man/man4/pts.4

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.Dd August 20, 2008
.Dt PTS 4
.Os
.Sh NAME
.Nm pts
.Nd pseudo-terminal driver
.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
The following
.Xr ioctl 2
calls apply only to pseudo-terminals:
.Bl -tag -width TIOCPTMASTER
.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 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
.Dv VSTOP
is
.Ql ^S
and
.Dv VSTART
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 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 TIOCGPTN
Obtain device unit number, which can be used to generate the filename of
the pseudo-terminal slave device. This
.Xr ioctl 2
should not be used directly. Instead, the
.Xr ptsname 3
function should be used.
.It Dv TIOCPTMASTER
Determine whether the file descriptor is pointing to a pseudo-terminal
master device.
This
.Xr ioctl 2
should not be used directly. It is used to implement routines like
.Xr grantpt 3 .
.El
.Pp
The maximum number of pseudo-terminals is limited to 1000.
It is not possible to use more than 1000 pseudo-terminals, as all software
which use
.Xr utmp 5
will not be able to handle pseudo-terminals with number superior to 999.
.Sh FILES
The files used by this
pseudo-terminals implementation are:
.Pp
.Bl -tag -width ".Pa /dev/pts/[num]"
.It Pa /dev/ptmx
Control device, returns a file descriptor to a new master pseudo-terminal
when opened. This device should not be opened directly. It's only
available for binary compatibility. New devices should only be created
though
.Xr posix_openpt 2 .
.It Pa /dev/pts/[num]
Pseudo-terminal slave devices.
.El
.Sh SYSCTL VARIABLES
The following
.Xr sysctl 8
variables can be used to modify or monitor
.Nm
behavior.
.Bl -tag -width indent
.It Va kern.pts_maxdev
Highest pseudo-terminal unit number to be allocated.
Because
.Xr utmp 5
is restricted to an 8-byte line name size,
.Nm
will not create any pseudo-terminals with a unit number above 999 by
default.
After increasing
.Dv UT_LINESIZE ,
this variable can be changed to allow more than 1000 pseudo-terminals to
be allocated simultaneously.
.El
.Sh DIAGNOSTICS
None.
.Sh SEE ALSO
.Xr grantpt 3 ,
.Xr posix_openpt 2 ,
.Xr ptsname 3 ,
.Xr pty 4 ,
.Xr tty 4
.Sh HISTORY
A
pseudo-terminal driver appeared in
.Bx 4.2 .
In
.Fx 8.0 ,
it was replaced with the
.Nm
driver.