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.\"     @(#)pty.4	8.2 (Berkeley) 11/30/93
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.Dd $Mdocdate: May 31 2007 $
.Dt PTY 4
.Os
.Sh NAME
.Nm pty
.Nd pseudo terminal driver
.Sh SYNOPSIS
.Cd "pseudo-device pty" Op Ar count
.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
In configuring, if an optional
.Ar count
is given in
the specification, space for that number of pseudo terminal pairs is
preallocated.
If the count is missing or is less than 2, a default count of 8 is used.
This is not a hard limit--space for additional pseudo terminal pairs
is allocated on demand up to the limit imposed by the
.Li kern.tty.maxptys
.Xr sysctl 8
(992 by default).
.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 non-zero parameter
and disabled by specifying (by reference) a zero 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
and
.Xr rlogind
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 a copy of the new
.Xr termios 4
structure.
.Pp
This is used by telnet daemons to implement TELNET "line mode",
allowing them 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 parameter and disabled by specifying (by reference)
a zero 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
.Pp
The standard way to allocate
.Nm
devices is through
.Xr openpty 3 ,
a function which internally uses a
.Dv PTMGET
.Xr ioctl 2
call on the
.Pa /dev/ptm
device.
The
.Dv PTMGET
command allocates a free pseudo terminal, changes its ownership to
the caller, revokes the access privileges for all previous users,
opens the file descriptors for the master and slave devices and returns
them to the caller in
.Fa struct ptmget .
.Bd -literal -offset indent
struct ptmget {
	int	cfd;
	int	sfd;
	char	cn[16];
	char	sn[16];
};
.Ed
.Pp
The
.Va cfd
and
.Va sfd
fields are the file descriptors for the controlling and slave terminals.
The
.Va cn
and
.Va sn
fields are the file names of the controlling and slave devices.
.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
.It Pa /dev/ptm
pseudo terminal management device
.El
.Sh SEE ALSO
.Xr openpty 3 ,
.Xr tty 4 ,
.Xr sysctl 8
.Sh HISTORY
The
.Nm
driver appeared in
.Bx 4.2 .
The
.Pa /dev/ptm
device was added in
.Ox 3.5 .
.Sh CAVEATS
The
.Pa ptm
device will only work on systems where the
.Pa /dev
directory has been properly populated with
.Nm
device nodes following the naming convention used in
.Ox .
Since
.Pa ptm
impersonates the super user for some operations it needs to perform
to complete the allocation of a pseudo terminal, the
.Pa /dev
directory must also be writeable by the super user.