal,
using a protocol similar to that of
.Dv TIOCPKT .
The
.Dv TIOCUCNTL
and
.Dv TIOCPKT
modes are mutually exclusive.
This mode is enabled from the primary side of a pseudo terminal
by specifying (by reference)
a nonzero parameter and disabled by specifying (by reference)
a zero parameter.
Each subsequent
.Xr read 2
from the primary side will return data written on the replica part of
the pseudo terminal preceded by a zero byte,
or a single byte reflecting a user control operation on the replica side.
A user control command consists of a special
.Xr ioctl 2
operation with no data; the command is given as
.Dv UIOCCMD Ns (n) ,
where
.Ar n
is a number in the range 1-255.
The operation value
.Ar n
will be received as a single byte on the next
.Xr read 2
from the primary side.
The
.Xr ioctl 2
.Dv UIOCCMD Ns (0)
is a no-op that may be used to probe for
the existence of this facility.
As with
.Dv TIOCPKT
mode, command operations may be detected with a
.Xr select 2
for exceptional conditions.
.El
.Sh FILES
.Bl -tag -width /dev/tty[p-sP-S][a-z0-9]x -compact
.It Pa /dev/pty[p-sP-S][a-z0-9]
primary pseudo terminals
.It Pa /dev/tty[p-sP-S][a-z0-9]
replica pseudo terminals
.El
.Sh DIAGNOSTICS
None.
.Sh HISTORY
The
.Nm
driver appeared in
.Bx 4.2 .