PTY(4D) 4D PTY(4D)

NAME


pty - legacy pseudo-terminal driver

SYNOPSIS


/dev/pty[p-r]*

/dev/tty[p-r]*

DESCRIPTION


This driver provides support for legacy static pseudo-terminal devices.
Modern software does not use this driver, preferring instead the STREAMS-
based ptm(4D) and pts(4D) pseudo-terminal drivers, consumed through the
portable posix_openpt(3C) interface.

The pty driver provides support for a pair of devices collectively known as
a pseudo-terminal. The two devices comprising a pseudo-terminal are known
as a manager and a subsidiary. The subsidiary device distinguishes between
the B0 baud rate and other baud rates specified in the c_cflag field of the
termios structure, and the CLOCAL flag in that member. It does not support
any of the other termio(4I) device control functions specified by flags in
the c_cflag field of the termios structure and by the IGNBRK, IGNPAR,
PARMRK, or INPCK flags in the c_iflag field of the termios structure, as
these functions apply only to asynchronous serial ports. All other
termio(4I) functions must be performed by STREAMS modules pushed atop the
driver; when a subsidiary device is opened, the ldterm(4M) and ttcompat(4M)
STREAMS modules are automatically pushed on top of the stream, providing
the standard termio(4I) interface.

Instead of having a hardware interface and associated hardware that
supports the terminal functions, the functions are implemented by another
process manipulating the manager device of the pseudo-terminal.

The manager and the subsidiary devices of the pseudo-terminal are tightly
connected. Any data written on the manager device is given to the
subsidiary device as input, as though it had been received from a hardware
interface. Any data written on the subsidiary terminal can be read from
the manager device (rather than being transmitted from a UAR).

The driver is statically configured to provide 48 pseudo-terminal pairs.
Software that requires dynamic pseudo-terminal devices, or a greater number
of devices, must be converted to use ptm(4D).

IOCTLS


The standard set of termio(4I) ioctls are supported by the subsidiary
device. None of the bits in the c_cflag field have any effect on the
pseudo-terminal, except that if the baud rate is set to B0, it will appear
to the process on the manager device as if the last process on the
subsidiary device had closed the line; thus, setting the baud rate to B0
has the effect of "hanging up" the pseudo-terminal, just as it has the
effect of "hanging up" a real terminal.

There is no notion of "parity" on a pseudo-terminal, so none of the flags
in the c_iflag field that control the processing of parity errors have any
effect. Similarly, there is no notion of a break, so none of the flags
that control the processing of breaks, and none of the ioctls that generate
breaks, have any effect.

Input flow control is automatically performed; a process that attempts to
write to the manager device will be blocked if too much unconsumed data is
buffered on the subsidiary device. The input flow control provided by the
IXOFF flag in the c_iflag field is not supported.

The delays specified in the c_oflag field are not supported.

As there are no modems involved in a pseudo-terminal, the ioctls that
return or alter the state of modem control lines are silently ignored.

A few special ioctls are provided on the manager devices of pseudo-
terminals to provide the functionality needed by applications programs to
emulate real hardware interfaces:

TIOCSTOP
The argument is ignored. Output to the pseudo-terminal is
suspended, as if a STOP character had been typed.

TIOCSTART
The argument is ignored. Output to the pseudo-terminal is
restarted, as if a START character had been typed.

TIOCPKT
The argument is a pointer to an int. If the value of the int is
non-zero, packet mode is enabled; if the value of the int is zero,
packet mode is disabled. When a pseudo-terminal is in packet mode,
each subsequent read(2) from the manager device will return data
written on the subsidiary device 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 using ^S.

TIOCPKT_START
Whenever output to the terminal is restarted.

TIOCPKT_DOSTOP
Whenever XON/XOFF flow control is enabled after being
disabled; it is considered "enabled" when the IXON flag in
the c_iflag field is set, the VSTOP member of the c_cc
array is ^S and the VSTART member of the c_cc array is ^Q.

TIOCPKT_NOSTOP
Whenever XON/XOFF flow control is disabled after being
enabled.

TIOCREMOTE
The argument is a pointer to an int. If the value of the int is
non-zero, remote mode is enabled; if the value of the int is zero,
remote mode is disabled. This mode can be enabled or disabled
independently of packet mode. When a pseudo-terminal is in remote
mode, input to the subsidiary device of the pseudo-terminal is flow
controlled and not input edited (regardless of the mode the
subsidiary side of the pseudo-terminal).

Each write to the manager device produces a record boundary for the
process reading the subsidiary device. 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 EOF character. Note: this means that a
process writing to a pseudo-terminal manager in remote mode must
keep track of line boundaries, and write only one line at a time to
the manager.

If, for example, it were to buffer up several newline characters
and write them to the manager with one write(2), it would appear to
a process reading from the subsidiary as if a single line
containing several newline characters had been typed (as if, for
example, a user had typed the literal next (LNEXT) character before
typing all but the last of those newline characters). Remote mode
can be used when doing remote line editing in a window manager, or
whenever flow controlled input is required.

FILES


/dev/pty[p-r][0-9a-f] Pseudo-terminal manager devices.

/dev/tty[p-r][0-9a-f] Pseudo-terminal subsidiary devices.

SEE ALSO


rlogin(1), posix_openpty(3C), ptm(4D), termio(4I), ldterm(4M),
ttcompat(4M), rlogind(8)

NOTES


This is a legacy device and should not be used by new software.

It is apparently not possible to send an EOT by writing zero bytes in
TIOCREMOTE mode.

OmniOS February 5, 2022 OmniOS