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pty - pseudo-terminal interfaces
A pseudo-terminal 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
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 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 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 emulators, 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-
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
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 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
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-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.
/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)
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.
select(2), setsid(2), forkpty(3), openpty(3), termios(3), pts(4),
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description of the project, and information about reporting bugs, can
be found at http://www.kernel.org/doc/man-pages/.