PTY(4) Device Drivers Manual 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 primary
device and a replica 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:
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 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:
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 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.
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 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.
FILES
/dev/pty[p-sP-S][a-z0-9] primary pseudo terminals
/dev/tty[p-sP-S][a-z0-9] replica pseudo terminals
DIAGNOSTICS
None.
HISTORY
The pty driver appeared in 4.2BSD.
BSD 4.2 November 30, 1993 BSD 4.2