tty.4

操作系统设计与实现源码

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 there 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 (2).
.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 ([e-mail])