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PTY(4) Device Drivers Manual PTY(4)

pty — pseudo terminal driver

pseudo-device pty [count]

The pty driver provides support for a device-pair termed a . A pseudo terminal is a pair of character devices, a device and a device. The replica 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 replica device has, instead, another process manipulating it through the primary half of the pseudo terminal. That is, anything written on the primary device is given to the replica device as input and anything written on the replica device is presented as input on the primary device.

In configuring, if an optional count is given in the specification, that number of pseudo terminal pairs are configured; the default count is 32.

The following ioctl(2) calls apply only to pseudo terminals:

Stops output to a terminal (e.g. like typing ‘^S’). Takes no parameter.
Restarts output (stopped by TIOCSTOP or by typing ‘^S’). Takes no parameter.
Enable/disable 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 primary side of a pseudo terminal, each subsequent read(2) from the terminal will return data written on the replica 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:
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 a la ‘^S’.
whenever output to the terminal is restarted.
whenever is ‘^S’ and is ‘^Q’.
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 primary 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.

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 primary side of a pseudo terminal by specifying (by reference) a nonzero parameter and disabled by specifying (by reference) a zero parameter. Each subsequent read(2) from the primary side will return data written on the replica part of the pseudo terminal preceded by a zero byte, or a single byte reflecting a user control operation on the replica 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 primary 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.

/dev/pty[p-sP-S][a-z0-9]
primary pseudo terminals
/dev/tty[p-sP-S][a-z0-9]
replica pseudo terminals

None.

The pty driver appeared in 4.2BSD.

November 30, 1993 BSD 4.2