From Linux Man Pages
pty - pseudo-terminal interfaces
DESCRIPTION
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 mas-
ter 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 pro-
cess that is connected to the master end. Pseudo-terminals are used by applications such as network login ser-
vices (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-terminals.
Linux provides both BSD-style and (standardised) 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 program-specific initialisations, changing the owner-
ship and permissions of the slave device using grantpt(3), and unlocking the slave using unlockpt(3)), the corre-
sponding 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 num-
ber 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-charac-
ter 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-ter-
minal 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.
NOTES
A description of the TIOCPKT ioctl(), which controls packet mode operation, can be found in tty_ioctl(4).
The BSD ioctl()s TIOCSTOP, TIOCSTART, TIOCUCNTL, TIOCREMOTE have not been implemented under Linux.
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)
RELATED
select(2), setsid(2), forkpty(3), openpty(3), termios(3), pts(4), tty(4), tty_ioctl(4)
CATEGORY