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 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
sshd(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 key (usually ‘^Z
’) which
generates the terminal stop signal (SIGTSTP
) and has
the effect of stopping the entire job. The shell will notice that the job
stopped, 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.
Input Processing and Reading Data
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
() or
fcntl
().
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.
Canonical Mode Input Processing
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.
Noncanonical Mode Input Processing
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) function 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(), the result is as if data had been received immediately after the
read().
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() 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 aSIGINT
signal which is sent to all processes in the foreground process group for which the terminal is the controlling terminal. IfISIG
is set, theINTR
character is discarded when processed. QUIT
- Special character on input and is recognized if the
ISIG
flag is enabled. Generates aSIGQUIT
signal which is sent to all processes in the foreground process group for which the terminal is the controlling terminal. IfISIG
is set, theQUIT
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 aNL
,EOF
, orEOL
character. IfICANON
is set, theERASE
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 aNL
,EOF
, orEOL
character. IfICANON
is set, theKILL
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 theEOF
is discarded. Thus, if there are no bytes waiting (that is, theEOF
occurred at the beginning of a line), a byte count of zero is returned from the read(), representing an end-of-file indication. IfICANON
is set, theEOF
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, likeNL
. SUSP
- If the
ISIG
flag is enabled, receipt of theSUSP
character causes aSIGTSTP
signal to be sent to all processes in the foreground process group for which the terminal is the controlling terminal, and theSUSP
character is discarded when processed. STOP
- Special character on both input and output and is recognized if the
IXON
(output control) orIXOFF
(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. IfIXON
is set, theSTOP
character is discarded when processed. START
- Special character on both input and output and is recognized if the
IXON
(output control) orIXOFF
(input control) flag is set. Can be used to resume output that has been suspended by aSTOP
character. IfIXON
is set, theSTART
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}. WhenICANON
andICRNL
are set andIGNCR
is not set, this character is translated into aNL
, and has the same effect as aNL
character.
The following special characters are extensions defined by this system and are not a part of 1003.1 termios.
EOL2
- Secondary
EOL
character. Same function asEOL
. 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 theALTWERASE
flag is not set, first any preceding whitespace is erased, and then the maximal sequence of non-whitespace characters. IfALTWERASE
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 theSIGTSTP
signal is delivered when one of the processes in the foreground process group issues a read() 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 aSIGINFO
signal to be sent to the foreground process group of the terminal. Also, if theNOKERNINFO
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() 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() 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
.
Input Modes
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 */
IUCLC
- /* translate upper case to lower case */
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.
If IUCLC
is set, characters will be
translated from upper to lower case on input.
The initial input control value after open() 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
) */ OXTABS
- /* expand tabs to spaces */
ONOEOT
- /* discard
EOT
's (^D) on output */ OCRNL
- /* map CR to NL */
OLCUC
- /* translate lower case to upper case */
ONOCR
- /* No CR output at column 0 */
ONLRET
- /* 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 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 OCRNL
is set, carriage returns are
translated to newlines.
If OLCUC
is set, lower case is translated
to upper case on output.
If ONOCR
is set, no CR character is output
when at column 0.
If ONLRET
is set, NL also performs CR on
output, and reset current column 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
- /* same as
CCTS_OFLOW
*/ 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() function waits
for the modem connection to complete. However, if the
O_NONBLOCK
flag is set or if
CLOCAL
has been set, the open() function returns
immediately without waiting for the connection.
If CCTS_OFLOW
(CRTSCTS
) is set, transmission is stopped when
CTS
is lowered and resumed when
CTS
is raised.
If MDMBUF
is set then output flow control
is controlled by the state of Carrier Detect.
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 */
ECHOK
- /* echo NL after line kill */
ECHO
- /* enable echoing */
ECHONL
- /* echo
NL
even ifECHO
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 alternate
WERASE
algorithm */ IEXTEN
- /* enable
DISCARD
andLNEXT
*/ EXTPROC
- /* external processing */
TOSTOP
- /* stop background jobs from output */
FLUSHO
- /* output being flushed (state) */
NOKERNINFO
- /* no kernel output from
VSTATUS
*/ PENDIN
- /* XXX retype pending input (state) */
NOFLSH
- /* don't flush after interrupt */
XCASE
- /* canonical upper/lower case */
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 is
not done.
If XCASE
and
ICANON
is set, an upper case character is preserved
on input if prefixed by a \ character. In addition, this prefix is added to
upper case characters on output.
In addition, the following special character translations are in effect:
for: | use: |
` |
\' |
| |
\! |
~ |
\^ |
{ |
\( |
} |
\) |
\ |
\\ |
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
<sys/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() is set according to the values in the header
<sys/ttydefaults.h>
.