sched.7: Minor wording improvement in text introducing system calls

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.TH PTY 7 2015-07-23 "Linux" "Linux Programmer's Manual"
.SH NAME
pty \- pseudoterminal interfaces
.SH 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
.IR master ;
the other end is called the
.IR 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 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
.RB ( 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.
Pseudoterminals are used by applications such as network login services
.RB ( ssh "(1), " rlogin "(1), " telnet (1)),
terminal emulators,
.BR script (1),
.BR screen (1),
and
.BR 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
pseudoterminals.

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 kernel 2.6.4, BSD-style pseudoterminals are considered deprecated
(they can be disabled when configuring the kernel);
UNIX 98 pseudoterminals should be used in new applications.
.SS UNIX 98 pseudoterminals
An unused UNIX 98 pseudoterminal master is opened by calling
.BR posix_openpt (3).
(This function opens the master clone device,
.IR /dev/ptmx ;
see
.BR pts (4).)
After performing any program-specific initializations,
changing the ownership and permissions of the slave device using
.BR grantpt (3),
and unlocking the slave using
.BR unlockpt (3)),
the corresponding slave device can be opened by passing
the name returned by
.BR ptsname (3)
in a call to
.BR 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
.RB ( 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
.IR /proc/sys/kernel/pty/max ,
and a corresponding file,
.IR /proc/sys/kernel/pty/nr ,
indicates how many pseudoterminals are currently in use.
For further details on these two files, see
.BR proc (5).
.SS BSD pseudoterminals
BSD-style pseudoterminals are provided as precreated pairs, with
names of the form
.I /dev/ptyXY
(master) and
.I /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,
.I /dev/ptyp1
and
.I /dev/ttyp1
constitute a BSD pseudoterminal pair.
A process finds an unused pseudoterminal pair by trying to
.BR 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 opened.
.SH FILES
.I /dev/ptmx
(UNIX 98 master clone device)
.br
.I /dev/pts/*
(UNIX 98 slave devices)
.br
.I /dev/pty[p-za-e][0-9a-f]
(BSD master devices)
.br
.I /dev/tty[p-za-e][0-9a-f]
(BSD slave devices)
.SH NOTES
A description of the
.B TIOCPKT
.BR ioctl (2),
which controls packet mode operation, can be found in
.BR tty_ioctl (4).

The BSD
.BR ioctl (2)
operations
.BR TIOCSTOP ,
.BR TIOCSTART ,
.BR TIOCUCNTL ,
and
.BR TIOCREMOTE
have not been implemented under Linux.
.SH SEE ALSO
.BR select (2),
.BR setsid (2),
.BR forkpty (3),
.BR openpty (3),
.BR termios (3),
.BR pts (4),
.BR tty (4),
.BR tty_ioctl (4)