NAME
pty —
pseudo terminal driver
SYNOPSIS
pseudo-device pty
DESCRIPTION
The
pty 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.
Pseudo terminal pairs are allocated on as-needed basis, maximum number of them
is controlled via
kern.maxptys sysctl (defaults to 992).
The following
ioctl(2) calls apply
only to pseudo terminals:
-
-
TIOCEXT
- Enable/disable “external processing”. This
affects delivery of
TIOCPKT_IOCTL
packets.
External processing is enabled by specifying (by reference) a nonzero
int parameter and disabled by specifying (by
reference) a zero int parameter.
TIOCEXT
is reset to its default (disabled) when the
slave closes the pty.
-
-
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
int parameter and disabled by specifying (by
reference) a zero int 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.
-
-
TIOCPKT_IOCTL
- When this bit is set, the slave has changed the
termios(4) structure
(TTY state), and the remainder of the data read from the master side
of the pty is the new
termios(4) structure.
The master side of the pty can also use
tcgetattr(3) to read
the new termios(4)
structure.
The master will not read packets with the bit
TIOCPKT_IOCTL
set until it has activated
“external processing” using
TIOCEXT
.
This is used by
telnetd(8) to implement
TELNET "line mode" - it allows the
telnetd(8) to detect
tty(4) state changes by the
slave, and negotiate the appropriate TELNET protocol equivalents with
the remote peer.
-
-
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 int parameter and disabled by
specifying (by reference) a zero int 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.
-
-
TIOCREMOTE
- A mode for the master half of a pseudo terminal,
independent of
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.
TIOCREMOTE
can be used when doing remote line
editing in a window manager, or whenever flow controlled input is
required.
FILES
- /dev/pty[p-zP-T][0-9a-zA-Z]
- master pseudo terminals
- /dev/tty[p-zP-T][0-9a-zA-Z]
- slave pseudo terminals
DIAGNOSTICS
None.
SEE ALSO
ioctl(2),
read(2),
select(2),
write(2),
openpty(3),
tty(4)
HISTORY
The
pty driver appeared in
4.2BSD.