748 lines
19 KiB
Groff
748 lines
19 KiB
Groff
.TH TTY 4
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.SH NAME
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tty, termios \- terminals
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.SH DESCRIPTION
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The
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.B tty
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driver family takes care of all user input and output. It governs the
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keyboard, the console, the serial lines, and pseudo ttys. Input on any of
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these devices undergoes "input processing", and output undergoes "output
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processing" according to the standard termios terminal interface.
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.SS "Input processing"
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Each terminal device has an input queue. This queue is used to store
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preprocessed input characters, and to perform the backspacing and erase
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functions. Some special characters like a newline make the contents of the
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queue available to a process reading from the terminal. Characters up to
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and including the newline, or another so-called "line break", may be read by
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a process. The process need not read all characters at once. An input line
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may be read byte by byte if one wants to. A line break just makes
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characters available for reading, thats all.
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.PP
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When data is made available depends on whether the tty is in canonical mode
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or not. In canonical mode the terminal processes input line by line. A
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line ends with a newline
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.RB ( NL ),
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end-of-file
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.RB ( EOF ),
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or end-of-line
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.RB ( EOL ).
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Characters that have not been delimited by such a line break may be erased
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one by one with the
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.B ERASE
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character or all at once with the
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.B KILL
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character. Once a line break is typed the characters become available to a
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reading process and can no longer be erased. Once read they are removed
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from the input queue. Several lines may be gathered in the input queue if
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no reader is present to read them, but a new reader will only receive one
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line. Two line breaks are never returned in one read call. The input queue
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has a maximum length of
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.B MAX_CANON
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characters. Any more characters are discarded. One must use
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.B ERASE
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or
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.B KILL
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to make the terminal functioning again if the input queue fills up. If
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nonblocking I/O is set then \-1 is returned with
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.B errno
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set to
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.B EAGAIN
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if the reader would otherwise be blocked.
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.PP
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In non-canonical mode (raw mode for short) all characters are immediately
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available to the reader in principle. One may however tune the terminal to
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bursty input with the
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.B MIN
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and
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.B TIME
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parameters, see the raw I/O parameters section below. In raw mode no
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characters are discarded if the input queue threatens to overflow if the
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device supports flow control.
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.SS "Output processing"
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Characters written to a terminal device may undergo output processing, which
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is usually just inserting a carriage returns before newlines. A writer
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may return before all characters are output if the characters can be stored
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in the output buffers. If not then the writer may be blocked until space is
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available. If non-blocking I/O is set then only the count of the number of
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bytes that can be processed immediately is returned. If no characters can
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be written at all then \-1 is returned with
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.B errno
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set to
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.BR EAGAIN .
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.SS "Special characters"
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Some characters have special functions in some of the terminal modes. These
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characters are as follows, with the MINIX 3 defaults shown in parentheses:
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.TP 5
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.BR INTR " (^?)"
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Special input character that is recognized if
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.B ISIG
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is set. (For
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.B ISIG
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and other flags see the various modes sections below.) It causes a
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.B SIGINT
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signal to be sent to all processes in the terminal process group. (See the
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section on session leaders below.)
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.TP
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.BR QUIT " (^\e)"
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Special input character if
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.B ISIG
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is set. Causes a
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.B SIGQUIT
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signal to be sent to the terminal process group.
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.TP
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.BR ERASE " (^H)"
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Special input character if
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.B ICANON
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is set. Erases the last character in the current line.
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.TP
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.BR KILL " (^U)"
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Special input character if
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.B ICANON
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is set. Erases the entire line.
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.TP
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.BR EOF " (^D)"
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Special input character if
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.B ICANON
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is set. It is a line break character that is not itself returned to a
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reader. If EOF is typed with no input present then the read returns zero,
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which normally causes the reader to assume that end-of-file is reached.
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.TP
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.BR CR " (^M)"
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Special input character if
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.B IGNCR
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or
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.B ICRNL
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is set. It is a carriage return ('\er'). If
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.B IGNCR
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is set then
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.B CR
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is discarded. If
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.B ICRNL
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is set and
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.B IGNCR
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is not set then
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.B CR
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is changed into an
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.B NL
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and has the same function as
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.BR NL.
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.TP
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.BR NL " (^J)"
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Special input character if
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.B ICANON
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is set. It is both a newline ('\en') and a line break.
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.br
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Special output character if
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.B OPOST
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and
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.B ONLCR
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are set. A
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.B CR NL
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sequence is output instead of just
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.BR NL .
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(MINIX 3 specific, but almost mandatory on any UNIX-like system.)
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.TP
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.BR TAB " (^I)"
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Special character on output if
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.B OPOST
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and
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.B XTABS
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are set. It is transformed into the number of spaces necessary to reach a
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column position that is a multiple of eight. (Only needed for terminals
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without hardware tabs.)
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.TP
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.BR EOL " (undefined)"
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Special input character if
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.B ICANON
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is set. It is an additional line break.
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.TP
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.BR SUSP " (^Z)"
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Special input character if job control is implemented and
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.B ISIG
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is set. It causes a
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.B SIGTSTP
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signal to be send to the terminal process group. (MINIX 3 does not have job
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control.)
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.TP
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.BR STOP " (^S)"
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Special input character if
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.B IXON
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is set. It suspends terminal output and is then discarded.
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.TP
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.BR START " (^Q)"
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Special output character if
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.B IXON
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is set. It starts terminal output if suspended and is then discarded. If
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.B IXANY
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is also set then any other character also starts terminal output, but they
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are not discarded.
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.TP
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.BR REPRINT " (^R)"
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Special input character if
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.B IEXTEN
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and
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.B ECHO
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are set. Reprints the input queue from the last line break onwards. A
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reprint also happens automatically if the echoed input has been messed up by
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other output and
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.B ERASE
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is typed.
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.TP
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.BR LNEXT " (^V)"
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Special input character if
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.B IEXTEN
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is set. It is the "literal next" character that causes the next character
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to be input without any special processing.
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.TP
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.BR DISCARD " (^O)"
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Special input character if
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.B IEXTEN
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is set. Causes output to be discarded until it is typed again. (Implemented
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only under Minix-vmd.)
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.PP
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All of these characters except
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.BR CR ,
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.B NL
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and
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.B TAB
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may be changed or disabled under MINIX 3. (Changes to
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.B START
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and
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.B STOP
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may be ignored under other termios implementations.) The
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.B REPRINT
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and
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.B LNEXT
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characters are MINIX 3 extensions that are commonly present in other
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implementations. \s-2POSIX\s+2 is unclear on whether
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.BR IEXTEN,
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.BR IGNCR
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and
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.BR ICRNL
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should be active in non-canonical mode, but under MINIX 3 they are.
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.SS "Terminal attributes"
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The attributes of a terminal, such as whether the mode should be canonical or
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non-canonical, are controlled by routines that use the
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.B termios
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structure as defined in
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.BR <termios.h> :
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.PP
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.RS
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.nf
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.ta +4n +10n +15n
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struct termios {
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tcflag_t c_iflag; /* input modes */
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tcflag_t c_oflag; /* output modes */
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tcflag_t c_cflag; /* control modes */
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tcflag_t c_lflag; /* local modes */
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speed_t c_ispeed; /* input speed */
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speed_t c_ospeed; /* output speed */
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cc_t c_cc[NCCS]; /* control characters */
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};
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.fi
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.RE
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.PP
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The types
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.BR tcflag ,
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.B speed_t
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and
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.B cc_t
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are defined in
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.B <termios.h>
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as unsigned integral types.
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.SS "Input Modes"
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The
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.B c_iflag
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field contains the following single bit flags that control input processing:
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.TP 5
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.B ICRNL
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Map
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.B CR
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to
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.B NL
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on input.
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.TP
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.B IGNCR
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Ignore
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.B CR
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on input. This flag overrides
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.BR ICRNL .
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.TP
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.B INLCR
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Map
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.B NL
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to
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.B CR
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on input. This is done after the
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.B IGNCR
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check.
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.TP
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.B IXON
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Enable start/stop output control.
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.TP
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.B IXOFF
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Enable start/stop input control. (Not implemented.)
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.TP
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.B IXANY
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Allow any character to restart output. (MINIX 3 specific.)
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.TP
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.B ISTRIP
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Strip characters to seven bits.
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.TP
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.B IGNPAR
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Ignore characters with parity errors. (Not implemented.)
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.TP
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.B INPCK
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Enable input parity checking. (Not implemented.)
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.TP
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.B PARMRK
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Mark parity errors by preceding the faulty character with '\e377', '\e0'.
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The character '\e377' is preceded by another '\e377' to avoid ambiguity.
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(Not implemented.)
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.TP
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.B BRKINT
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Send the signal
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.B SIGINT
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to the terminal process group when receiving a break condition. (Not
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implemented.)
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.TP
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.B IGNBRK
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Ignore break condition. If neither
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.B BRKINT
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or
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.B IGNBRK
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is set a break is input as a single '\e0', or if
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.B PARMRK
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is set as '\e377', '\e0', '\e0'.
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(Breaks are always ignored.)
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.TP
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.B SCANCODES
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Send input as keyboard scancodes rather than processed ASCII characters. This
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flag only applies to consoles; to check whether the flag is supported use the
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.B tcgetattr
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function after setting it and test whether
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.B c_iflag
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still contains the
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.B SCANCODES
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flag. Scancodes are provided directly and without any processing. As a
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consequence, the
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.B ISTRIP
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,
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.B IGNCR
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,
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.B ICRNL
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,
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.B INLCR
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,
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.B ICANON
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and
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.B IEXTEN
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flags no longer influence input if this flag is specified. However, they may
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still influence echoing if enabled. (MINIX 3 specific.)
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.SS "Output Modes"
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The
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.B c_oflag
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field contains the following single bit flags that control output processing:
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.TP
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.B OPOST
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Perform output processing. This flag is the "main switch" on output
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processing. All other flags are MINIX 3 specific.
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.TP
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.B ONLCR
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Transform an
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.B NL
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to a
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.B CR NL
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sequence on output. Note that a key labeled "RETURN" or "ENTER" usually
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sends a
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.BR CR .
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In line oriented mode this is normally transformed into
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.B NL
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by
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.BR ICRNL .
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.B NL
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is the normal UNIX line delimiter ('\en'). On output an
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.B NL
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is transformed into the
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.B CR NL
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sequence that is necessary to reach the first column of the next line.
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(This is a common output processing function for UNIX-like systems, but not
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always separately switchable by an
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.B ONLCR
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flag.)
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.TP
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.B XTABS
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Transform a
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.B TAB
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into the number of spaces necessary to reach a column position that is a
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multiple of eight.
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.TP
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.B ONOEOT
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Discard
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.B EOT
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(^D) characters. (Minix-vmd only.)
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.SS "Control Modes"
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The
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.B c_cflag
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field contains the following single bit flags and bit field for basic
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hardware control:
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.TP
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.B CLOCAL
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Ignore modem status lines.
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.TP
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.B CREAD
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Enable receiver. (The receiver is always enabled.)
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.TP
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.B CSIZE
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Number of bits per byte.
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.B CSIZE
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masks off the values
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.BR CS5 ,
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.BR CS6 ,
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.BR CS7
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and
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.BR CS8
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that indicate that 5, 6, 7 or 8 bits are used.
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.TP
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.B CSTOPB
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Send two stop bits instead of one. Two stop bits are normally used at 110
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baud or less.
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.TP
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.B PARENB
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Enable parity generation.
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.TP
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.B PARODD
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Generate odd parity if parity is generated, otherwise even parity.
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.TP
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.B HUPCL
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Drop the modem control lines on the last close of the terminal line. (Not
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implemented.)
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.SS "Local Modes"
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|
The
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.B c_lflag
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field contains the following single bit flags that control various functions:
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|
.TP
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.B ECHO
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Enable echoing of input characters. Most input characters are echoed as
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they are. Control characters are echoed as
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.BI "^" X
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where
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.I X
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is the letter used to say that the control character is
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.BI CTRL\- X\fR.
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The
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.BR CR ,
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.BR NL
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and
|
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.BR TAB
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characters are echoed with their normal effect unless they are escaped by
|
|
.BR LNEXT .
|
|
.TP
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|
.B ECHOE
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|
If
|
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.B ICANON
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and
|
|
.B ECHO
|
|
are set then echo
|
|
.B ERASE
|
|
and
|
|
.B KILL
|
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as one or more backspace-space-backspace sequences to wipe out the last
|
|
character or the entire line, otherwise they are echoed as they are.
|
|
.TP
|
|
.B ECHOK
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|
If
|
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.B ICANON
|
|
and
|
|
.B ECHO
|
|
are set and
|
|
.B ECHOE
|
|
is not set then output an
|
|
.B NL
|
|
after the
|
|
.B KILL
|
|
character. (For hardcopy terminals it is best to unset
|
|
.B ECHOE
|
|
and to set
|
|
.BR ECHOK .)
|
|
.TP
|
|
.B ECHONL
|
|
Echo
|
|
.B NL
|
|
even if
|
|
.B ECHO
|
|
is not set, but
|
|
.B ICANON
|
|
is set.
|
|
.TP
|
|
.B ICANON
|
|
Canonical input. This enables line oriented input and erase and kill
|
|
processing.
|
|
.TP
|
|
.B IEXTEN
|
|
Enable implementation defined input extensions.
|
|
.TP
|
|
.B ISIG
|
|
Enable the signal characters
|
|
.BR INTR ,
|
|
.BR QUIT
|
|
and
|
|
.BR SUSP .
|
|
.TP
|
|
.B NOFLSH
|
|
Disable the flushing of the input and output queues that is normally done if
|
|
a signal is sent.
|
|
.TP
|
|
.B TOSTOP
|
|
Send a
|
|
.B SIGTTOU
|
|
signal if job control is implemented and a background process tries to
|
|
write. (MINIX 3 has no job control.)
|
|
.SS "Input and output speed"
|
|
The input and output speed are encoded into the
|
|
.B c_ispeed
|
|
and
|
|
.B c_ospeed
|
|
fields.
|
|
.B <termios.h>
|
|
defines the symbols
|
|
.BR B0 ,
|
|
.BR B50 ,
|
|
.BR B75 ,
|
|
.BR B110 ,
|
|
.BR B134 ,
|
|
.BR B150 ,
|
|
.BR B200 ,
|
|
.BR B300 ,
|
|
.BR B600 ,
|
|
.BR B1200 ,
|
|
.BR B1800 ,
|
|
.BR B2400 ,
|
|
.BR B4800 ,
|
|
.BR B9600 ,
|
|
.BR B19200 ,
|
|
.BR B38400 ,
|
|
.BR B57600
|
|
and
|
|
.BR B115200
|
|
as values used to indicate the given baud rates. The zero baud rate,
|
|
.BR B0 ,
|
|
if used for the input speed causes the input speed to be equal to the
|
|
output speed. Setting the output speed to zero hangs up the line. One
|
|
should use the functions
|
|
.BR cfgetispeed() ,
|
|
.BR cfgetospeed() ,
|
|
.BR cfsetispeed()
|
|
and
|
|
.BR cfsetospeed()
|
|
to get or set a speed, because the
|
|
.B c_ispeed
|
|
and
|
|
.B c_ospeed
|
|
fields may not be visible under other implementations. (The
|
|
.B c_ispeed
|
|
and
|
|
.B c_ospeed
|
|
fields and the
|
|
.B B57600
|
|
and
|
|
.B B115200
|
|
symbols are MINIX 3 specific.)
|
|
.SS "Special characters"
|
|
The
|
|
.B c_cc
|
|
array contains the special characters that can be modified. The array has
|
|
length
|
|
.B NCCS
|
|
and is subscripted by the symbols
|
|
.BR VEOF ,
|
|
.BR VEOL ,
|
|
.BR VERASE ,
|
|
.BR VINTR ,
|
|
.BR VKILL ,
|
|
.BR VMIN ,
|
|
.BR VQUIT ,
|
|
.BR VTIME ,
|
|
.BR VSUSP ,
|
|
.BR VSTART ,
|
|
.BR VSTOP ,
|
|
.BR VREPRINT ,
|
|
.BR VLNEXT
|
|
and
|
|
.BR VDISCARD .
|
|
All these symbols are defined in
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.BR <termios.h> .
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Some implementations may give the same values to the
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.B VMIN
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and
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.B VTIME
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subscripts and the
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.B VEOF
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and
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.B VEOL
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subscripts respectively, and may ignore changes to
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.B START
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and
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.BR STOP .
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(Under MINIX 3 all special characters have their own
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.I c_cc
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slot and can all be modified.)
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.SS "Raw I/O Parameters"
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The
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.B MIN
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and
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.B TIME
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parameters can be used to adjust a raw connection to bursty input.
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.B MIN
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represents a minimum number of bytes that must be received before a read
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call returns.
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.B TIME
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is a timer of 0.1 second granularity that can be used to time out a read.
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Setting either of these parameters to zero has special meaning, which leads
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to the following four possibilities:
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.TP 5
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.B "MIN > 0, TIME > 0"
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.B TIME
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is an inter-byte timer that is started (and restarted) when a byte is
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received. A read succeeds when either the minimum number of characters
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is received or the timer expires. Note that the timer starts
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.B after
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the first character, so the read returns at least one byte.
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.TP
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.B "MIN > 0, TIME = 0"
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Now the timer is disabled, and a reader blocks indefinitely until at least
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.B MIN
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characters are received.
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.TP
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.B "MIN = 0, TIME > 0"
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.B TIME
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is now a read timer that is started when a read is executed. The read will
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return if the read timer expires or if at least one byte is input. (Note
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that a value of zero may be returned to the reader.)
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.TP
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.B "MIN = 0, TIME = 0"
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The bytes currently available are returned. Zero is returned if no bytes
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are available.
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.SS "User Level Functions"
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Termios attributes are set or examined, and special functions can be
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performed by using the functions described in
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.BR termios (3).
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.SS "Session Leaders and Process Groups"
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With the use of the
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.B setsid()
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function can a process become a session leader. A session leader forms a
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process group with a process group id equal to the process id of the session
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leader. If a session leader opens a terminal device file then this terminal
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becomes the controlling tty of the session leader. Unless the terminal is
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already the controlling tty of another process, or unless the
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.B O_NOCTTY
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flag is used to prevent the allocation of a controlling tty. The process
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group of the session leader is now remembered as the terminal process group
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for signals sent by the terminal driver. All the children and grandchildren
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of the session leader inherit the controlling terminal and process group
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until they themselves use
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.BR setsid() .
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.PP
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The controlling tty becomes inaccessible to the children of the session
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leader when the session leader exits, and a hangup signal is sent to all
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the members of the process group. The input and output queues are flushed
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on the last close of a terminal and all attributes are reset to the default
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state.
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.PP
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A special device
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.B /dev/tty
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is a synonym for the controlling tty of a process. It allows a process to
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reach the terminal even when standard input, output and error are
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redirected. Opening this device can also be used as a test to see if a
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process has a controlling tty or not.
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.PP
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For MINIX 3 a special write-only device
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.B /dev/log
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exists for processes that want to write messages to the system console.
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Unlike the console this device is still accessible when a session leader
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exits.
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.PP
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Minix-vmd also has a
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.B /dev/log
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device, but this device is read-write. All messages written to the log
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device or to the console when X11 is active can be read from
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.BR /dev/log .
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The system tries to preserve the log buffer over a reboot so that panic
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messages reappear in the log if the system happens to crash.
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.SS "Pseudo Terminals"
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Pseudo ttys allow a process such as a remote login daemon to set up a
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terminal for a remote login session. The login session uses a device like
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.B /dev/ttyp0
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for input and output, and the remote login daemon uses the device
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.B /dev/ptyp0
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to supply input to or take output from the login session and transfer this
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to or from the originating system. So the character flow may be: Local
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user input sent to the remote system is written to
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.B /dev/ptyp0
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by the remote login daemon, undergoes input processing and appears on
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.B /dev/ttyp0
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as input to the login session. Output from the login session to
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.B /dev/ttyp0
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undergoes output processing, is read from
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.B /dev/ptyp0
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by the remote login daemon and is send over to the local system to be
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displayed for the user. (So there are only four data streams to worry about
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in a pseudo terminal.)
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.PP
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A pseudo terminal can be allocated by trying to open all the controlling
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devices
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.BI /dev/pty nn
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one by one until it succeeds. Further opens will fail once a pty is open.
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The process should now fork, the child should become session leader, open
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the tty side of the pty and start a login session.
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.PP
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If the tty side is eventually closed down then reads from the pty side will
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return zero and writes return \-1 with
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.B errno
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set to
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.BR EIO .
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If the pty side is closed first then a
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.B SIGHUP
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signal is sent to the session leader and further reads from the tty side
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return zero and writes return \-1 with
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.B errno
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set to
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.BR EIO .
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(Special note: A line erase may cause up to three times the size of the
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tty input queue to be sent to the pty reader as backspace overstrikes. Some
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of this output may get lost if the pty reader cannot accept it all at once
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in a single read call.)
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.SH FILES
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The list below shows all devices that MINIX 3 and Minix-vmd have. Not all of
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these devices are configured in by default, as indicated by the numbers
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(i/j/k, l/m/n) that tell the minimum, default and maximum possible number of
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these devices for MINIX 3 (i/j/k) and Minix-vmd (l/m/n).
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.TP 20
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.B /dev/console
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System console.
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.TP
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.B /dev/ttyc[1-7]
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Virtual consoles. (0/1/7, 0/1/7)
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.TP
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.BR /dev/log
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Console log device.
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.TP
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.B /dev/tty0[0-3]
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Serial lines. (0/2/2, 4/4/4)
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.TP
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.B /dev/tty[p-w][0-f]
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|
Pseudo ttys. (0/0/64, 1/32/128)
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.TP
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.B /dev/pty[p-w][0-f]
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Associated pseudo tty controllers.
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.SH "SEE ALSO"
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.BR stty (1),
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.BR termios (3),
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.BR setsid (2),
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.BR read (2),
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.BR write (2).
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.SH BUGS
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|
A fair number of flags are not implemented under MINIX 3 (yet). Luckily they
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are very limited utility and only apply to RS-232, not to the user interface.
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.SH AUTHOR
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Kees J. Bot (kjb@cs.vu.nl)
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