pty.4freebsd

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Version: 300173 (debian - 07/07/09)

Section: 4 (Pilotes et protocoles réseau)


BSD mandoc

NAME

pty - pseudo terminal driver

SYNOPSIS

device pty

DESCRIPTION

The 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.

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

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 parameter and disabled by specifying (by reference) a zero 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.

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 parameter and disabled by specifying (by reference) a zero 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.

There is currently two systems available: the original BSD and a SysVR4 pts-like implementation. It is possible to switch between the two implementations by setting the kern.pts.enable sysctl. Setting it to 0 will use the BSD to non-zero the pts implementation. It defaults to 0. It is possible to set the maximum number of ptys which can be allocated at the same time with the kern.pts.max sysctl. It defaults to 1000. It is not recommended to use more than 1000 pseudo-terminals, as all software which use utmp(5) will not be able to handle pseudo-terminals with number superior to 999.

The pts implementation also supports the TIOCGPTN ioctl(2) call, which takes a pointer to an Vt unsigned int as a parameter and provides the number of the pty.

FILES

The files used by the BSD pseudo terminals implementation are:
/dev/pty[p-sP-S][0-9a-v]
master pseudo terminals
/dev/tty[p-sP-S][0-9a-v]
slave pseudo terminals

The files used by the pts implementation are:

/dev/ptmx
control device, returns a file descriptor to a new master pseudo terminal when opened.
/dev/pty[num]
master pseudo terminals
/dev/pts/[num]
slave pseudo terminals

DIAGNOSTICS

None.

SEE ALSO

tty(4)

HISTORY

The driver appeared in BSD 4.2