minix/man/man4/tty.4

.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
Enable receiver.  (The receiver is always enabled.)
.TP
.B CSIZE
Number of bits per byte.
.B CSIZE
masks off the values
.BR CS5 ,
.BR CS6 ,
.BR CS7
and
.BR CS8
that indicate that 5, 6, 7 or 8 bits are used.
.TP
.B CSTOPB
Send two stop bits instead of one.  Two stop bits are normally used at 110
baud or less.
.TP
.B PARENB
Enable parity generation.
.TP
.B PARODD
Generate odd parity if parity is generated, otherwise even parity.
.TP
.B HUPCL
Drop the modem control lines on the last close of the terminal line.  (Not
implemented.)
.SS "Local Modes"
The
.B c_lflag
field contains the following single bit flags that control various functions:
.TP
.B ECHO
Enable echoing of input characters.  Most input characters are echoed as
they are.  Control characters are echoed as 
.BI "^" X
where
.I X
is the letter used to say that the control character is
.BI CTRL\- X\fR.
The
.BR CR ,
.BR NL
and
.BR TAB
characters are echoed with their normal effect unless they are escaped by
.BR LNEXT .
.TP
.B ECHOE
If
.B ICANON
and
.B ECHO
are set then echo
.B ERASE
and
.B KILL
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
If
.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 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 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
.BR <termios.h> .
Some implementations may give the same values to the
.B VMIN
and
.B VTIME
subscripts and the
.B VEOF
and
.B VEOL
subscripts respectively, and may ignore changes to
.B START
and
.BR STOP .
(Under Minix all special characters have their own
.I c_cc
slot and can all be modified.)
.SS "Raw I/O Parameters"
The
.B MIN
and
.B TIME
parameters can be used to adjust a raw connection to bursty input.
.B MIN
represents a minimum number of bytes that must be received before a read
call returns.
.B TIME
is a timer of 0.1 second granularity that can be used to time out a read.
Setting either of these parameters to zero has special meaning, which leads
to the following four possibilities:
.TP 5
.B "MIN > 0, TIME > 0"
.B TIME
is an inter-byte timer that is started (and restarted) when a byte is
received.  A read succeeds when either the minimum number of characters
is received or the timer expires.  Note that the timer starts
.B after
the first character, so the read returns at least one byte.
.TP
.B "MIN > 0, TIME = 0"
Now the timer is disabled, and a reader blocks indefinitely until at least
.B MIN
characters are received.
.TP
.B "MIN = 0, TIME > 0"
.B TIME
is now a read timer that is started when a read is executed.  The read will
return if the read timer expires or if at least one byte is input.  (Note
that a value of zero may be returned to the reader.)
.TP
.B "MIN = 0, TIME = 0"
The bytes currently available are returned.  Zero is returned if no bytes
are available.
.SS "User Level Functions"
Termios attributes are set or examined, and special functions can be
performed by using the functions described in
.BR termios (3).
.SS "Session Leaders and Process Groups"
With the use of the
.B setsid()
function can a process become a session leader.  A session leader forms a
process group with a process group id equal to the process id of the session
leader.  If a session leader opens a terminal device file then this terminal
becomes the controlling tty of the session leader.  Unless the terminal is
already the controlling tty of another process, or unless the
.B O_NOCTTY
flag is used to prevent the allocation of a controlling tty.  The process
group of the session leader is now remembered as the terminal process group
for signals sent by the terminal driver.  All the children and grandchildren
of the session leader inherit the controlling terminal and process group
until they themselves use
.BR setsid() .
.PP
The controlling tty becomes inaccessible to the children of the session
leader when the session leader exits, and a hangup signal is sent to all
the members of the process group.  The input and output queues are flushed
on the last close of a terminal and all attributes are reset to the default
state.
.PP
A special device
.B /dev/tty
is a synonym for the controlling tty of a process.  It allows a process to
reach the terminal even when standard input, output and error are
redirected.  Opening this device can also be used as a test to see if a
process has a controlling tty or not.
.PP
For Minix a special write-only device
.B /dev/log
exists for processes that want to write messages to the system console.
Unlike the console this device is still accessible when a session leader
exits.
.PP
Minix-vmd also has a
.B /dev/log
device, but this device is read-write.  All messages written to the log
device or to the console when X11 is active can be read from
.BR /dev/log .
The system tries to preserve the log buffer over a reboot so that panic
messages reappear in the log if the system happens to crash.
.SS "Pseudo Terminals"
Pseudo ttys allow a process such as a remote login daemon to set up a
terminal for a remote login session.  The login session uses a device like
.B /dev/ttyp0
for input and output, and the remote login daemon uses the device
.B /dev/ptyp0
to supply input to or take output from the login session and transfer this
to or from the originating system.  So the character flow may be:  Local
user input sent to the remote system is written to
.B /dev/ptyp0
by the remote login daemon, undergoes input processing and appears on
.B /dev/ttyp0
as input to the login session.  Output from the login session to
.B /dev/ttyp0
undergoes output processing, is read from
.B /dev/ptyp0
by the remote login daemon and is send over to the local system to be
displayed for the user.  (So there are only four data streams to worry about
in a pseudo terminal.)
.PP
A pseudo terminal can be allocated by trying to open all the controlling
devices
.BI /dev/pty nn
one by one until it succeeds.  Further opens will fail once a pty is open.
The process should now fork, the child should become session leader, open
the tty side of the pty and start a login session.
.PP
If the tty side is eventually closed down then reads from the pty side will
return zero and writes return \-1 with
.B errno
set to
.BR EIO .
If the pty side is closed first then a
.B SIGHUP
signal is sent to the session leader and further reads from the tty side
return zero and writes return \-1 with
.B errno
set to
.BR EIO .
(Special note:  A line erase may cause up to three times the size of the
tty input queue to be sent to the pty reader as backspace overstrikes.  Some
of this output may get lost if the pty reader cannot accept it all at once
in a single read call.)
.SS "Backwards compatibility"
The
.BR TIOCGETP ,
.BR TIOCSETP ,
.BR TIOCGETC
and
.BR TIOCSETC
ioctl functions that are used by the old
.B sgtty
terminal interface are still supported by the terminal driver by emulation.
Note that these old functions cannot control all termios attributes, so the
terminal must be in a relatively sane state to avoid problems.
.SH FILES
The list below shows all devices that Minix and Minix-vmd have.  Not all of
these devices are configured in by default, as indicated by the numbers
(i/j/k, l/m/n) that tell the minimum, default and maximum possible number of
these devices for Minix (i/j/k) and Minix-vmd (l/m/n).
.TP 20
.B /dev/console
System console.
.TP
.B /dev/ttyc[1-7]
Virtual consoles.  (0/1/7, 0/1/7)
.TP
.BR /dev/log
Console log device.
.TP
.B /dev/tty0[0-3]
Serial lines.  (0/2/2, 4/4/4)
.TP
.B /dev/tty[p-w][0-f]
Pseudo ttys.  (0/0/64, 1/32/128)
.TP
.B /dev/pty[p-w][0-f]
Associated pseudo tty controllers.
.SH "SEE ALSO"
.BR stty (1),
.BR termios (3),
.BR setsid (2),
.BR read (2),
.BR write (2).
.SH BUGS
A fair number of flags are not implemented under Minix (yet).  Luckily they
are very limited utility and only apply to RS-232, not to the user interface.
.SH AUTHOR
Kees J. Bot (kjb@cs.vu.nl)