PTY(4) — System Manager’s Manual — Special Files

NAME

pty − pseudo terminal driver

SYNOPSIS

pseudo-device pty

DESCRIPTION

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 master and a slave. The slave device provides an interface identical to that described in tty(4), but instead of having a hardware interface such as the Zilog chip and associated hardware used by zs(4S) supporting the terminal functions, the functions of the terminal are implemented by another process manipulating the master side of the pseudo-terminal.

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

In configuring, if no optional “count” is given in the specification, 16 pseudo terminal pairs are configured.

A few special ioctl’s are provided on the control-side devices of pseudo-terminals to provide the functionality needed by applications programs to emulate real hardware interfaces:

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.

There are also two independent modes which can be used by applications programs:

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 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.

This mode is used by rlogin(1C) and rlogind(8C) to implement a remote-echoed, locally ^S/^Q flow-controlled remote login with proper back-flushing of output when interrupts occur; it can be used by other similar programs.

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-r][0-9a-f]master pseudo terminals
/dev/tty[p-r][0-9a-f]slave pseudo terminals

DIAGNOSTICS

None.