TTY(IV) 5/27/74 TTY(IV)
NAME
tty - general typewriter interface
DESCRIPTION
This section describes both a particular special file, and
the general nature of the typewriter interface.
The file /dev/tty is, in each process, a synonym for the
control typewriter associated with that process. It is
useful for programs or Shell sequences which wish to be sure
of writing messages on the typewriter no matter how output
has been redirected. It can also be used for programs which
demand a file name for output, when typed output is desired
and it is tiresome to find out which typewriter is currently
in use.
As for typewriters in general: all of the low-speed
asynchronous communications ports use the same general
interface, no matter what hardware is involved. The
remainder of this section discusses the common features of
the interface; the KL, DC, and DH writeups (IV) describe
peculiarities of the individual devices.
When a typewriter file is opened, it causes the process to
wait until a connection is established. In practice user's
programs seldom open these files; they are opened by init
and become a user's input and output file. The very first
typewriter file open in a process becomes the control
typewriter for that process. The control typewriter plays a
special role in handling quit or interrupt signals, as
discussed below. The control typewriter is inherited by a
child process during a fork.
A terminal associated with one of these files ordinarily
operates in full-duplex mode. Characters may be typed at
any time, even while output is occurring, and are only lost
when the system's character input buffers become completely
choked, which is rare, or when the user has accumulated the
maximum allowed number of input characters which have not
yet been read by some program. Currently this limit is 256
characters. When the input limit is reached all the saved
characters are thrown away without notice.
These special files have a number of modes which can be
changed by use of the stty system call (II). When first
opened, the interface mode is 300 baud; either parity
accepted; 10 bits/character (one stop bit); and newline
action character. Modes that can be changed by stty include
the interface speed (if the hardware permits); acceptance of
even parity, odd parity, or both; a raw mode in which all
characters may be read one at a time; a carriage return (CR)
mode in which CR is mapped into newline on input and either
CR or line feed (LF) cause echoing of the sequence LF-CR;
mapping of upper case letters into lower case; suppression
of echoing; a variety of delays after function characters;
and the printing of tabs as spaces. See getty (VIII) for
the way that terminal speed and type are detected.
Normally, typewriter input is processed in units of lines.
This means that a program attempting to read will be
suspended until an entire line has been typed. Also, no
matter how many characters are requested in the read call,
at most one line will be returned. It is not however
necessary to read a whole line at once; any number of
characters may be requested in a read, even one, without
losing information.
During input, erase and kill processing is normally done.
By default, the character `#' erases the last character
typed, except that it will not erase beyond the beginning of
a line or an EOT. By default, the character `@' kills the
entire line up to the point where it was typed, but not
beyond an EOT. Both these characters operate on a keystroke
basis independently of any backspacing or tabbing that may
have been done. Either `@' or `#' may be entered literally
by preceding it by `\'; the erase or kill character remains,
but the `\' disappears. These two characters may be changed
to others.
When desired, all upper-case letters are mapped into the
corresponding lower-case letter. The upper-case letter may
be generated by preceding it by `\'. In addition, the
following escape sequences are generated on output and
accepted on input:
for use
` \'
| \!
~ \^
{ \(
} \)
In raw mode, the program reading is awakened on each
character. No erase or kill processing is done; and the
EOT, quit and interrupt characters are not treated
specially. The input parity bit is passed back to the
reader, but parity is still generated for output characters.
The ASCII EOT (control-D) character may be used to generate
an end of file from a typewriter. When an EOT is received,
all the characters waiting to be read are immediately passed
to the program, without waiting for a new-line, and the EOT
is discarded. Thus if there are no characters waiting,
which is to say the EOT occurred at the beginning of a line,
zero characters will be passed back, and this is the
standard end-of-file indication. The EOT is passed back
unchanged in raw mode.
When the carrier signal from the dataset drops (usually
because the user has hung up his terminal) a hangup signal
is sent to all processes with the typewriter as control
typewriter. Unless other arrangements have been made, this
signal causes the processes to terminate. If the hangup
signal is ignored, any read returns with an end-of-file
indication. Thus programs which read a typewriter and test
for end-of-file on their input can terminate appropriately
when hung up on.
Two characters have a special meaning when typed. The ASCII
DEL character (sometimes called `rubout') is not passed to a
program but generates an interrupt signal which is sent to
all processes with the associated control typewriter.
Normally each such process is forced to terminate, but
arrangements may be made either to ignore the signal or to
receive a trap to an agreed-upon location. See signal (II).
The ASCII character FS generates the quit signal. Its
treatment is identical to the interrupt signal except that
unless a receiving process has made other arrangements it
will not only be terminated but a core image file will be
generated. If you find it hard to type this character, try
control-\ or control-shift-L.
When one or more characters are written, they are actually
transmitted to the terminal as soon as previously-written
characters have finished typing. Input characters are
echoed by putting them in the output queue as they arrive.
When a process produces characters more rapidly than they
can be typed, it will be suspended when its output queue
exceeds some limit. When the queue has drained down to some
threshold the program is resumed. Even parity is always
generated on output. The EOT character is not transmitted
(except in raw mode) to prevent terminals which respond to
it from hanging up.
FILES
/dev/tty
SEE ALSO
dc (IV), kl (IV), dh (IV), getty (VIII), stty (I, II), gtty
(I, II), signal (II)
BUGS
Half-duplex terminals are not supported. On raw-mode
output, parity should be transmitted as specified in the
characters written.