tty.4

操作系统设计与实现源码

.TH TTY 4
.SH NAME
tty, termios \- terminals
.SH DESCRIPTION
The
.B tty
driver family takes care of all user input and output.  It governs the
keyboard, the console, the serial lines, and pseudo ttys.  Input on any of
these devices undergoes "input processing", and output undergoes "output
processing" according to the standard termios terminal interface.
.SS "Input processing"
Each terminal device has an input queue.  This queue is used to store
preprocessed input characters, and to perform the backspacing and erase
functions.  Some special characters like a newline make the contents of the
queue available to a process reading from the terminal.  Characters up to
and including the newline, or another so-called "line break", may be read by
a process.  The process need not read all characters at once.  An input line
may be read byte by byte if one wants to.  A line break just makes
characters available for reading, thats all.
.PP
When data is made available depends on whether the tty is in canonical mode
or not.  In canonical mode the terminal processes input line by line.  A
line ends with a newline
.RB ( NL ),
end-of-file
.RB ( EOF ),
or end-of-line
.RB ( EOL ).
Characters that have not been delimited by such a line break may be erased
one by one with the
.B ERASE
character or all at once with the
.B KILL
character.  Once a line break is typed the characters become available to a
reading process and can no longer be erased.  Once read they are removed
from the input queue.  Several lines may be gathered in the input queue if
no reader is present to read them, but a new reader will only receive one
line.  Two line breaks are never returned in one read call.  The input queue
has a maximum length of
.B MAX_CANON
characters.  Any more characters are discarded.  One must use
.B ERASE
or
.B KILL
to make the terminal functioning again if the input queue fills up.  If
nonblocking I/O is set then \-1 is returned with
.B errno
set to
.B EAGAIN
if the reader would otherwise be blocked.
.PP
In non-canonical mode (raw mode for short) all characters are immediately
available to the reader in principle.  One may however tune the terminal to
bursty input with the
.B MIN
and
.B TIME
parameters, see the raw I/O parameters section below.  In raw mode no
characters are discarded if the input queue threatens to overflow if the
device supports flow control.
.SS "Output processing"
Characters written to a terminal device may undergo output processing, which
is usually just inserting a carriage returns before newlines.  A writer
may return before all characters are output if the characters can be stored
in the output buffers.  If not then the writer may be blocked until space is
available.  If non-blocking I/O is set then only the count of the number of
bytes that can be processed immediately is returned.  If no characters can
be written at all then \-1 is returned with
.B errno
set to
.BR EAGAIN .
.SS "Special characters"
Some characters have special functions in some of the terminal modes.  These
characters are as follows, with the Minix defaults shown in parentheses:
.TP 5
.BR INTR " (^?)"
Special input character that is recognized if
.B ISIG
is set.  (For
.B ISIG
and other flags see the various modes sections below.)  It causes a
.B SIGINT
signal to be sent to all processes in the terminal process group.  (See the
section on session leaders below.)
.TP
.BR QUIT " (^\e)"
Special input character if
.B ISIG
is set.  Causes a
.B SIGQUIT
signal to be sent to the terminal process group.
.TP
.BR ERASE " (^H)"
Special input character if
.B ICANON
is set.  Erases the last character in the current line.
.TP
.BR KILL " (^U)"
Special input character if
.B ICANON
is set.  Erases the entire line.
.TP
.BR EOF " (^D)"
Special input character if
.B ICANON
is set.  It is a line break character that is not itself returned to a
reader.  If EOF is typed with no input present then the read returns zero,
which normally causes the reader to assume that end-of-file is reached.
.TP
.BR CR " (^M)"
Special input character if
.B IGNCR
or
.B ICRNL
is set.  It is a carriage return ('\er').  If
.B IGNCR
is set then
.B CR
is discarded.  If
.B ICRNL
is set and
.B IGNCR
is not set then
.B CR
is changed into an
.B NL
and has the same function as
.BR NL.
.TP
.BR NL " (^J)"
Special input character if
.B ICANON
is set.  It is both a newline ('\en') and a line break.
.br
Special output character if
.B OPOST
and
.B ONLCR
are set.  A
.B CR NL
sequence is output instead of just
.BR NL .
(Minix specific, but almost mandatory on any UNIX-like system.)
.TP
.BR TAB " (^I)"
Special character on output if
.B OPOST
and
.B XTABS
are set.  It is transformed into the number of spaces necessary to reach a
column position that is a multiple of eight.  (Only needed for terminals
without hardware tabs.)
.TP
.BR EOL " (undefined)"
Special input character if
.B ICANON
is set.  It is an additional line break.
.TP
.BR SUSP " (^Z)"
Special input character if job control is implemented and
.B ISIG
is set.  It causes a
.B SIGTSTP
signal to be send to the terminal process group.  (Minix does not have job
control.)
.TP
.BR STOP " (^S)"
Special input character if
.B IXON
is set.  It suspends terminal output and is then discarded.
.TP
.BR START " (^Q)"
Special output character if
.B IXON
is set.  It starts terminal output if suspended and is then discarded.  If
.B IXANY
is also set then any other character also starts terminal output, but they
are not discarded.
.TP
.BR REPRINT " (^R)"
Special input character if
.B IEXTEN
and
.B ECHO
are set.  Reprints the input queue from the last line break onwards.  A
reprint also happens automatically if the echoed input has been messed up by
other output and
.B ERASE
is typed.
.TP
.BR LNEXT " (^V)"
Special input character if
.B IEXTEN
is set.  It is the "literal next" character that causes the next character
to be input without any special processing.
.TP
.BR DISCARD " (^O)"
Special input character if
.B IEXTEN
is set.  Causes output to be discarded until it is typed again.  (Implemented
only under Minix-vmd.)
.PP
All of these characters except
.BR CR ,
.B NL
and
.B TAB
may be changed or disabled under Minix.  (Changes to
.B START
and
.B STOP
may be ignored under other termios implementations.)  The
.B REPRINT
and
.B LNEXT
characters are Minix extensions that are commonly present in other
implementations.  \s-2POSIX\s+2 is unclear on whether
.BR IEXTEN,
.BR IGNCR
and
.BR ICRNL
should be active in non-canonical mode, but under Minix they are.
.SS "Terminal attributes"
The attributes of a terminal, such as whether the mode should be canonical or
non-canonical, are controlled by routines that use the
.B termios
structure as defined in
.BR <termios.h> :
.PP
.RS
.nf
.ta +4n +10n +15n
struct termios {
	tcflag_t	c_iflag;	/* input modes */
	tcflag_t	c_oflag;	/* output modes */
	tcflag_t	c_cflag;	/* control modes */
	tcflag_t	c_lflag;	/* local modes */
	speed_t	c_ispeed;	/* input speed */
	speed_t	c_ospeed;	/* output speed */
	cc_t	c_cc[NCCS];	/* control characters */
};
.fi
.RE
.PP
The types
.BR tcflag ,
.B speed_t
and
.B cc_t
are defined in
.B <termios.h>
as unsigned integral types.
.SS "Input Modes"
The
.B c_iflag
field contains the following single bit flags that control input processing:
.TP 5
.B ICRNL
Map
.B CR
to
.B NL
on input.
.TP
.B IGNCR
Ignore
.B CR
on input.  This flag overrides
.BR ICRNL .
.TP
.B INLCR
Map
.B NL
to
.B CR
on input.  This is done after the
.B IGNCR
check.
.TP
.B IXON
Enable start/stop output control.
.TP
.B IXOFF
Enable start/stop input control.  (Not implemented.)
.TP
.B IXANY
Allow any character to restart output.  (Minix specific.)
.TP
.B ISTRIP
Strip characters to seven bits.
.TP
.B IGNPAR
Ignore characters with parity errors.  (Not implemented.)
.TP
.B INPCK
Enable input parity checking.  (Not implemented.)
.TP
.B PARMRK
Mark parity errors by preceding the faulty character with '\e377', '\e0'.
The character '\e377' is preceded by another '\e377' to avoid ambiguity.
(Not implemented.)
.TP
.B BRKINT
Send the signal
.B SIGINT
to the terminal process group when receiving a break condition.  (Not
implemented.)
.TP
.B IGNBRK
Ignore break condition.  If neither
.B BRKINT
or
.B IGNBRK
is set a break is input as a single '\e0', or if
.B PARMRK
is set as '\e377', '\e0', '\e0'.
(Breaks are always ignored.)
.SS "Output Modes"
The
.B c_oflag
field contains the following single bit flags that control output processing:
.TP
.B OPOST
Perform output processing.  This flag is the "main switch" on output
processing.  All other flags are Minix specific.
.TP
.B ONLCR
Transform an
.B NL
to a
.B CR NL
sequence on output.  Note that a key labeled "RETURN" or "ENTER" usually
sends a
.BR CR .
In line oriented mode this is normally transformed into
.B NL
by
.BR ICRNL .
.B NL
is the normal UNIX line delimiter ('\en').  On output an
.B NL
is transformed into the
.B CR NL
sequence that is necessary to reach the first column of the next line.
(This is a common output processing function for UNIX-like systems, but not
always separately switchable by an
.B ONLCR
flag.)
.TP
.B XTABS
Transform a
.B TAB
into the number of spaces necessary to reach a column position that is a
multiple of eight.
.TP
.B ONOEOT
Discard
.B EOT
(^D) characters.  (Minix-vmd only.)
.SS "Control Modes"
The
.B c_cflag
field contains the following single bit flags and bit field for basic
hardware control:
.TP
.B CLOCAL
Ignore modem status lines.
.TP
.B CREAD