PTY(4) BSD Kernel Interfaces Manual PTY(4)NAMEpty — pseudo terminal driver
pseudo-device pty [count]
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 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
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 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
TIOCPKT_FLUSHWRITE whenever the write queue for the terminal
TIOCPKT_STOP whenever output to the terminal is
stopped a la ‘^S’.
TIOCPKT_START whenever output to the terminal is
TIOCPKT_DOSTOP whenever t_stopc is ‘^S’ and t_startc is
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‐
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 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.
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.
/dev/pty[p-sP-S][a-z0-9] master pseudo terminals
/dev/tty[p-sP-S][a-z0-9] slave pseudo terminals
The pty driver appeared in 4.2BSD.
4.2 Berkeley Distribution November 30, 1993 4.2 Berkeley Distribution