TTY(4) TTY(4)
NAME
tty - general terminal interface
DESCRIPTION
This section describes both a particular special file, and
the general nature of the terminal interface.
The file /dev/tty is, in each process, a synonym for the
control terminal associated with that process. It is useful
for programs that wish to be sure of writing messages on the
terminal no matter how output has been redirected. It can
also be used for programs that demand a file name for out-
put, when typed output is desired and it is tiresome to find
out which terminal is currently in use.
As for terminals in general: all of the low-speed asyn-
chronous communications ports use the same general inter-
face, no matter what hardware is involved. The remainder of
this section discusses the common features of the interface.
When a terminal 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
terminal file open in a process becomes the control terminal
for that process. The control terminal plays a special role
in handling quit or interrupt signals, as discussed below.
The control terminal is inherited by a child process during
a fork, even if the control terminal is closed. The set of
processes that thus share a control terminal is called a
process group; all members of a process group receive cer-
tain signals together, see DEL below and kill(2).
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 that 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.
Normally, terminal input is processed in units of lines.
This means that a program attempting to read will be sus-
pended 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.
TTY(4) TTY(4)
There are special modes, discussed below, that permit the
program to read each character as typed without waiting for
a full line.
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 fol-
lowing escape sequences can be generated on output and
accepted on input:
for use
` \'
| \!
~ \^
{ \(
} \)
Certain ASCII control characters have special meaning.
These characters are not passed to a reading program except
in raw mode where they lose their special character. Also,
it is possible to change these characters from the default;
see below.
EOT (Control-D) may be used to generate an end of file from
a terminal. When an EOT is received, all the charac-
ters 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.
DEL (Rubout) is not passed to a program but generates an
interrupt signal which is sent to all processes with
the associated control terminal. 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(2).
FS (Control-\ or control-shift-L) generates the quit
TTY(4) TTY(4)
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.
DC3 (Control-S) delays all printing on the terminal until
something is typed in.
DC1 (Control-Q) restarts printing after DC3 without gener-
ating any input to a program.
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 terminal as control termi-
nal. 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 that read a terminal and test for end-of-file
on their input can terminate appropriately when hung up on.
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 that respond to it
from hanging up.
Several ioctl(2) calls apply to terminals. Most of them use
the following structure, defined in <sgtty.h>:
struct sgttyb {
char sg_ispeed;
char sg_ospeed;
char sg_erase;
char sg_kill;
int sg_flags;
};
The sg_ispeed and sg_ospeed fields describe the input and
output speeds of the device according to the following
table, which corresponds to the DEC DH-11 interface. If
other hardware is used, impossible speed changes are
ignored. Symbolic values in the table are as defined in
<sgtty.h>.
B0 0 (hang up dataphone)
B50 1 50 baud
TTY(4) TTY(4)
B75 2 75 baud
B110 3 110 baud
B134 4 134.5 baud
B150 5 150 baud
B200 6 200 baud
B300 7 300 baud
B600 8 600 baud
B1200 9 1200 baud
B1800 10 1800 baud
B2400 11 2400 baud
B4800 12 4800 baud
B9600 13 9600 baud
EXTA 14 External A
EXTB 15 External B
In the current configuration, only 110, 150, 300 and 1200
baud are really supported on dial-up lines. Code conversion
and line control required for IBM 2741's (134.5 baud) must
be implemented by the user's program. The half-duplex line
discipline required for the 202 dataset (1200 baud) is not
supplied; full-duplex 212 datasets work fine.
The sg_erase and sg_kill fields of the argument structure
specify the erase and kill characters respectively.
(Defaults are # and @.)
The sg_flags field of the argument structure contains sev-
eral bits that determine the system's treatment of the ter-
minal:
ALLDELAY 0177400 Delay algorithm selection
BSDELAY 0100000 Select backspace delays (not implemented):
BS0 0
BS1 0100000
VTDELAY 0040000 Select form-feed and vertical-tab delays:
FF0 0
FF1 0100000
CRDELAY 0030000 Select carriage-return delays:
CR0 0
CR1 0010000
CR2 0020000
CR3 0030000
TBDELAY 0006000 Select tab delays:
TAB0 0
TAB1 0001000
TAB2 0004000
XTABS 0006000
NLDELAY 0001400 Select new-line delays:
NL0 0
NL1 0000400
NL2 0001000
NL3 0001400
TTY(4) TTY(4)
EVENP 0000200 Even parity allowed on input (most terminals)
ODDP 0000100 Odd parity allowed on input
RAW 0000040 Raw mode: wake up on all characters, 8-bit interface
CRMOD 0000020 Map CR into LF; echo LF or CR as CR-LF
ECHO 0000010 Echo (full duplex)
LCASE 0000004 Map upper case to lower on input
CBREAK 0000002 Return each character as soon as typed
TANDEM 0000001 Automatic flow control
The delay bits specify how long transmission stops to allow
for mechanical or other movement when certain characters are
sent to the terminal. In all cases a value of 0 indicates
no delay.
Backspace delays are currently ignored but might be used for
Terminet 300's.
If a form-feed/vertical tab delay is specified, it lasts for
about 2 seconds.
Carriage-return delay type 1 lasts about .08 seconds and is
suitable for the Terminet 300. Delay type 2 lasts about .16
seconds and is suitable for the VT05 and the TI 700. Delay
type 3 is unimplemented and is 0.
New-line delay type 1 is dependent on the current column and
is tuned for Teletype model 37's. Type 2 is useful for the
VT05 and is about .10 seconds. Type 3 is unimplemented and
is 0.
Tab delay type 1 is dependent on the amount of movement and
is tuned to the Teletype model 37. Type 3, called XTABS, is
not a delay at all but causes tabs to be replaced by the
appropriate number of spaces on output.
Characters with the wrong parity, as determined by bits 200
and 100, are ignored.
In raw mode, every character is passed immediately to the
program without waiting until a full line has been typed.
No erase or kill processing is done; the end-of-file indica-
tor (EOT), the interrupt character (DEL) and the quit char-
acter (FS) are not treated specially. There are no delays
and no echoing, and no replacement of one character for
another; characters are a full 8 bits for both input and
output (parity is up to the program).
Mode 020 causes input carriage returns to be turned into
new-lines; input of either CR or LF causes LF-CR both to be
echoed (for terminals with a new-line function).
CBREAK is a sort of half-cooked (rare?) mode. Programs can
TTY(4) TTY(4)
read each character as soon as typed, instead of waiting for
a full line, but quit and interrupt work, and output delays,
case-translation, CRMOD, XTABS, ECHO, and parity work nor-
mally. On the other hand there is no erase or kill, and no
special treatment of \ or EOT.
TANDEM mode causes the system to produce a stop character
(default DC3) whenever the input queue is in danger of over-
flowing, and a start character (default DC1) when the input
queue has drained sufficiently. It is useful for flow con-
trol when the `terminal' is actually another machine that
obeys the conventions.
Several ioctl calls have the form:
#include <sgtty.h>
ioctl(fildes, code, arg)
struct sgttyb *arg;
The applicable codes are:
TIOCGETP
Fetch the parameters associated with the terminal, and
store in the pointed-to structure.
TIOCSETP
Set the parameters according to the pointed-to struc-
ture. The interface delays until output is quiescent,
then throws away any unread characters, before changing
the modes.
TIOCSETN
Set the parameters but do not delay or flush input.
Switching out of RAW or CBREAK mode may cause some gar-
bage input.
With the following codes the arg is ignored.
TIOCEXCL
Set ``exclusive-use'' mode: no further opens are per-
mitted until the file has been closed.
TIOCNXCL
Turn off ``exclusive-use'' mode.
TIOCHPCL
When the file is closed for the last time, hang up the
terminal. This is useful when the line is associated
with an ACU used to place outgoing calls.
TIOCFLUSH
TTY(4) TTY(4)
All characters waiting in input or output queues are
flushed.
The following codes affect characters that are special to
the terminal interface. The argument is a pointer to the
following structure, defined in <sgtty.h>:
struct tchars {
char t_intrc; /* interrupt */
char t_quitc; /* quit */
char t_startc; /* start output */
char t_stopc; /* stop output */
char t_eofc; /* end-of-file */
char t_brkc; /* input delimiter (like nl) */
};
The default values for these characters are DEL, FS, DC1,
DC3, EOT, and -1. A character value of -1 eliminates the
effect of that character. The t_brkc character, by default
-1, acts like a new-line in that it terminates a `line,' is
echoed, and is passed to the program. The `stop' and
`start' characters may be the same, to produce a toggle
effect. It is probably counterproductive to make other spe-
cial characters (including erase an kill) identical.
The calls are:
TIOCSETC
Change the various special characters to those given in
the structure.
TIOCSETP
Set the special characters to those given in the struc-
ture.
FILES
/dev/tty
/dev/tty*
/dev/console
SEE ALSO
getty(8), stty (1), signal(2), ioctl(2)
BUGS
Half-duplex terminals are not supported.
The terminal handler has clearly entered the race for ever-
greater complexity and generality. It's still not complex
and general enough for TENEX fans.