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.\"     @(#)pty.4	8.2 (Berkeley) 11/30/93
.\"
.Dd November 30, 2013
.Dt PTY 4
.Os
.Sh NAME
.Nm pty
.Nd pseudo terminal driver
.Sh SYNOPSIS
.Cd pseudo-device pty
.Sh DESCRIPTION
The
.Nm
driver provides support for a device-pair termed a
.Em pseudo terminal .
A pseudo terminal is a pair of character devices, a
.Em master
device and a
.Em slave
device.
The slave 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 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.
.Pp
Pseudo terminal pairs are allocated on
as-needed basis, maximum number of them is controlled via
.Em kern.maxptys
sysctl (defaults to 992).
.Pp
The following
.Xr ioctl 2
calls apply only to pseudo terminals:
.Bl -tag -width TIOCREMOTE
.It Dv TIOCEXT
Enable/disable
.Dq "external processing" .
This affects delivery of
.Dv TIOCPKT_IOCTL
packets.
External processing is enabled by specifying (by reference) a nonzero
.Vt int
parameter and disabled by specifying (by reference) a zero
.Vt int
parameter.
.Pp
.Dv TIOCEXT
is reset to its default (disabled) when the slave closes the
.Nm .
.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
.Vt int
parameter and disabled by specifying (by reference) a zero
.Vt int
parameter.
When applied to the master side of a pseudo
terminal, each subsequent
.Xr 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
.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 .
.Pp
While this mode is in use, the presence of control status information
to be read from the master 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 TIOCPKT_IOCTL
When this bit is set, the slave has changed the
.Xr termios 4
structure (TTY state),
and the remainder of the data read from
the master side of the
.Nm
is the new
.Xr termios 4
structure.
The master side of the
.Nm
can also use
.Xr tcgetattr 3
to read the new
.Xr termios 4
structure.
.Pp
The master will not read packets with the bit
.Dv TIOCPKT_IOCTL
set until it has activated
.Dq "external processing"
using
.Dv TIOCEXT .
.Pp
This is used by
.Xr telnetd 8
to implement TELNET "line mode" - it allows the
.Xr telnetd 8
to detect
.Xr tty 4
state changes by the slave, and negotiate the appropriate TELNET
protocol equivalents with the remote peer.
.El
.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 master side of a pseudo terminal
by specifying (by reference)
a nonzero
.Vt int
parameter and disabled by specifying (by reference)
a zero
.Vt int
parameter.
Each subsequent
.Xr 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
.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 master 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.
.It Dv TIOCREMOTE
A mode for the master half of a pseudo terminal, independent
of
.Dv 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.
.Dv TIOCREMOTE
can be used when doing remote line
editing in a window manager, or whenever flow controlled input
is required.
.El
.Sh FILES
.Bl -tag -width /dev/tty[p-zP-T][0-9a-zA-Z]x -compact
.It Pa /dev/pty[p-zP-T][0-9a-zA-Z]
master pseudo terminals
.It Pa /dev/tty[p-zP-T][0-9a-zA-Z]
slave pseudo terminals
.El
.Sh DIAGNOSTICS
None.
.Sh SEE ALSO
.Xr ioctl 2 ,
.Xr read 2 ,
.Xr select 2 ,
.Xr write 2 ,
.Xr openpty 3 ,
.Xr tty 4
.Sh HISTORY
The
.Nm
driver appeared in
.Bx 4.2 .