tty.4

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.\" SCCSID: @(#)tty.4	2.3	6/8/87
.TH tty 4
.SH Name
tty \- general terminal interface
.SH Description
.PP
.B "Terminal Subsystem"
.sp
The terminal subsystem is the part of the operating system
that allows users to read and write characters over 
asynchronous terminal lines.  An important aspect of this
subsystem is to provide a means for the user to set and receive terminal
attributes.  Terminal attributes involve such things as line
speed (baud rate), character length, parity, flow control,
modem control,
as well as numerous character-processing capabilities.
.PP
The ULTRIX terminal interface allows the user to specify terminal
attributes in different ways.  Several
different terminal interfaces exist to provide standard
compliant terminal control.
.PP
.B "Terminal Interface Definitions"
.sp
The
.MS tty 4
reference page
describes the standard Berkeley terminal interface. 
This interface is backward compatible with earlier versions of 
the ULTRIX operating system.
.sp
The
.MS termio 4
reference page
describes the terminal interface as defined by the
System V Interface Definition.  
.sp
The
.MS termios 4
reference page
describes the termios termio interface as defined by the IEEE
P1003 POSIX specification.
.PP
Functionally, the three terminal interfaces are quite similar.  Major 
differences lie in how terminal attributes are specified. This includes
the use of different data structures to represent terminal attributes, as
well as the means of setting and receiving attributes.
.PP
It is possible to use combinations of the three terminal 
interface definitions.  Under these circumstances, the attributes of one
interface are mapped into the corresponding attributes of the other
interfaces.  Combining aspects of different interfaces is discouraged,
because it prevents the development of portable programs.
For example, a program intended for use with the System V termio
terminal interface fails to be a standard compliant program if it
sets terminal attributes that are specific to
either of the other two terminal
interfaces.
.PP
Combinations of the different terminal interfaces should be used with
extreme caution to avoid unwanted side-effects.  For example, a program may
have initially set up its terminal environment using the System V termio 
interface,
.MS termio 4 .
Suppose that the initial line settings are seven bits even parity with input and
output processing performed.  If this same program were to set the line to
RAW mode as specified in
.MS tty 4 ,
the line would be set to eight bits no parity, with no input 
or output processing.
These settings would be reflected in the parameters as specified in
.MS termio 4 .
This simple example is meant to illustrate the subtle side effects that can
result from the use of combinations of terminal interfaces.
.PP
Using combinations of terminal interfaces can also cause problems if
attributes that are not common to all interfaces are used.  For example,
the Berkeley terminal interface allows the user to set the value of the
start and stop characters.  The System V termio interface defines that the
start and stop characters shall be Control-Q and Control-S.  As a result, if
a terminal changes the start and stop characters using the Berkeley
terminal interface, those characters are reset if the terminal 
parameters are set using the System V termio interface.
.sp
.B "Terminal Interface Usage"
.br
.sp
The three interfaces have been developed to provide standard 
compliant terminal behavior.  The interface type should be specified at
the time of program compilation.  As described in
.MS cc 1 ,
to compile a System V\-compliant program, the \-Y option (or setting PROG_ENV
equal to SYSTEM_FIVE) should be used.  Similarly, the \-YPOSIX option should
be used to compile a POSIX\-compliant program.  Without the \-Y or \-YPOSIX
compile option, the program intends to use the Berkeley terminal 
interface.  Refer to 
.MS intro 2
for specific details on compatibility modes.
.sp 1
.SH Berkeley Terminal Interface
.NXR "terminal interface"
.NXA "tty keyword" "terminal interface"
.NXA "terminal interface" "bk line discipline"
.NXA "terminal" "Hazeltine terminal"
.PP
.B Line Disciplines.
.NXR "terminal interface" "line disciplines"
.PP
The system provides different
.I "line disciplines"
for controlling communications lines.
In this version of the system, there are several disciplines available:
.IP "old" 8
The old (Version 7) terminal driver.
This is sometimes used when using the
standard shell
.MS sh 1
and for compatibility with other standard Version 7 UNIX systems.
.IP "new"
The standard terminal driver, with features for job control.  
This must be used when using 
.MS csh 1 .
.IP "net"
A line discipline used for networking and loading data into
the system over communications lines.  It allows high speed input
at low overhead and is described in 
.MS bk 4 .
.IP "termio"
This line discipline is intended for use by System V programs that
use the termio interface, as described in
.MS termio 4 .
The termio line discipline is also used by programs that require a
POSIX IEEE P1003 termios interface as described in
.MS termios 4 .
.PP
Line discipline switching is accomplished with the TIOCSETD 
.PN ioctl:
.PP
.B "int ldisc = LDISC; ioctl(filedes, TIOCSETD, &ldisc);"
.PP
LDISC is OTTYDISC for the standard tty driver,
NTTYDISC for the new
driver, NETLDISC for the networking discipline,
and TERMIODISC for System V termio and POSIX termios.
The standard tty driver is discipline 0 by convention.
Other disciplines may exist for special purposes.
The current line discipline can be obtained with the TIOCGETD ioctl.
Pending input is discarded when the line discipline is changed.
.PP
All of the low-speed asynchronous
communications ports can use any
of the available line disciplines, no matter what
hardware is involved.
The remainder of this section discusses the
old and new disciplines.
.PP
.B "The Control Terminal"
.NXR "terminal interface" "control terminals and"
.PP
When a terminal file is opened, it causes the process to wait until a
connection is established.  In practice, user programs seldom open
these files; they are opened by
.MS getty 8
or
.MS rlogind 8c
and become a user's standard input and output file.
.PP
If a process that has no control terminal opens a terminal file, then
that terminal file becomes the control terminal for that process.
The control terminal is thereafter inherited by a child process during a
.MS fork 2 ,
even if the control terminal is closed.
.PP
The file 
.PN /dev/tty
is, in each process, a synonym for a
.I "control terminal"
associated with that process.  It is useful for programs that want 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 output, when typed output is desired
and it is tiresome to find out which terminal
is currently in use.
.PP
A process can remove the association it
has with its controlling terminal by 
opening the file 
.PN /dev/tty
and issuing:
.IP
\fBioctl(fildes, TIOCNOTTY, 0)\fR
.PP
This is often desirable in server processes.
.PP
.B "Process Groups"
.NXR "process group" "associating with terminal"
.PP
Command processors such as 
.MS csh 1
can arbitrate the terminal between different
.I jobs
by placing related jobs in a single process group and associating this
process group with the terminal.  A terminal's associated process group
may be set using the TIOCSPGRP 
.MS ioctl 2 :
.IP
\fBioctl(fildes, TIOCSPGRP, &pgrp)\fR
.PP
The process group can be examined using
using TIOCGPGRP, rather than TIOCSPGRP, returning the current
process group in
.I pgrp.
The new terminal driver aids in this arbitration by restricting access
to the terminal by processes that are not in the current process group;
see
.B "Job Access Control."
.PP
.B "Modes"
.NXR "terminal" "modes"
.PP
The terminal drivers have three major modes, characterized by the
amount of processing on the input and output characters:
.IP cooked 10
.NXR "cooked mode" "defined"
The normal mode.
In this mode, lines of input are collected and input editing
is done.
The edited line is made available when it is completed by
a newline or when the
.I t_brkc 
character (normally undefined) or
.I t_eofc 
character (normally an EOT, CTRL/D) is entered.
A carriage return is usually made synonymous with newline in this mode
and replaced with a newline whenever it is typed.
All driver functions
(input editing, interrupt generation,
output processing such as delay generation and tab expansion, and
so forth) are available in this mode.
.IP CBREAK 10
.NXR "CBREAK mode" "defined"
This mode eliminates the character, word, and 
line editing input facilities,
making the input character available to the user program as it is typed.
Flow control, literal-next, and
interrupt processing are still done in this mode.
Output processing is done.
.IP RAW 10
.NXR "RAW mode" "defined"
This mode eliminates all input processing and makes all input characters
available as they are typed; no output processing is done either.
.PP
The style of input processing can also be different when
the terminal is put in nonblocking I/O mode. 
For further information, see the FNDELAY flag described in
.MS fcntl 2 .
In this case, a 
.MS read 2
from the control terminal never blocks.  Rather, it returns
an error indication (EWOULDBLOCK), if there is no
input available.
.PP
A process may also request a SIGIO signal be sent it whenever input
is present
and also whenever output queues fall below the low-water mark.
To enable this mode, the FASYNC flag should be set using
.MS fcntl 2 .
.PP
.B "Input Editing"
.NXR "terminal" "editing input"
.PP
An ULTRIX terminal 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.
This limit is 256 characters.
In RAW mode, the terminal
driver throws away all input and output without notice
when the limit is reached.
In CBREAK mode or cooked mode, 
it refuses to accept any further input and, if
in the new line discipline, rings the terminal bell.
.PP
Input characters are normally accepted in either even or odd parity,
with the parity bit being stripped off before the character is given to
the program.  By clearing either the EVEN or ODD bit in the flags word, 
it is possible to have input characters
with that parity discarded (see the \fBSummary\fR).
.PP
In all of the line disciplines, it is possible to simulate terminal
input using the TIOCSTI ioctl, which takes, as its third argument,
the address of a character.  The system pretends that this character
was typed on the argument terminal,
which must be the control terminal, except
for the superuser. (This call is not in standard Version 7 UNIX.)
.PP
Input characters are normally echoed by putting them in an output queue
as they arrive.  This may be disabled by clearing the ECHO bit in the
flags word using the
.MS stty 3
call or the TIOCSETN or TIOCSETP ioctl
(see the \fBSummary\fR).
.PP
In cooked mode, terminal input is processed in units of lines.
A program attempting
to read is normally suspended until an entire line has been
received
(but, see the description of SIGTTIN in \fBJob Access Control\fR 
and FIONREAD in \fBSummary of modes\fR.)
No matter how many characters are requested
in the read call, at most one line is returned.
It is not, however, necessary to read a whole line at
once; any number of characters can be
requested in a read, even one, without losing information.
.PP
During input, line editing is normally done, with the erase character 
.I sg_erase
(by default, the number sign (#))
logically erasing the last character typed and the 
.I sg_kill
character (by default, the at sign (@))
logically erasing the entire current input line.
These are often reset on CRTs,
with CTRL/H replacing the number sign (#),
and CTRL/U replacing the at sign (@).
These characters
never erase beyond the beginning of the current input line or an EOF.
These characters may be entered literally, by
preceding them with a backslash (\e\|). 
In the old Teletype driver, both the backslash (\e\|) and
the character entered literally appear on the screen; in the new
driver, the (\e\|) normally disappears.
.PP
The drivers normally treat either
a carriage return or a newline character
as terminating an input line,
replacing the return with a newline and echoing
a return and a line feed.
If the CRMOD bit is cleared in the local mode word, then the processing
for carriage return is disabled, and it is simply echoed as a return
and does not terminate cooked mode input.
.PP
In the new driver, there is a literal-next character (normally CTRL/V), 
which can be typed
in both cooked and CBREAK mode preceding any
character to prevent its special meaning to the terminal handler. 
This is to be preferred to the
use of the backslash (\e\|) escaping erase and
kill characters, but the backslash (\e\|) is retained with its
old function in the new driver for historical reasons.
.PP
The new terminal driver also provides two other editing characters in
normal mode.  The word-erase character, 
normally CTRL/W, erases the preceding
word, but not any spaces before it.  For the purposes of CTRL/W, a word
is defined as a sequence of nonblank characters, with tabs counted as
blanks.
Finally, the reprint character,
normally CTRL/R, retypes the pending input beginning
on a new line.
Retyping occurs automatically in cooked mode, if characters
that would normally be erased 
from the screen are fouled by program output.
.PP
.B "Input Echoing and Redisplay"
.NXR "terminal" "echoing input"
.PP
In the old terminal driver,
the erase character is simply echoed.  When a kill character
is typed, it is echoed, followed by a newline (even if the character is
not killing the line, because it was preceded by a backslash (\e\|).)
.PP
The new terminal driver has several modes for handling the echoing of
terminal input, controlled by bits in a local mode word.
.PP
.I "Hardcopy terminals."
When a hardcopy terminal is in use, the LPRTERA bit is normally set in
the local mode word.  Characters that are logically erased are
then printed out backwards,
preceded by a backslash (\e\|) and followed by a slash (/) in this mode.
.PP
.I "CRT terminals"
When a CRT terminal is in use, the
LCRTBS bit is normally set in the local
mode word.  The terminal driver echoes the proper number of erase
characters when input is erased. In the normal case, where the erase
character is a CTRL/H, this causes the cursor of the terminal to back up
to where it was before the logically erased character was typed.
If the input has become fouled due to interspersed asynchronous output,
the input is automatically retyped.
.PP
.I "Erasing characters from a CRT"
When a CRT terminal is in use, the LCRTERA bit can be set to cause