PTY(7) Linux Programmer's Manual PTY(7)
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 pseudotermi-
nal 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 exam-
ple, 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. Pseudotermi-
nals 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).
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-
Linux provides both BSD-style and (standardized) System V-style pseu-
doterminals. System V-style terminals are commonly called UNIX 98
pseudoterminals on Linux systems. Since kernel 2.6.4, BSD-style pseu-
doterminals are considered deprecated (they can be disabled when con-
figuring the kernel); UNIX 98 pseudoterminals should be used in new
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
using grantpt(3), and unlocking the slave using unlockpt(3)), the cor-
responding 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. In kernels up to and including 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 kernel 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 cur-
rently 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
UNIX 98 master clone device
UNIX 98 slave devices
BSD master devices
BSD slave devices
A description of the TIOCPKT ioctl(2), which controls packet mode oper-
ation, can be found in ioctl_tty(2).
The BSD ioctl(2) operations TIOCSTOP, TIOCSTART, TIOCUCNTL, and TIOCRE-
MOTE have not been implemented under Linux.
ioctl_tty(2), select(2), setsid(2), forkpty(3), openpty(3), termios(3),
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