PTY(7D) Devices PTY(7D)

pty — legacy pseudo-terminal driver

/dev/pty[p-r]*

/dev/tty[p-r]*

This driver provides support for legacy static pseudo-terminal devices. Modern software does not use this driver, preferring instead the STREAMS-based ptm(7D) and pts(7D) 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 . The two devices comprising a pseudo-terminal are known as a and a . 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(7I) 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(7I) functions must be performed by STREAMS modules pushed atop the driver; when a subsidiary device is opened, the ldterm(7M) and ttcompat(7M) STREAMS modules are automatically pushed on top of the stream, providing the standard termio(7I) 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(7D).

The standard set of termio(7I) 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:

The argument is ignored. Output to the pseudo-terminal is suspended, as if a character had been typed.
The argument is ignored. Output to the pseudo-terminal is restarted, as if a character had been typed.
The argument is a pointer to an int. If the value of the int is non-zero, 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:
Whenever the read queue for the terminal is flushed.
Whenever the write queue for the terminal is flushed.
Whenever output to the terminal is stopped using ^S.
Whenever output to the terminal is restarted.
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
Whenever XON/XOFF flow control is disabled after being enabled.
The argument is a pointer to an int. If the value of the int is non-zero, 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 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 () 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.

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

rlogin(1), rlogind(1M), posix_openpty(3C), ptm(7D), termio(7I), ldterm(7M), ttcompat(7M)

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

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

February 5, 2022 illumos