termio(4) Kernel Interfaces Manual termio(4)
Name
termio - System V terminal interface
Description
This section specificly describes the System V terminal interface. A general description of the available terminal interfaces is provided
in
When a terminal file is opened, it normally causes the process to wait until a connection is established. In practice, users' programs
seldom open these files; they are opened by 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 later. The control terminal
is inherited by a child process during a A process can break this association by changing its process group using
A terminal associated with one of these files ordinarily operates in full-duplex mode. Characters can be typed at any time, even while
output is occurring. They 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
(MAX_INPUT) characters, as defined in When the input limit is reached, all the saved characters are thrown away 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 character. This means that a program attempting to read is suspended until an entire line has been typed. 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 can be requested in a read, even one, without losing information.
Some characters have special meaning when input. For example, during input erase and kill, processing is normally done. The erase charac-
ter erases the last character typed, except that it will not erase beyond the beginning of a line. Typically the erase character is the
number sign (#). The kill character kills (deletes) the entire input line, and optionally outputs a newline character. The default kill
character is the at sign (@). Both characters operate on a key-stroke basis, independently of any backspacing or tabbing. Both the erase
and kill characters can be entered literally by preceding them with the escape character (). In this case the escape character is not
read. The erase and kill characters can be changed.
Certain characters have special functions on input. These functions and their default character values are summarized as follows:
(Rubout or ASCII DEL)
generates an interrupt signal that is sent to all processes with the associated control terminal. Normally, each such process is
forced to terminate, but arrangements can be made either to ignore the signal or to receive a trap to an agreed-upon location; see
(CTRL/| 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 is not only terminated but a core image file (called core) is created in the current working directory.
The number sign (#) erases the preceding character.
It will not erase beyond the start of a line, as delimited by an NL, EOF, or EOL character.
The at sign (@) deletes the entire line,
as delimited by an NL, EOF, or EOL character.
(CTRL/D or ASCII EOT) can
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 newline, 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 are passed back, which is the standard end-of-file indication.
(ASCII LF) is the normal line delimiter.
It can not be changed or escaped.
(ASCII NUL) is an additional line delimiter, like NL.
It is not normally used.
(CTRL/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.
(CTRL/Q or ASCII DC1)
is used to resume 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 can not be changed or escaped.
Used to control terminal I/O when the ICANON flag is not set in the c_lflag.
Input processing behaves as described in the MIN/TIME Interaction section that follows.
Used to control terminal I/O when the ICANON flag is not set in the c_lflag.
Input processing behaves as described in the MIN/TIME Interaction section that follows.
The character values for INTR, QUIT, ERASE, KILL, EOF, MIN, TIME, and EOL can be changed to suit individual tastes. The ERASE, KILL, and
EOF characters can be escaped by a preceding backslash () character, in which case no special function is performed.
When one or more characters are written, they are transmitted to the terminal as soon as previously-written characters have finished typ-
ing. 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 is suspended when its output queue exceeds some limit. When the queue has drained down to some threshold, the pro-
gram is resumed.
When the carrier signal from the data-set drops, a hang-up signal, SIGHUP, 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 appro-
priately when hung up on.
Several system calls apply to terminal files. The primary calls use the following structure, defined in
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 initial values for each function are as follows:
VINTR DEL
VQUIT FS
VERASE #
VKILL @
VEOF EOT
VEOL NUL
VMIN 6
VTIME 1
The c_iflag field describes the basic terminal input control:
IGNBRK Ignore break condition.
BRKINT Signal interrupt on break.
IGNPAR Ignore characters with parity errors.
PARMRK Mark parity errors.
INPCK Enable input parity check.
ISTRIP Strip character.
INLCR Map NL to CR on input.
IGNCR Ignore CR.
ICRNL Map CR to NL on input.
IUCLC Map uppercase to lowercase on input.
IXON Enable start/stop output control.
IXANY Enable any character to restart output.
IXOFF 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 generates an interrupt signal and flushes 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 that 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 that 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 gen-
eration, without input parity errors.
If ISTRIP is set, valid input characters are first stripped to seven bits. 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 letter is translated into the corresponding lowercase letter.
If IXON is set, start/stop output control is enabled. A received STOP character suspends output and a received START character restarts
output. All start/stop characters are ignored and not read. If IXANY is set, any input character restarts output that has been suspended.
If IXOFF is set, the system transmits 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 Postprocess output.
OLCUC Map lower case to upper on output.
ONLCR Map NL to CR-NL on output.
OCRNL Map CR to NL on output.
ONOCR No CR output at column 0.
ONLRET NL performs CR function.
OFILL Use fill characters for delay.
OFDEL Fill is DEL or else NUL.
NLDLY Select newline delays:
NL0 Newline delay type 0.
NL1 Newline delay type 1.
CRDLY Select carriage-return delays:
CR0 Carriage-return delay type 0.
CR1 Carriage-return delay type 1.
CR2 Carriage-return delay type 2.
CR3 Carriage-return delay type 3.
TABDLY Select horizontal-tab delays:
TAB0 Horizontal-tab delay type 0.
TAB1 Horizontal-tab delay type 1.
TAB2 Horizontal-tab delay type 2.
TAB3 Expand tabs to spaces.
BSDLY Select backspace delays:
BS0 Backspace delay type 0.
BS1 Backspace delay type 1.
VTDLY Select vertical-tab delays:
VT0 Vertical-tab delay type 0.
VT1 Vertical-tab delay type 1.
FFDLY Select form-feed delays:
FF0 Form-feed delay type 0.
FF1 Form-feed delay type 1.
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 lowercase letter is transmitted as the corresponding uppercase letter. This function is often used in conjunction with
IUCLCS.
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 is set to 0 and the delays specified for CR are used. Otherwise, the NL
character is assumed to do just the line-feed function; the column pointer remains 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 termi-
nal. In all cases, a value of 0 indicates no delay. If OFILL is set, fill characters is 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 form-feed or vertical-tab delay is specified, it lasts for about two seconds.
A 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 is transmitted.
A carriage-return delay type 1 is dependent on the current column position. The type 2 delay is about 0.10 seconds; the type 3 delay is
about 0.15 seconds. If OFILL is set, delay type 1 transmits two fill characters; type 2 transmits four fill characters.
A horizontal-tab delay type 1 is dependent on the current column position. The type 2 delay is about 0.10 seconds. Type 3 specifies that
tabs are to be expanded into spaces. If OFILL is set, two fill characters is transmitted for any delay.
A backspace delay lasts about 0.05 seconds. If SM 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 Baud rate:
B0 Hang up
B50 50 baud
B75 75 baud
B110 110 baud
B134 134.5 baud
B150 150 baud
B200 200 baud
B300 300 baud
B600 600 baud
B1200 1200 baud
B1800 1800 baud
B2400 2400 baud
B4800 4800 baud
B9600 9600 baud
B19200 19200 baud
B38400 38400 baud
EXTA External A (Same as B19200)
EXTB External B (Same as B38400)
CSIZE Character size:
CS5 5 bits
CS6 6 bits
CS7 7 bits
CS8 8 bits
CSTOPB Send two stop bits, otherwise one.
CREAD Enable receiver.
PARENB Parity enable.
PARODD Odd parity, otherwise even.
HUPCL Hang up on last close.
CLOCAL Local line, otherwise dial-up.
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, only 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-termi-
nal-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.
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 Enable signals.
ICANON Canonical input (erase and kill processing).
XCASE Canonical upper/lower presentation.
ECHO Enable echo.
ECHOE Echo erase character as BS-SP-BS.
ECHOK Echo NL after kill character.
ECHONL Echo NL.
NOFLSH 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 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 can be disabled individually by changing the
value of the control character to an unlikely or impossible value (for example, 0).
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 is not
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 backslash (), and is output pre-
ceded by a backslash (). In this mode, the following escape sequences are generated on output and accepted on input:
for ` | ~ { }
use ' ! ^ ( )
For example, A is input as a,
as \n, and N as \
.
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 clears 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 is echoed after the kill character to emphasize that the line is deleted. Note that an escape character
preceding the erase or kill character removes any special function. If ECHONL is set, the NL character is 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, terminals that respond to EOT are prevented 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 c_line field of the argument structure is used to specify the line discipline. Support is provided for the basic termio line disci-
pline only. For this reason, the value of this field is irrelevant and should be set to zero (0) by convention.
The primary 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 input queue and set the new parameters.
Additional 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, 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.
MIN/TIME Interaction
MIN represents the minimum number of characters that should be received when the read is satisfied (that is, the characters are returned to
the user). TIME is a timer of 0.10 second granularity used to time-out bursty and short-term data transmissions. The four possible values
for MIN and TIME and their interactions follow:
1. MIN > 0, TIME > 0. In this case, TIME serves as an intercharacter timer activated after the first character is received and reset upon
receipt of each character. MIN and TIME interact as follows:
As soon as one character is received the intercharacter timer is started.
If MIN characters are received before the intercharacter timer expires, the read is satisfied.
If the timer expires before MIN characters are received, the characters received to that point are returned to the user.
A operation will sleep until the MIN and TIME mechanism are activated by the receipt of the first character; thus, at least one character
must be returned.
2. MIN > 0, TIME = 0. In this case, because TIME = 0, the timer plays no role and only MIN is significant. A operation is not satisfied
until MIN characters are received.
3. MIN = 0, TIME > 0. In this case, because MIN = 0, TIME no longer serves as an intercharacter timer, but now serves as a read timer
that is activated as soon as the operation is processed. A operation is satisfied as soon as a single character is received or the timer
expires, in which case, the operation would not return any characters.
4. MIN = 0, TIME = 0. In this case, return is immediate. If characters are present, they are returned to the user.
termio(4)