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PTY(7) Linux Programmer's Manual PTY(7)
NAME
pty - pseudo-terminal interfaces
DESCRIPTION
A pseudo-terminal is a pair of virtual character devices that provide a bidirectional com-
munication channel. One end of the channel is called the master; the other end is called
the slave. The slave end of the pseudo-terminal 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 pseudo-terminal 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 pro-
cess 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 pseudo-terminal can
be read by the process that is connected to the master end. Pseudo-terminals are used by
applications such as network login services (ssh(1), rlogin(1), telnet(1)), terminal emu-
lators, script(1), screen(1), and expect(1).
Historically, two pseudo-terminal APIs have evolved: BSD and System V. SUSv1 standardized
a pseudo-terminal API based on the System V API, and this API should be employed in all
new programs that use pseudo-terminals.
Linux provides both BSD-style and (standardized) System V-style pseudo-terminals. System
V-style terminals are commonly called Unix 98 pseudo-terminals on Linux systems. Since
kernel 2.6.4, BSD-style pseudo-terminals are considered deprecated (they can be disabled
when configuring the kernel); Unix 98 pseudo-terminals should be used in new applications.
Unix 98 pseudo-terminals
An unused Unix 98 pseudo-terminal master is opened by calling posix_openpt(3). (This
function opens the master clone device, /dev/ptmx; see pts(4).) After performing any pro-
gram-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 pseudo-terminals. 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 pseudo-terminals 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 pseudo-terminals are currently in use. For further details on these two files, see
proc(5).
BSD pseudo-terminals
BSD-style pseudo-terminals are provided as pre-created 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 pseudo-terminal pair. A process finds an unused pseudo-
terminal pair by trying to open(2) each pseudo-terminal master until an open succeeds.
The corresponding pseudo-terminal slave (substitute "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
A description of the TIOCPKT ioctl(2), which controls packet mode operation, can be found
in tty_ioctl(4).
The BSD ioctl(2) operations TIOCSTOP, TIOCSTART, TIOCUCNTL, and TIOCREMOTE have not been
implemented under Linux.
SEE ALSO
select(2), setsid(2), forkpty(3), openpty(3), termios(3), pts(4), tty(4), tty_ioctl(4)
COLOPHON
This page is part of release 3.05 of the Linux man-pages project. A description of the
project, and information about reporting bugs, can be found at http://www.ker-
nel.org/doc/man-pages/.
Linux 2005-10-10 PTY(7)
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