pty - pseudoterminal interfaces
A pseudoterminal (sometimes abbreviated "pty") is a pair of virtual
character devices that provide a bidirectional communication channel. One end
of the channel is called the
master; the other end is called the
slave.
The slave end of the pseudoterminal provides an interface that behaves exactly
like a classical terminal. A process that expects to be connected to a
terminal, can open the slave end of a pseudoterminal and then be driven by a
program that has opened the master end. Anything that is written on the master
end is provided to the process on the slave end as though it was input typed
on a terminal. For example, writing the interrupt character (usually
control-C) to the master device would cause an interrupt signal
(
SIGINT) to be generated for the foreground process group that is
connected to the slave. Conversely, anything that is written to the slave end
of the pseudoterminal can be read by the process that is connected to the
master end.
Data flow between master and slave is handled asynchronously, much like data
flow with a physical terminal. Data written to the slave will be available at
the master promptly, but may not be available immediately. Similarly, there
may be a small processing delay between a write to the master, and the effect
being visible at the slave.
Historically, two pseudoterminal APIs have evolved: BSD and System V. SUSv1
standardized a pseudoterminal API based on the System V API, and this API
should be employed in all new programs that use pseudoterminals.
Linux provides both BSD-style and (standardized) System V-style pseudoterminals.
System V-style terminals are commonly called UNIX 98 pseudoterminals on Linux
systems.
Since Linux 2.6.4, BSD-style pseudoterminals are considered deprecated: support
can be disabled when building the kernel by disabling the
CONFIG_LEGACY_PTYS option. (Starting with Linux 2.6.30, that option is
disabled by default in the mainline kernel.) UNIX 98 pseudoterminals should be
used in new applications.
An unused UNIX 98 pseudoterminal master is opened by calling
posix_openpt(3). (This function opens the master clone device,
/dev/ptmx; see
pts(4).) After performing any program-specific
initializations, changing the ownership and permissions of the slave device
using
grantpt(3), and unlocking the slave using
unlockpt(3)),
the corresponding slave device can be opened by passing the name returned by
ptsname(3) in a call to
open(2).
The Linux kernel imposes a limit on the number of available UNIX 98
pseudoterminals. Up to and including Linux 2.6.3, this limit is configured at
kernel compilation time (
CONFIG_UNIX98_PTYS), and the permitted number
of pseudoterminals can be up to 2048, with a default setting of 256. Since
Linux 2.6.4, the limit is dynamically adjustable via
/proc/sys/kernel/pty/max, and a corresponding file,
/proc/sys/kernel/pty/nr, indicates how many pseudoterminals are
currently in use. For further details on these two files, see
proc(5).
BSD-style pseudoterminals are provided as precreated pairs, with names of the
form
/dev/ptyXY (master) and
/dev/ttyXY (slave), where X is a
letter from the 16-character set [p-za-e], and Y is a letter from the
16-character set [0-9a-f]. (The precise range of letters in these two sets
varies across UNIX implementations.) For example,
/dev/ptyp1 and
/dev/ttyp1 constitute a BSD pseudoterminal pair. A process finds an
unused pseudoterminal pair by trying to
open(2) each pseudoterminal
master until an open succeeds. The corresponding pseudoterminal slave
(substitute "tty" for "pty" in the name of the master) can
then be opened.
- /dev/ptmx
- UNIX 98 master clone device
- /dev/pts/*
- UNIX 98 slave devices
- /dev/pty[p-za-e][0-9a-f]
- BSD master devices
- /dev/tty[p-za-e][0-9a-f]
- BSD slave devices
Pseudoterminals are used by applications such as network login services
(
ssh(1),
rlogin(1),
telnet(1)), terminal emulators such
as
xterm(1),
script(1),
screen(1),
tmux(1),
unbuffer(1), and
expect(1).
A description of the
TIOCPKT ioctl(2), which controls packet mode
operation, can be found in
ioctl_tty(2).
The BSD
ioctl(2) operations
TIOCSTOP,
TIOCSTART,
TIOCUCNTL, and
TIOCREMOTE have not been implemented under Linux.
ioctl_tty(2),
select(2),
setsid(2),
forkpty(3),
openpty(3),
termios(3),
pts(4),
tty(4)