PTY(4)

NAME
pty - pseudo terminal driver

SYNOPSIS
pseudo-device pty [count]

DESCRIPTION
The 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.

In configuring, if an optional count is given in the specification, that number of pseudo terminal pairs is initially configured; the default count is 16. Additional 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
    Enable/disable “external processing”. This affects delivery of 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
    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.

TIOCPKT
    Enable/disable packet mode. Packet mode is enabled by specifying (by reference) a nonzero int parameter and disabled by specifying (by reference) a zero int parameter. When applied to the master side of a pseudo terminal, each subsequent read(2) 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’.

        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
        When this bit is set, the slave has changed the termios(4) structure (TTY state), and the remainder of the data read from the master side of the 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
    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 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
    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-zP-T][0-9a-zA-Z]
    master pseudo terminals
/dev/tty[p-zP-T][0-9a-zA-Z]
    slave pseudo terminals

DIAGNOSTICS
None.

SEE ALSO
ioctl(2), read(2), select(2), write(2), openpty(3), tty(4)

HISTORY
The pty driver appeared in 4.2BSD.