pty(7) Miscellaneous Information Manual pty(7)
NAME
pty - pseudoterminal interfaces
DESCRIPTION
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 con-
nected 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 fore-
ground process group that is connected to the slave. Conversely, any-
thing 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 pseudotermi-
nals.
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 CON-
FIG_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.
UNIX 98 pseudoterminals
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 initial-
izations, changing the ownership and permissions of the slave device us-
ing grantpt(3), and unlocking the slave using unlockpt(3)), the corre-
sponding slave device can be opened by passing the name returned by pt-
sname(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 config-
ured 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/ker-
nel/pty/nr, indicates how many pseudoterminals are currently in use.
For further details on these two files, see proc(5).
BSD pseudoterminals
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 un-
used pseudoterminal pair by trying to open(2) each pseudoterminal master
until an open succeeds. The corresponding pseudoterminal slave (substi-
tute "tty" for "pty" in the name of the master) can then be opened.
FILES
/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
NOTES
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 opera-
tion, can be found in ioctl_tty(2).
The BSD ioctl(2) operations TIOCSTOP, TIOCSTART, TIOCUCNTL, and TIOCRE-
MOTE have not been implemented under Linux.
SEE ALSO
ioctl_tty(2), select(2), setsid(2), forkpty(3), openpty(3), termios(3),
pts(4), tty(4)
Linux man-pages 6.9.1 2024-05-02 pty(7)
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