PTY(4) BSD Kernel Interfaces Manual PTY(4)NAMEpty — 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 pre‐
sented 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 dis‐
abled 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 speci‐
fying (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 out‐
put; it can be used by other similar pro‐
grams.
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 acti‐
vated “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 appropri‐
ate 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-termi‐
nal, 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 dis‐
abled 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 oper‐
ation 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 exis‐
tence of this facility. As with TIOCPKT mode, command opera‐
tions may be detected with a select(2) for exceptional condi‐
tions.
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 ALSOioctl(2), read(2), select(2), write(2), openpty(3), tty(4)HISTORY
The pty driver appeared in 4.2BSD.
BSD February 1, 2011 BSD
