NAME
termios —
general terminal line
discipline
SYNOPSIS
#include <termios.h>
DESCRIPTION
This describes a general terminal line discipline that is supported on tty
asynchronous communication ports.
Opening a Terminal Device
File
When a terminal file is opened, it normally causes the process to wait until a
connection is established. For most hardware, the presence of a connection is
indicated by the assertion of the hardware
CARRIER DETECT
(CD) line. If the
termios structure associated with the
terminal file has the
CLOCAL flag set in the cflag, or
if the
O_NONBLOCK flag is set in the
open(2) call, then the open will
succeed even without a connection being present.
In practice, applications seldom open these files; they are opened by special
programs, such as
getty(8) or
rlogind(8), and become an
application's standard input, output, and error files.
Job Control in a Nutshell
Every process is associated with a particular process group and session. The
grouping is hierarchical: every member of a particular process group is a
member of the same session. This structuring is used in managing groups of
related processes for purposes of
job control; that is, the
ability from the keyboard (or from program control) to simultaneously stop or
restart a complex command (a command composed of one or more related
processes).
The grouping into process groups allows delivering of signals that stop or start
the group as a whole, along with arbitrating which process group has access to
the single controlling terminal. The grouping at a higher layer into sessions
is to restrict the job control related signals and system calls to within
processes resulting from a particular instance of a "login".
Typically, a session is created when a user logs in, and the login terminal is
set up to be the controlling terminal; all processes spawned from that login
shell are in the same session, and inherit the controlling terminal. A job
control shell operating interactively (that is, reading commands from a
terminal) normally groups related processes together by placing them into the
same process group. A set of processes in the same process group is
collectively referred to as a "job".
When the foreground process group of the terminal is the same as the process
group of a particular job, that job is said to be in the
foreground. When the process group of the terminal is
different than the process group of a job (but is still the controlling
terminal), that job is said to be in the
background.
Normally the shell reads a command and starts the job that implements that
command. If the command is to be started in the foreground (typical), it sets
the process group of the terminal to the process group of the started job,
waits for the job to complete, and then sets the process group of the terminal
back to its own process group (it puts itself into the foreground).
If the job is to be started in the background (as denoted by the shell operator
"&"), it never changes the process group of the terminal and
doesn't wait for the job to complete (that is, it immediately attempts to read
the next command).
If the job is started in the foreground, the user may type a character (usually
‘
^Z’) which generates the terminal stop
signal (
SIGTSTP) and has the affect of stopping the
entire job. The shell will notice that the job stopped (see
wait(2)), and will resume running
after placing itself in the foreground.
The shell also has commands for placing stopped jobs in the background, and for
placing stopped or background jobs into the foreground.
Orphaned Process Groups
An orphaned process group is a process group that has no process whose parent is
in a different process group, yet is in the same session. Conceptually it
means a process group that doesn't have a parent that could do anything if it
were to be stopped. For example, the initial login shell is typically in an
orphaned process group. Orphaned process groups are immune to keyboard
generated stop signals and job control signals resulting from reads or writes
to the controlling terminal.
The Controlling Terminal
A terminal may belong to a process as its controlling terminal. Each process of
a session that has a controlling terminal has the same controlling terminal. A
terminal may be the controlling terminal for at most one session. The
controlling terminal for a session is allocated by the session leader by
issuing the
TIOCSCTTY ioctl. A controlling terminal is
never acquired by merely opening a terminal device file. When a controlling
terminal becomes associated with a session, its foreground process group is
set to the process group of the session leader.
The controlling terminal is inherited by a child process during a
fork(2) function call. A process
relinquishes its controlling terminal when it creates a new session with the
setsid(2) function; other
processes remaining in the old session that had this terminal as their
controlling terminal continue to have it. A process does not relinquish its
controlling terminal simply by closing all of its file descriptors associated
with the controlling terminal if other processes continue to have it open.
When a controlling process terminates, the controlling terminal is disassociated
from the current session, allowing it to be acquired by a new session leader.
Subsequent access to the terminal by other processes in the earlier session
will be denied, with attempts to access the terminal treated as if modem
disconnect had been sensed.
Terminal Access Control
If a process is in the foreground process group of its controlling terminal,
read operations are allowed. Any attempts by a process in a background process
group to read from its controlling terminal causes a
SIGTTIN signal to be sent to the process's group
unless one of the following special cases apply: If the reading process is
ignoring or blocking the
SIGTTIN signal, or if the
process group of the reading process is orphaned, the
read(2) returns -1 with
errno set to EIO and no signal
is sent. The default action of the
SIGTTIN signal is
to stop the process to which it is sent.
If a process is in the foreground process group of its controlling terminal,
write operations are allowed. Attempts by a process in a background process
group to write to its controlling terminal will cause the process group to be
sent a
SIGTTOU signal unless one of the following
special cases apply: If
TOSTOP is not set, or if
TOSTOP is set and the process is ignoring or blocking
the
SIGTTOU signal, the process is allowed to write to
the terminal and the
SIGTTOU signal is not sent. If
TOSTOP is set, and the process group of the writing
process is orphaned, and the writing process is not ignoring or blocking
SIGTTOU, the
write(2) returns -1 with errno
set to
EIO and no signal is sent.
Certain calls that set terminal parameters are treated in the same fashion as
write, except that
TOSTOP is ignored; that is, the
effect is identical to that of terminal writes when
TOSTOP is set.
A terminal device associated with a terminal device file may operate in
full-duplex mode, so that data may arrive even while output is occurring. Each
terminal device file has associated with it an input queue, into which
incoming data is stored by the system before being read by a process. The
system imposes a limit, {
MAX_INPUT}, on the number of
bytes that may be stored in the input queue. The behavior of the system when
this limit is exceeded depends on the setting of the
IMAXBEL flag in the
termios
c_iflag. If this flag is set, the terminal is sent an
ASCII
BEL character each time a character is received
while the input queue is full. Otherwise, the input queue is flushed upon
receiving the character.
Two general kinds of input processing are available, determined by whether the
terminal device file is in canonical mode or noncanonical mode. Additionally,
input characters are processed according to the
c_iflag
and
c_lflag fields. Such processing can include echoing,
which in general means transmitting input characters immediately back to the
terminal when they are received from the terminal. This is useful for
terminals that can operate in full-duplex mode.
The manner in which data is provided to a process reading from a terminal device
file is dependent on whether the terminal device file is in canonical or
noncanonical mode.
Another dependency is whether the
O_NONBLOCK flag is set
by
open(2) or
fcntl(2). If the
O_NONBLOCK flag is clear, then the read request is
blocked until data is available or a signal has been received. If the
O_NONBLOCK flag is set, then the read request is
completed, without blocking, in one of three ways:
- If there is enough data available to satisfy the entire
request, and the read completes successfully the number of bytes read is
returned.
- If there is not enough data available to satisfy the
entire request, and the read completes successfully, having read as much
data as possible, the number of bytes read is returned.
- If there is no data available, the read returns -1, with
errno set to
EAGAIN.
When data is available depends on whether the input processing mode is canonical
or noncanonical.
In canonical mode input processing, terminal input is processed in units of
lines. A line is delimited by a newline
‘
\n’ character, an end-of-file
(
EOF) character, or an end-of-line
(
EOL) character. See the
Special Characters section for
more information on
EOF and
EOL. This means that a read request will not return
until an entire line has been typed, or a signal has been received. Also, no
matter how many bytes 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 bytes, even one, may be requested in a read without losing
information.
{
MAX_CANON} is a limit on the number of bytes in a line.
The behavior of the system when this limit is exceeded is the same as when the
input queue limit {
MAX_INPUT}, is exceeded.
Erase and kill processing occur when either of two special characters, the
ERASE and
KILL characters (see
the
Special Characters section),
is received. This processing affects data in the input queue that has not yet
been delimited by a newline
NL,
EOF, or
EOL character. This
un-delimited data makes up the current line. The
ERASE
character deletes the last character in the current line, if there is any. The
KILL character deletes all data in the current line,
if there is any. The
ERASE and
KILL characters have no effect if there is no data in
the current line. The
ERASE and
KILL characters themselves are not placed in the input
queue.
In noncanonical mode input processing, input bytes are not assembled into lines,
and erase and kill processing does not occur. The values of the
VMIN and
VTIME members of the
c_cc array are used to determine how to process the
bytes received.
VMIN represents the minimum number of bytes that should
be received when the
read(2)
system call successfully returns.
VTIME is a timer of
0.1 second granularity that is used to time out bursty and short term data
transmissions. If
VMIN is greater than
{
MAX_INPUT}, the response to the request is undefined.
The four possible values for
VMIN and
VTIME and their interactions are described below.
Case A: VMIN > 0, VTIME
> 0
In this case
VTIME serves as an inter-byte timer and is
activated after the first byte is received. Since it is an inter-byte timer,
it is reset after a byte is received. The interaction between
VMIN and
VTIME is as follows:
as soon as one byte is received, the inter-byte timer is started. If
VMIN bytes are received before the inter-byte timer
expires (remember that the timer is reset upon receipt of each byte), the read
is satisfied. If the timer expires before
VMIN bytes
are received, the characters received to that point are returned to the user.
Note that if
VTIME expires at least one byte is
returned because the timer would not have been enabled unless a byte was
received. In this case (
VMIN > 0,
VTIME > 0) the read blocks until the
VMIN and
VTIME mechanisms are
activated by the receipt of the first byte, or a signal is received. If data
is in the buffer at the time of the
read(2), the result is as if data
had been received immediately after the
read(2).
Case B: VMIN > 0, VTIME =
0
In this case, since the value of
VTIME is zero, the
timer plays no role and only
VMIN is significant. A
pending read is not satisfied until
VMIN bytes are
received (i.e., the pending read blocks until
VMIN
bytes are received), or a signal is received. A program that uses this case to
read record-based terminal
I/O may block indefinitely
in the read operation.
Case C: VMIN = 0, VTIME >
0
In this case, since
VMIN = 0,
VTIME no longer represents an inter-byte timer. It now
serves as a read timer that is activated as soon as the read function is
processed. A read is satisfied as soon as a single byte is received or the
read timer expires. Note that in this case if the timer expires, no bytes are
returned. If the timer does not expire, the only way the read can be satisfied
is if a byte is received. In this case the read will not block indefinitely
waiting for a byte; if no byte is received within
VTIME*0.1 seconds after the read is initiated, the
read returns a value of zero, having read no data. If data is in the buffer at
the time of the read, the timer is started as if data had been received
immediately after the read.
Case D: VMIN = 0, VTIME = 0
The minimum of either the number of bytes requested or the number of bytes
currently available is returned without waiting for more bytes to be input. If
no characters are available, read returns a value of zero, having read no
data.
Writing Data and Output
Processing
When a process writes one or more bytes to a terminal device file, they are
processed according to the
c_oflag field (see the
Output Modes section). The
implementation may provide a buffering mechanism; as such, when a call to
write(2) completes, all of the
bytes written have been scheduled for transmission to the device, but the
transmission will not necessarily have been completed.
Special Characters
Certain characters have special functions on input or output or both. These
functions are summarized as follows:
-
-
INTR
- Special character on input and is recognized if the
ISIG flag (see the
Local Modes section) is enabled.
Generates a SIGINT signal which is sent to all
processes in the foreground process group for which the terminal is the
controlling terminal. If ISIG is set, the
INTR character is discarded when processed.
-
-
QUIT
- Special character on input and is recognized if the
ISIG flag is enabled. Generates a
SIGQUIT signal which is sent to all processes in
the foreground process group for which the terminal is the controlling
terminal. If ISIG is set, the
QUIT character is discarded when processed.
-
-
ERASE
- Special character on input and is recognized if the
ICANON flag is set. Erases the last character in
the current line; see
Canonical Mode Input
Processing. It does not erase beyond the start of a line, as delimited
by an NL, EOF, or
EOL character. If ICANON
is set, the ERASE character is discarded when
processed.
-
-
KILL
- Special character on input and is recognized if the
ICANON flag is set. Deletes the entire line, as
delimited by a NL, EOF, or
EOL character. If ICANON
is set, the KILL character is discarded when
processed.
-
-
EOF
- Special character on input and is recognized if the
ICANON flag is set. When received, all the bytes
waiting to be read are immediately passed to the process, without waiting
for a newline, and the EOF is discarded. Thus, if
there are no bytes waiting (that is, the EOF
occurred at the beginning of a line), a byte count of zero is returned
from the read(2), representing
an end-of-file indication. If ICANON is set, the
EOF character is discarded when processed.
-
-
NL
- Special character on input and is recognized if the
ICANON flag is set. It is the line delimiter
‘\n’.
-
-
EOL
- Special character on input and is recognized if the
ICANON flag is set. Is an additional line
delimiter, like NL.
-
-
SUSP
- If the
ISIG flag is enabled,
receipt of the SUSP character causes a
SIGTSTP signal to be sent to all processes in the
foreground process group for which the terminal is the controlling
terminal, and the SUSP character is discarded when
processed.
-
-
STOP
- Special character on both input and output and is
recognized if the
IXON (output control) or
IXOFF (input control) flag is set. Can be used to
temporarily suspend output. It is useful with fast terminals to prevent
output from disappearing before it can be read. If
IXON is set, the STOP
character is discarded when processed.
-
-
START
- Special character on both input and output and is
recognized if the
IXON (output control) or
IXOFF (input control) flag is set. Can be used to
resume output that has been suspended by a STOP
character. If IXON is set, the
START character is discarded when processed.
-
-
CR
- Special character on input and is recognized if the
ICANON flag is set; it is the
‘\r’, as denoted in the C Standard
{2}. When ICANON and ICRNL
are set and IGNCR is not set, this character is
translated into a NL, and has the same effect as a
NL character.
The following special characters are extensions defined by this system and are
not a part of
IEEE Std 1003.1 (“POSIX.1”)
termios.
-
-
EOL2
- Secondary
EOL character. Same
function as EOL.
-
-
WERASE
- Special character on input and is recognized if the
ICANON flag is set. Erases the last word in the
current line according to one of two algorithms. If the
ALTWERASE flag is not set, first any preceding
whitespace is erased, and then the maximal sequence of non-whitespace
characters. If ALTWERASE is set, first any
preceding whitespace is erased, and then the maximal sequence of
alphabetic/underscores or non alphabetic/underscores. As a special case in
this second algorithm, the first previous non-whitespace character is
skipped in determining whether the preceding word is a sequence of
alphabetic/underscores. This sounds confusing but turns out to be quite
practical.
-
-
REPRINT
- Special character on input and is recognized if the
ICANON flag is set. Causes the current input edit
line to be retyped.
-
-
DSUSP
- Has similar actions to the
SUSP
character, except that the SIGTSTP signal is
delivered when one of the processes in the foreground process group issues
a read(2) to the controlling
terminal.
-
-
LNEXT
- Special character on input and is recognized if the
IEXTEN flag is set. Receipt of this character
causes the next character to be taken literally.
-
-
DISCARD
- Special character on input and is recognized if the
IEXTEN flag is set. Receipt of this character
toggles the flushing of terminal output.
-
-
STATUS
- Special character on input and is recognized if the
ICANON flag is set. Receipt of this character
causes a SIGINFO signal to be sent to the
foreground process group of the terminal. Also, if the
NOKERNINFO flag is not set, it causes the kernel
to write a status message to the terminal that displays the current load
average, the name of the command in the foreground, its process ID, the
symbolic wait channel, the number of user and system seconds used, the
percentage of CPU the process is getting, and the resident set size of the
process.
The
NL and
CR characters cannot
be changed. The values for all the remaining characters can be set and are
described later in the document under Special Control Characters.
Special character functions associated with changeable special control
characters can be disabled individually by setting their value to
{
_POSIX_VDISABLE}; see
Special Control
Characters.
If two or more special characters have the same value, the function performed
when that character is received is undefined.
Modem Disconnect
If a modem disconnect is detected by the terminal interface for a controlling
terminal, and if
CLOCAL is not set in the
c_cflag field for the terminal, the
SIGHUP signal is sent to the controlling process
associated with the terminal. Unless other arrangements have been made, this
causes the controlling process to terminate. Any subsequent call to the
read(2) function returns the value
zero, indicating end of file. Thus, processes that read a terminal file and
test for end-of-file can terminate appropriately after a disconnect. Any
subsequent
write(2) to the
terminal device returns -1, with
errno set to
EIO, until the device is closed.
GENERAL TERMINAL INTERFACE
Closing a Terminal Device
File
The last process to close a terminal device file causes any output to be sent to
the device and any input to be discarded. Then, if
HUPCL is set in the control modes, and the
communications port supports a disconnect function, the terminal device
performs a disconnect.
Parameters That Can Be Set
Routines that need to control certain terminal I/O characteristics do so by
using the
termios structure as defined in the header
<termios.h>. This structure contains
minimally four scalar elements of bit flags and one array of special
characters. The scalar flag elements are named:
c_iflag,
c_oflag,
c_cflag, and
c_lflag. The character array is named
c_cc, and its maximum index is
NCCS.
Values of the
c_iflag field describe the basic terminal
input control, and are composed of following masks:
IGNBRK
- /* ignore BREAK condition */
BRKINT
- /* map BREAK to SIGINT */
IGNPAR
- /* ignore (discard) parity errors */
PARMRK
- /* mark parity and framing errors */
INPCK
- /* enable checking of parity errors */
ISTRIP
- /* strip 8th bit off chars */
INLCR
- /* map NL into CR */
IGNCR
- /* ignore CR */
ICRNL
- /* map CR to NL (ala CRMOD) */
IXON
- /* enable output flow control */
IXOFF
- /* enable input flow control */
IXANY
- /* any char will restart after stop */
IMAXBEL
- /* ring bell on input queue full */
In the context of asynchronous serial data transmission, a break condition is
defined as a sequence of zero-valued bits that continues for more than the
time to send one byte. The entire sequence of zero-valued bits is interpreted
as a single break condition, even if it continues for a time equivalent to
more than one byte. In contexts other than asynchronous serial data
transmission the definition of a break condition is implementation defined.
If
IGNBRK is set, a break condition detected on input is
ignored, that is, not put on the input queue and therefore not read by any
process. If
IGNBRK is not set and
BRKINT is set, the break condition flushes the input
and output queues and if the terminal is the controlling terminal of a
foreground process group, the break condition generates a single
SIGINT signal to that foreground process group. If
neither
IGNBRK nor
BRKINT is
set, a break condition is read as a single
‘
\0’, or if
PARMRK
is set, as ‘
\377’,
‘
\0’,
‘
\0’.
If
IGNPAR is set, a byte with a framing or parity error
(other than break) is ignored.
If
PARMRK is set, and
IGNPAR is
not set, a byte with a framing or parity error (other than break) is given to
the application as the three-character sequence
‘
\377’,
‘
\0’, X, where
‘
\377’,
‘
\0’ is a two-character flag preceding
each sequence and 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 ‘
\377’ is given to the
application as ‘
\377’,
‘
\377’. If neither
PARMRK nor
IGNPAR is set, a
framing or parity error (other than break) is given to the application as a
single character ‘
\0’.
If
INPCK is set, input parity checking is enabled. If
INPCK is not set, input parity checking is disabled,
allowing output parity generation without input parity errors. Note that
whether input parity checking is enabled or disabled is independent of whether
parity detection is enabled or disabled (see
Control Modes). If parity detection is
enabled but input parity checking is disabled, the hardware to which the
terminal is connected recognizes the parity bit, but the terminal special file
does not check whether this bit is set correctly or not.
If
ISTRIP is set, valid input bytes 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). If
IGNCR is not set
and
ICRNL is set, a received
CR character is translated into a
NL character.
If
IXON is set, start/stop output control is enabled. A
received
STOP character suspends output and a received
START character restarts output. If
IXANY is also set, then any character may restart
output. When
IXON is set,
START and
STOP characters are
not read, but merely perform flow control functions. When
IXON is not set, the
START and
STOP characters are read.
If
IXOFF is set, start/stop input control is enabled.
The system shall transmit one or more
STOP characters,
which are intended to cause the terminal device to stop transmitting data, as
needed to prevent the input queue from overflowing and causing the undefined
behavior described in
Input Processing and
Reading Data, and shall transmit one or more
START
characters, which are intended to cause the terminal device to resume
transmitting data, as soon as the device can continue transmitting data
without risk of overflowing the input queue. The precise conditions under
which
STOP and START characters are transmitted are
implementation defined.
If
IMAXBEL is set and the input queue is full,
subsequent input shall cause an ASCII
BEL character to
be transmitted to the output queue.
The initial input control value after
open(2) is implementation defined.
Output Modes
Values of the
c_oflag field describe the basic terminal
output control, and are composed of the following masks:
OPOST
- /* enable following output processing */
ONLCR
- /* map NL to CR-NL (ala
CRMOD)
*/
OCRNL
- /* map CR to NL */
OXTABS
- /* expand tabs to spaces */
ONOEOT
- /* discard
EOT's (^D) on output
*/
ONOCR
- /* do not transmit CRs on column 0 */
ONLRET
- /* on the terminal NL performs the CR function */
If
OPOST is set, the remaining flag masks are
interpreted as follows; otherwise characters are transmitted without change.
If
ONLCR is set, newlines are translated to carriage
return, linefeeds.
If
OCRNL is set, carriage returns are translated to
newlines.
If
OXTABS is set, tabs are expanded to the appropriate
number of spaces (assuming 8 column tab stops).
If
ONOEOT is set, ASCII
EOT's
are discarded on output.
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.
Control Modes
Values of the
c_cflag field describe the basic terminal
hardware control, and are composed of the following masks. Not all values
specified are supported by all hardware.
CSIZE
- /* character size mask */
CS5
- /* 5 bits (pseudo) */
CS6
- /* 6 bits */
CS7
- /* 7 bits */
CS8
- /* 8 bits */
CSTOPB
- /* send 2 stop bits */
CREAD
- /* enable receiver */
PARENB
- /* parity enable */
PARODD
- /* odd parity, else even */
HUPCL
- /* hang up on last close */
CLOCAL
- /* ignore modem status lines */
CCTS_OFLOW
- /*
CTS flow control of output
*/
CRTSCTS
- /* logically the same as
CCTS_OFLOW
| CCTS_IFLOW */
CRTS_IFLOW
- /* RTS flow control of input */
MDMBUF
- /* flow control output via Carrier */
The
CSIZE bits specify the byte size in bits for both
transmission and reception. The
c_cflag is masked with
CSIZE and compared with the values
CS5,
CS6,
CS7, or
CS8. 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
stop bits are normally used.
If
CREAD is set, the receiver is enabled. Otherwise, no
character is received. Not all hardware supports this bit. In fact, this flag
is pretty silly and if it were not part of the
termios
specification it would be omitted.
If
PARENB is set, parity generation and detection are
enabled and a parity bit is added to each character. If parity is enabled,
PARODD specifies odd parity if set, otherwise even
parity is used.
If
HUPCL is set, the modem control lines for the port
are lowered when the last process with the port open closes the port or the
process terminates. The modem connection is broken.
If
CLOCAL is set, a connection does not depend on the
state of the modem status lines. If
CLOCAL is clear,
the modem status lines are monitored.
Under normal circumstances, a call to the
open(2) function waits for the
modem connection to complete. However, if the
O_NONBLOCK flag is set or if
CLOCAL has been set, the
open(2) function returns
immediately without waiting for the connection.
If the
tty(4)
TIOCFLAG_CLOCAL flag has been set on the port then the
CLOCAL flag will automatically be set on every open.
The
CCTS_OFLOW and
CRTS_IFLOW
flags are currently unused. Only
CRTSCTS, which has
the combined effect, is implemented. Note that
CRTSCTS
support is hardware and driver dependent. Check the specific port driver
manual page to see if hardware flow control is supported on the port you are
using.
If the
tty(4)
TIOCFLAG_CRTSCTS flag has been set on the port then
the
CRTSCTS flag will automatically be set on every
open.
If
MDMBUF is set then output flow control is controlled
by the state of Carrier Detect.
If the
tty(4)
TIOCFLAG_MDMBUF flag has been set on the port then the
MDMBUF flag will automatically be set on every open.
If the object for which the control modes are set is not an asynchronous serial
connection, some of the modes may be ignored; for example, if an attempt is
made to set the baud rate on a network connection to a terminal on another
host, the baud rate may or may not be set on the connection between that
terminal and the machine it is directly connected to.
Local Modes
Values of the
c_lflag field describe the control of
various functions, and are composed of the following masks.
ECHOKE
- /* visual erase for line kill */
ECHOE
- /* visually erase chars */
ECHO
- /* enable echoing */
ECHONL
- /* echo
NL even if
ECHO is off */
ECHOPRT
- /* visual erase mode for hardcopy */
ECHOCTL
- /* echo control chars as ^(Char) */
ISIG
- /* enable signals
INTR,
QUIT, [D]SUSP */
ICANON
- /* canonicalize input lines */
ALTWERASE
- /* use alternative
WERASE algorithm
*/
IEXTEN
- /* enable
DISCARD and
LNEXT */
EXTPROC
- /* external processing */
TOSTOP
- /* stop background jobs from output */
FLUSHO
- /* output being flushed (state) */
NOKERNINFO
- /* no kernel output from
VSTATUS
*/
PENDIN
- /* re-echo input buffer at next read */
NOFLSH
- /* don't flush output on signal */
If
ECHO is set, input characters are echoed back to the
terminal. If
ECHO is not set, input characters are not
echoed.
If
ECHOE and
ICANON are set, the
ERASE character causes the terminal to erase the last
character in the current line from the display, if possible. If there is no
character to erase, an implementation may echo an indication that this was the
case or do nothing.
If
ECHOK and
ICANON are set, the
KILL character causes the current line to be discarded
and the system echoes the ‘
\n’ character
after the
KILL character.
If
ECHOKE and
ICANON are set,
the
KILL character causes the current line to be
discarded and the system causes the terminal to erase the line from the
display.
If
ECHOPRT and
ICANON are set,
the system assumes that the display is a printing device and prints a
backslash and the erased characters when processing
ERASE characters, followed by a forward slash.
If
ECHOCTL is set, the system echoes control characters
in a visible fashion using a caret followed by the control character.
If
ALTWERASE is set, the system uses an alternative
algorithm for determining what constitutes a word when processing
WERASE characters (see
WERASE).
If
ECHONL and
ICANON are set,
the ‘
\n’ character echoes even if
ECHO is not set.
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, as described in
Canonical Mode Input
Processing.
If
ICANON is not set, read requests are satisfied
directly from the input queue. A read is not satisfied until at least
VMIN bytes have been received or the timeout value
VTIME expired between bytes. The time value represents
tenths of seconds. See
Noncanonical Mode
Input Processing for more details.
If
ISIG is set, each input character is checked against
the special control characters
INTR,
QUIT, and
SUSP (job control
only). 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.
If
IEXTEN is set, implementation-defined functions are
recognized from the input data. How
IEXTEN being set
interacts with
ICANON,
ISIG,
IXON, or
IXOFF is
implementation defined. If
IEXTEN is not set, then
implementation-defined functions are not recognized, and the corresponding
input characters are not processed as described for
ICANON,
ISIG,
IXON, and
IXOFF.
If
NOFLSH is set, the normal flush of the input and
output queues associated with the
INTR,
QUIT, and
SUSP characters are
not be done.
If
TOSTOP is set, the signal
SIGTTOU is sent to the process group of a process that
tries to write to its controlling terminal if it is not in the foreground
process group for that terminal. This signal, by default, stops the members of
the process group. Otherwise, the output generated by that process is output
to the current output stream. Processes that are blocking or ignoring
SIGTTOU signals are excepted and allowed to produce
output and the
SIGTTOU signal is not sent.
If
NOKERNINFO is set, the kernel does not produce a
status message when processing
STATUS characters (see
STATUS).
Special Control Characters
The special control characters values are defined by the array
c_cc. This table lists the array index, the
corresponding special character, and the system default value. For an accurate
list of the system defaults, consult the header file
<ttydefaults.h>.
| Index
Name |
Special Character |
Default Value |
VEOF |
EOF |
^D |
VEOL |
EOL |
_POSIX_VDISABLE |
VEOL2 |
EOL2 |
_POSIX_VDISABLE |
VERASE |
ERASE |
^?
‘\177’ |
VWERASE |
WERASE |
^W |
VKILL |
KILL |
^U |
VREPRINT |
REPRINT |
^R |
VINTR |
INTR |
^C |
VQUIT |
QUIT |
^\\
‘\34’ |
VSUSP |
SUSP |
^Z |
VDSUSP |
DSUSP |
^Y |
VSTART |
START |
^Q |
VSTOP |
STOP |
^S |
VLNEXT |
LNEXT |
^V |
VDISCARD |
DISCARD |
^O |
VMIN |
--- |
1 |
VTIME |
--- |
0 |
VSTATUS |
STATUS |
^T |
If the value of one of the changeable special control characters (see
Special Characters) is
{
_POSIX_VDISABLE}, that function is disabled; that is,
no input data is recognized as the disabled special character. If
ICANON is not set, the value of
{
_POSIX_VDISABLE} has no special meaning for the
VMIN and
VTIME entries of the
c_cc array.
The initial values of the flags and control characters after
open(2) is set according to the
values in the header
<sys/ttydefaults.h>.
SEE ALSO
tcsendbreak(3),
tcsetattr(3)