TERMIO(7) TERMIO(7)
NAME
termio - general terminal interface
DESCRIPTION
All of the asynchronous communications ports use the same
general interface, no matter what hardware is involved. The
remainder of this section discusses the common features of
this interface.
When a terminal file is opened, it normally causes the
process to wait until a connection is established. In
practice, users' programs seldom open terminal files; they
are opened by getty and become a user's standard input,
output, and error files. The very first terminal file
opened by the process group leader of a terminal file not
already associated with a process group becomes the control
terminal for that process group. The control terminal plays
a special role in handling quit and interrupt signals, as
discussed below. The control terminal is inherited by a
child process during a fork(2). A process can break this
association by changing its process group using setpgrp(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
full, 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, the buffer is
flushed and all the saved characters are thrown away without
notice.
Normally, terminal input is processed in units of lines. A
line is delimited by a new-line (ASCII LF) character, an
end-of-file (ASCII EOT) character, or an end-of-line
character. 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,
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TERMIO(7) TERMIO(7)
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 the
line. By default, the character @ kills (deletes) the
entire input line, and optionally outputs a new-line
character. Both these characters operate on a key-stroke
basis, independently of any backspacing or tabbing that may
have been done. Both the erase and kill characters may be
entered literally by preceding them with the escape
character (\). In this case the escape character is not
read. The erase and kill characters may be changed.
Certain characters have special functions on input. These
functions and their default character values are summarized
as follows:
INTR (Rubout or ASCII DEL) 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).
QUIT (Control-| or ASCII FS) generates a 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 (called core) will be created
in the current working directory.
SWTCH (Control-z or ASCII SUB) is used by the job control
facility, shl, to change the current layer to the
control layer.
ERASE (#) erases the preceding character. It will not
erase beyond the start of a line, as delimited by a
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TERMIO(7) TERMIO(7)
NL, EOF, or EOL character.
KILL (@) deletes the entire line, as delimited by a NL,
EOF, or EOL character.
EOF (Control-d or ASCII EOT) may be used to generate an
end-of-file from a terminal. When received, all the
characters waiting to be read are immediately passed
to the program, without waiting for a new-line, and
the EOF is discarded. Thus, if there are no
characters waiting, which is to say the EOF occurred
at the beginning of a line, zero characters will be
passed back, which is the standard end-of-file
indication.
NL (ASCII LF) is the normal line delimiter. It can not
be changed or escaped.
EOL (ASCII NUL) is an additional line delimiter, like
NL. It is not normally used.
EOL2 is another additional line delimiter.
STOP (Control-s or ASCII DC3) can be used to temporarily
suspend output. It is useful with CRT terminals to
prevent output from disappearing before it can be
read. While output is suspended, STOP characters
are ignored and not read.
START (Control-q or ASCII DC1) is used to resume output
which has been suspended by a STOP character. While
output is not suspended, START characters are
ignored and not read. The start/stop characters can
not be changed or escaped.
The character values for INTR, QUIT, SWTCH, ERASE, KILL,
EOF, and EOL may be changed to suit individual tastes. The
ERASE, KILL, and EOF characters may be escaped by a
preceding \ character, in which case no special function is
done.
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TERMIO(7) TERMIO(7)
When the carrier signal from the data-set drops, a hang-up
signal is sent to all processes that have this terminal as
the control terminal. Unless other arrangements have been
made, this signal causes the processes to terminate. If the
hang-up signal is ignored, any subsequent read returns with
an end-of-file indication. Thus, programs that read a
terminal and test for end-of-file can terminate
appropriately when hung up on.
When one or more characters are written, they are
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.
If 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.
Several ioctl(2) system calls apply to terminal files. The
primary calls use the following structure, defined in
<termio.h>:
#define NCC 8
struct termio {
unsigned short c_iflag;/* input modes */
unsigned short c_oflag;/* output modes */
unsigned short c_cflag;/* control modes */
unsigned short c_lflag;/* local modes */
char c_line; /* line discipline */
unsigned char c_cc[NCC];/* control chars */
};
The special control characters are defined by the array
c_cc. The relative positions and initial values for each
function are as follows:
0 VINTR DEL
1 VQUIT FS
2 VERASE #
3 VKILL @
4 VEOF EOT
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TERMIO(7) TERMIO(7)
5 VEOL NUL
6 reserved
7 SWTCH
The c_iflag field describes the basic terminal input
control:
IGNBRK 0000001 Ignore break condition.
BRKINT 0000002 Signal interrupt on break.
IGNPAR 0000004 Ignore characters with parity errors.
PARMRK 0000010 Mark parity errors.
INPCK 0000020 Enable input parity check.
ISTRIP 0000040 Strip character.
INLCR 0000100 Map NL to CR on input.
IGNCR 0000200 Ignore CR.
ICRNL 0000400 Map CR to NL on input.
IUCLC 0001000 Map upper-case to lower-case on input.
IXON 0002000 Enable start/stop output control.
IXANY 0004000 Enable any character to restart output.
IXOFF 0010000 Enable start/stop input control.
If IGNBRK is set, the break condition (a character framing
error with data all zeros) is ignored, that is, not put on
the input queue and therefore not read by any process.
Otherwise if BRKINT is set, the break condition will
generate an interrupt signal and flush both the input and
output queues. If IGNPAR is set, characters with other
framing and parity errors are ignored.
If PARMRK is set, a character with a framing or parity error
which is not ignored is read as the three-character
sequence: 0377, 0, X, where X is the data of the character
received in error. To avoid ambiguity in this case, if
ISTRIP is not set, a valid character of 0377 is read as
0377, 0377. If PARMRK is not set, a framing or parity error
which is not ignored is read as the character NUL (0).
If INPCK is set, input parity checking is enabled. If INPCK
is not set, input parity checking is disabled. This allows
output parity generation without input parity errors.
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TERMIO(7) TERMIO(7)
If ISTRIP is set, valid input characters are first stripped
to 7-bits, otherwise all 8-bits are processed.
If INLCR is set, a received NL character is translated into
a CR character. If IGNCR is set, a received CR character is
ignored (not read). Otherwise if ICRNL is set, a received
CR character is translated into a NL character.
If IUCLC is set, a received upper-case alphabetic character
is translated into the corresponding lower-case character.
If IXON is set, start/stop output control is enabled. A
received STOP character will suspend output and a received
START character will restart output. All start/stop
characters are ignored and not read. If IXANY is set, any
input character, will restart output which has been
suspended.
If IXOFF is set, the system will transmit START/STOP
characters when the input queue is nearly empty/full.
The initial input control value is all-bits-clear.
The c_oflag field specifies the system treatment of output:
OPOST 0000001 Postprocess output.
OLCUC 0000002 Map lower case to upper on output.
ONLCR 0000004 Map NL to CR-NL on output.
OCRNL 0000010 Map CR to NL on output.
ONOCR 0000020 No CR output at column 0.
ONLRET 0000040 NL performs CR function.
OFILL 0000100 Use fill characters for delay.
OFDEL 0000200 Fill is DEL, else NUL.
NLDLY 0000400 Select new-line delays:
NL0 0
NL1 0000400
CRDLY 0003000 Select carriage-return delays:
CR0 0
CR1 0001000
CR2 0002000
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TERMIO(7) TERMIO(7)
CR3 0003000
TABDLY 0014000 Select horizontal-tab delays:
TAB0 0
TAB1 0004000
TAB2 0010000
TAB3 0014000 Expand tabs to spaces.
BSDLY 0020000 Select backspace delays:
BS0 0
BS1 0020000
VTDLY 0040000 Select vertical-tab delays:
VT0 0
VT1 0040000
FFDLY 0100000 Select form-feed delays:
FF0 0
FF1 0100000
If OPOST is set, output characters are post-processed as
indicated by the remaining flags, otherwise characters are
transmitted without change.
If OLCUC is set, a lower-case alphabetic character is
transmitted as the corresponding upper-case character. This
function is often used in conjunction with IUCLC.
If ONLCR is set, the NL character is transmitted as the CR-
NL character pair. If OCRNL is set, the CR character is
transmitted as the NL character. If ONOCR is set, no CR
character is transmitted when at column 0 (first position).
If ONLRET is set, the NL character is assumed to do the
carriage-return function; the column pointer will be set to
0 and the delays specified for CR will be used. Otherwise
the NL character is assumed to do just the line-feed
function; the column pointer will remain unchanged. The
column pointer is also set to 0 if the CR character is
actually transmitted.
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. If OFILL is set, fill characters will be
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TERMIO(7) TERMIO(7)
transmitted for delay instead of a timed delay. This is
useful for high baud rate terminals which need only a
minimal delay. If OFDEL is set, the fill character is DEL,
otherwise NUL.
If a form-feed or vertical-tab delay is specified, it lasts
for about 2 seconds.
New-line delay lasts about 0.10 seconds. If ONLRET is set,
the carriage-return delays are used instead of the new-line
delays. If OFILL is set, two fill characters will be
transmitted.
Carriage-return delay type 1 is dependent on the current
column position, type 2 is about 0.10 seconds, and type 3 is
about 0.15 seconds. If OFILL is set, delay type 1 transmits
two fill characters, and type 2, four fill characters.
Horizontal-tab delay type 1 is dependent on the current
column position. Type 2 is about 0.10 seconds. Type 3
specifies that tabs are to be expanded into spaces. If
OFILL is set, two fill characters will be transmitted for
any delay.
Backspace delay lasts about 0.05 seconds. If OFILL is set,
one fill character will be transmitted.
The actual delays depend on line speed and system load.
The initial output control value is all bits clear.
The c_cflag field describes the hardware control of the
terminal:
CBAUD 0000017 Baud rate:
B0 0 Hang up
B50 0000001 50 baud
B75 0000002 75 baud
B110 0000003 110 baud
B134 0000004 134 baud
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TERMIO(7) TERMIO(7)
B150 0000005 150 baud
B200 0000006 200 baud
B300 0000007 300 baud
B600 0000010 600 baud
B1200 0000011 1200 baud
B1800 0000012 1800 baud
B2400 0000013 2400 baud
B4800 0000014 4800 baud
B9600 0000015 9600 baud
B19200 0000016 19200 baud
EXTA 0000016 External A
B38400 0000017 38400 baud
EXTB 0000017 External B
CSIZE 0000060 Character size:
CS5 0 5 bits
CS6 0000020 6 bits
CS7 0000040 7 bits
CS8 0000060 8 bits
CSTOPB 0000100 Send two stop bits, else one.
CREAD 0000200 Enable receiver.
PARENB 0000400 Parity enable.
PARODD 0001000 Odd parity, else even.
HUPCL 0002000 Hang up on last close.
CLOCAL 0004000 Local line, else dial-up.
RCV1EN 0010000
XMT1EN 0020000
LOBLK 0040000 Block layer output.
The CBAUD bits specify the baud rate. The zero baud rate,
B0, is used to hang up the connection. If B0 is specified,
the data-terminal-ready signal will not be asserted.
Normally, this will disconnect the line. For any particular
hardware, impossible speed changes are ignored.
The CSIZE bits specify the character size in bits for both
transmission and reception. This size does not include the
parity bit, if any. If CSTOPB is set, two stop bits are
used, otherwise one stop bit. For example, at 110 baud, two
stops bits are required.
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TERMIO(7) TERMIO(7)
If PARENB is set, parity generation and detection is enabled
and a parity bit is added to each character. If parity is
enabled, the PARODD flag specifies odd parity if set,
otherwise even parity is used.
If CREAD is set, the receiver is enabled. Otherwise no
characters will be received.
If HUPCL is set, the line will be disconnected when the last
process with the line open closes it or terminates. That
is, the data-terminal-ready signal will not be asserted.
If CLOCAL is set, the line is assumed to be a local, direct
connection with no modem control. Otherwise modem control
is assumed.
If LOBLK is set, the output of a job control layer will be
blocked when it is not the current layer. Otherwise the
output generated by that layer will be multiplexed onto the
current layer.
The initial hardware control value after open is B300, CS8,
CREAD, HUPCL.
The c_lflag field of the argument structure is used by the
line discipline to control terminal functions. The basic
line discipline (0) provides the following:
ISIG 0000001 Enable signals.
ICANON 0000002 Canonical input (erase and kill processing).
XCASE 0000004 Canonical upper/lower presentation.
ECHO 0000010 Enable echo.
ECHOE 0000020 Echo erase character as BS-SP-BS.
ECHOK 0000040 Echo NL after kill character.
ECHONL 0000100 Echo NL.
NOFLSH 0000200 Disable flush after interrupt or quit.
If ISIG is set, each input character is checked against the
special control characters INTR, SWTCH, and QUIT. If an
input character matches one of these control characters, the
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TERMIO(7) TERMIO(7)
function associated with that character is performed. If
ISIG is not set, no checking is done. Thus these special
input functions are possible only if ISIG is set. These
functions may be disabled individually by changing the value
of the control character to an unlikely or impossible value
(e.g., 0377).
If ICANON is set, canonical processing is enabled. This
enables the erase and kill edit functions, and the assembly
of input characters into lines delimited by NL, EOF, and
EOL. If ICANON is not set, read requests are satisfied
directly from the input queue. A read will not be satisfied
until at least MIN characters have been received or the
timeout value TIME has expired between characters. This
allows fast bursts of input to be read efficiently while
still allowing single character input. The MIN and TIME
values are stored in the position for the EOF and EOL
characters, respectively. The time value represents tenths
of seconds.
If XCASE is set, and if ICANON is set, an upper-case letter
is accepted on input by preceding it with a \ character, and
is output preceded by a \ character. In this mode, the
following escape sequences are generated on output and
accepted on input:
for: use:
` \'
| \!
~ \^
{ \(
} \)
\ \\
For example, A is input as \a, \n as \\n, and \N as \\\n.
If ECHO is set, characters are echoed as received.
When ICANON is set, the following echo functions are
possible. If ECHO and ECHOE are set, the erase character is
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TERMIO(7) TERMIO(7)
echoed as ASCII BS SP BS, which will clear the last
character from a CRT screen. If ECHOE is set and ECHO is
not set, the erase character is echoed as ASCII SP BS. If
ECHOK is set, the NL character will be echoed after the kill
character to emphasize that the line will be deleted. Note
that an escape character preceding the erase or kill
character removes any special function. If ECHONL is set,
the NL character will be echoed even if ECHO is not set.
This is useful for terminals set to local echo (so-called
half duplex). Unless escaped, the EOF character is not
echoed. Because EOT is the default EOF character, this
prevents terminals that respond to EOT from hanging up.
If NOFLSH is set, the normal flush of the input and output
queues associated with the quit, switch, and interrupt
characters will not be done.
The initial line-discipline control value is all bits clear.
The primary ioctl(2) system calls have the form:
ioctl (fildes, command, arg)
struct termio *arg;
The commands using this form are:
TCGETA Get the parameters associated with the
terminal and store in the termio structure
referenced by arg.
TCSETA Set the parameters associated with the
terminal from the structure referenced by
arg. The change is immediate.
TCSETAW Wait for the output to drain before setting
the new parameters. This form should be used
when changing parameters that will affect
output.
TCSETAF Wait for the output to drain, then flush the
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TERMIO(7) TERMIO(7)
input queue and set the new parameters.
Additional ioctl(2) calls have the form:
ioctl (fildes, command, arg)
int arg;
The commands using this form are:
TCSBRK Wait for the output to drain. If arg is 0,
then send a break (zero bits for 0.25
seconds).
TCXONC Start/stop control. If arg is 0, suspend
output; if 1, restart suspended output.
TCFLSH If arg is 0, flush the input queue; if 1,
flush the output queue; if 2, flush both the
input and output queues.
FILES
/dev/tty*
SEE ALSO
stty(1) in the User's Reference Manual.
fork(2), ioctl(2), setpgrp(2), signal(2) in the Programmer's
Reference Manual.
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