The
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 to a process 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.
The following
ioctl(2)
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(2)
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(2)
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(2)
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(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
ioctl(2)
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(2)
from the master side.
The
ioctl(2)
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(2)
for exceptional conditions.
There is currently two
systems available: the original
BSD
and a
SysVR4 pts-like implementation.
It is possible to switch between the two implementations by setting the
kern.pts.enable
sysctl.
Setting it to 0 will use the
BSD
to non-zero the pts implementation.
It defaults to 0.
It is possible to set the maximum number of ptys
which can be allocated at the same time with the
kern.pts.max
sysctl.
It defaults to 1000.
It is not recommended to use more than 1000 pseudo-terminals, as all software
which use
utmp(5)
will not be able to handle pseudo-terminals with number superior to 999.
The pts implementation also supports the
TIOCGPTNioctl(2)
call, which takes a pointer to an
Vt unsigned int
as a parameter and provides the
number of the pty.
FILES
The files used by the
BSD pseudo terminals implementation are:
/dev/pty[p-sP-S][0-9a-v]
master pseudo terminals
/dev/tty[p-sP-S][0-9a-v]
slave pseudo terminals
The files used by the pts implementation are:
/dev/ptmx
control device, returns a file descriptor to a new master pseudo terminal
when opened.
