PTY(4) | Device Drivers Manual | PTY(4) |
pty
—
pseudo-device pty
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
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
^S
’). Takes no parameter.TIOCSTART
TIOCSTOP
or by typing
‘^S
’). Takes no parameter.TIOCPKT
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
TIOCPKT_FLUSHWRITE
TIOCPKT_STOP
^S
’.TIOCPKT_START
TIOCPKT_DOSTOP
^S
’ and
t_startc is
‘^Q
’.TIOCPKT_NOSTOP
^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
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
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
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.pty
driver appeared in
4.2BSD.
November 30, 2013 | NetBSD 9.4 |