termio(7) CLIX termio(7)
NAME
termio - General terminal interface
DESCRIPTION
All asynchronous communications ports use the same general interface,
regardless of the hardware 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, user programs seldom open
terminal files; they are opened by getty and become a user's stdin,
stdout, and stderr files. The 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
(discussed below). The control terminal is inherited by a child process
during a fork() function. A process can break this association by
changing its process group using setpgrp() function.
A terminal associated with one of these files ordinarily operates in
full-duplex mode. Characters may be typed 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
discarded without notice.
Normally, terminal input is processed in units of lines. A line is
delimited by a newline (ASCII LF) character, an end-of-file (ASCII EOT)
character, or an end-of-line (ASCII EOL) 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. However, it is not necessary to
read a whole line at once; any number of characters may be requested in a
read (even one character) without losing information.
During input, erase and kill processing is normally performed. By
default, the number sign (#) erases the last character typed, except that
it will not erase beyond the beginning of the line. By default, the at
(@) character deletes the entire input line and optionally outputs a
newline character. Both characters operate on a key-stroke basis,
independent from any backspacing or tabbing. Both the erase and delete
characters may be entered literally by preceding them with the escape
character (\). In this case, the escape character is not read. The erase
and delete characters may be changed.
Certain characters have special functions on input. These functions and
their default character values are summarized as follows:
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termio(7) CLIX termio(7)
INTR (Rubout or ASCII DEL) Generates an interrupt signal sent to all
processes with the associated control terminal. Normally, each
process is forced to terminate, but arrangements may be made to
ignore the signal or to receive a trap to an agreed-on location.
(See signal(2).)
QUIT (<Ctrl-L> 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 (<Ctrl-Z> or ASCII SUB) Is used by the job control facility, shl,
to change the current layer to the control layer.
SUSP (<Ctrl-Z> or ASCII SUB) Generates a SIGTSTP signal sent to all
processes with the associated control terminal. Normally, each
process is forced to stop, but arrangements may be made either to
ignore the signal or to receive a trap to an agreed-upon location.
(See signal(2).)
ERASE (#) Erases the preceding character. It will not erase beyond the
start of a line, as delimited by a NL, EOF, or EOL character.
KILL (@) Deletes the entire line, as delimited by a NL, EOF, or EOL
character.
EOF (<Ctrl-D> or ASCII EOT) Generates an end-of-file from a terminal.
When received, all characters waiting to be read are immediately
passed to the program, without waiting for a newline, and the EOF
is discarded. Thus, if no characters are waiting (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 cannot be changed or
escaped.
EOL (ASCII NUL) Is an additional line delimiter, like NL. It is not
normally used.
EOL2 Is an additional line delimiter.
STOP (<Ctrl-S> or ASCII DC3) Temporarily suspend output. It is used
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 (<Ctrl-Q> or ASCII DC1) Resumes output that has been suspended by
a STOP character. While output is not suspended, START characters
are ignored and not read. The start/stop characters cannot be
changed or escaped.
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termio(7) CLIX termio(7)
The character values for INTR, QUIT, SWTCH, SUSP, ERASE, KILL, EOF, and
EOL may be changed. The ERASE, KILL, and EOF characters may be escaped by
a preceding backslash (\) character. In this case, no special function is
performed.
When the carrier signal from the data-set drops, a SIGHUP 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 SIGHUP 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 is written, the characters are transmitted to
the terminal as soon as previously-written characters finish 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
entered, it will be suspended when its output queue exceeds its limit.
When the queue has drained to a threshold, the program is resumed.
Several ioctl() functions apply to terminal files. The primary calls use
the following structure defined in <termio.h>:
#define NCC 9
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
1 VQUIT
2 VERASE
3 VKILL
4 VEOF
5 VEOL
6 VEOL2
4 VMIN
5 VTIME
7 VSWTCH
8 VSUSP
The c_iflag field describes the basic terminal input control:
IGNBRK 0000001 Ignore break condition.
BRKINT 0000002 Signal interrupt on break.
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termio(7) CLIX termio(7)
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 uppercase to lowercase 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 (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 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.
If ISTRIP is set, valid input characters are first stripped to seven bits.
(The high-order bit is turned to 0.) Otherwise, all eight 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 uppercase alphabetic character is translated
into the corresponding lowercase 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 processed and discarded (not placed
in the input stream). If IXANY is set, any input character, will restart
output that has been suspended.
If IXOFF is set, the system will transmit START/STOP characters when the
input queue is nearly empty/full.
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termio(7) CLIX termio(7)
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 lowercase to uppercase on output.
ONLCR 0000004 Map NL to CR-NL on output.
OCRNL 0000010 Map CR to NL on output.
ONOCR 0000020 No CR is 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 newline delays:
NL0 0000000
NL1 0000400
CRDLY 0003000 Select carriage-return delays:
CR0 0000000
CR1 0001000
CR2 0002000
CR3 0003000
TABDLY 0014000 Select horizontal-tab delays:
TAB0 0000000
TAB1 0004000
TAB2 0010000
TAB3 0014000 Expand tabs to spaces.
BSDLY 0020000 Select backspace delays:
BS0 0000000
BS1 0020000
VTDLY 0040000 Select vertical-tab delays:
VT0 0000000
VT1 0040000
FFDLY 0100000 Select formfeed delays:
FF0 0000000
FF1 0100000
If OPOST is set, output characters are postprocessed as indicated by the
remaining flags. Otherwise, characters are transmitted without change.
If OLCUC is set, a lowercase alphabetic character is transmitted as the
corresponding uppercase character. This function is often used 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 at column 0
(first position). If ONLRET is set, the NL character is assumed to
perform 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 perform only the linefeed function; the column pointer will
remain unchanged. The column pointer is also set to 0 if the CR character
is actually transmitted.
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termio(7) CLIX termio(7)
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 transmitted for delay instead of a timed delay. This
is useful for high baud rate terminals that need only a minimal delay. If
OFDEL is set, the fill character is DEL. Otherwise, it is NUL.
If a formfeed or vertical-tab delay is specified, it lasts for about two
seconds.
Newline delay lasts about 0.10 seconds. If ONLRET is set, the carriage-
return delays are used instead of the newline delays. If OFILL is set,
two fill characters will be transmitted.
Carriage-return delay type 1 depends 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 transmits four
fill characters.
Horizontal-tab delay type 1 depends 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 0000000 Hang up
B50 0000001 50 baud
B75 0000002 75 baud
B110 0000003 110 baud
B134 0000004 134 baud
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
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termio(7) CLIX termio(7)
EXTA 0000016 External A
B38400 0000017 38400 baud
EXTB 0000017 External B
CSIZE 0000060 Character size:
CS5 0000000 5 bits
CS6 0000020 6 bits
CS7 0000040 7 bits
CS8 0000060 8 bits
CSTOPB 0000100 Send two stop bits. Otherwise, send one.
CREAD 0000200 Enable receiver.
PARENB 0000400 Enable parity.
PARODD 0001000 Use odd parity. Otherwise, use even.
HUPCL 0002000 Hang up on last close.
CLOCAL 0004000 Set local line. Otherwise, set dial-up.
LOBLK 0040000 Block layer output.
The CBAUD bits specify the baud rate. The zero baud rate, B0, hangs 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 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 is used. For
example, at 110 baud, two stops bits are required.
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.
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termio(7) CLIX termio(7)
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.
TOSTOP 0000400 SIGTTOU on background output.
If ISIG is set, each input character is checked against the special
control characters INTR, SWTCH, SUSP, and QUIT. If an input character
matches one of these control characters, the function associated with that
character is performed. If ISIG is not set, no checking is performed.
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 (such as 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 and ICANON are set, an uppercase letter is accepted on input when
it is preceded by a \ character and is output preceded by a \ character.
In this mode, the following escape sequences are generated on output and
accepted at 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 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
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termio(7) CLIX termio(7)
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 (known as half duplex). Unless
escaped, the EOF character is not echoed. EOT being the default EOF
character 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
performed.
If TOSTOP is set, background processes attempting to write to a
controlling terminal will generate a SIGTTOU signal sent to all processes
with the associated control terminal.
The initial line-discipline control value is all bits clear.
The primary ioctl() functions have the following form:
ioctl(fildes, request, arg)
struct termio *arg;
The requests using this form are as follows:
TCGETA Gets the parameters associated with the terminal and stores in
the termio structure referenced by arg.
TCSETA Sets the parameters associated with the terminal from the
structure referenced by arg. The change is immediate.
TCSETAW Waits for the output to drain before setting the new parameters.
This form should be used when changing parameters that will
affect output.
TCSETAF Waits for the output to drain, flushes the input queue, and sets
the new parameters.
Additional ioctl() calls have the following form:
ioctl(fildes, request, arg)
int arg;
The requests using this form are as follows:
TCSBRK Waits for the output to drain. If arg is 0, sends a break (zero
bits for 0.25 seconds).
TCXONC Starts/stops control. If arg is 0, suspends output; if 1,
restarts suspended output.
TCFLSH If arg is 0, flushes the input queue; if 1, flushes the output
queue; and if 2, flushes both the input and output queues.
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termio(7) CLIX termio(7)
FILES
/dev/tty Terminal interface.
RELATED INFORMATION
Commands: stty(1)
Functions: setpgrp(2), signal(2), fork(2), ioctl(2)
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