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.
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.
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.
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.
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:
EAGAIN
.
When data is available depends on whether the input processing mode is canonical or noncanonical.
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.
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.
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).
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.
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.
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.
INTR
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
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
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
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
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
ICANON
flag is set.
It is the line delimiter
`\n'.
EOL
ICANON
flag is set.
Is an additional line delimiter,
like
NL
.
SUSP
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
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
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
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'') .
EOL2
EOL
character.
Same function as
EOL
.
WERASE
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
ICANON
flag is set.
Causes the current input edit line
to be retyped.
DSUSP
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
IEXTEN
flag is set.
Receipt of this character causes the next
character to be taken literally.
DISCARD
IEXTEN
flag is set.
Receipt of this character toggles the flushing
of terminal output.
STATUS
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.
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.
HUPCL
is set in the control modes, and the communications port supports a
disconnect function, the terminal device performs a disconnect.
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
.
c_iflag
field describe the basic
terminal input control, and are composed of
following masks:
IGNBRK
BRKINT
IGNPAR
PARMRK
INPCK
ISTRIP
INLCR
IGNCR
ICRNL
IXON
IXOFF
IXANY
IMAXBEL
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.
c_oflag
field describe the basic terminal output control,
and are composed of the following masks:
OPOST
ONLCR
CRMOD
)
*/
OCRNL
OXTABS
ONOEOT
EOT
's
(^D)
on output */
ONOCR
ONLRET
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.
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
CS5
CS6
CS7
CS8
CSTOPB
CREAD
PARENB
PARODD
HUPCL
CLOCAL
CCTS_OFLOW
CTS
flow control of output */
CRTSCTS
CCTS_OFLOW
|
CCTS_IFLOW
*/
CRTS_IFLOW
MDMBUF
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.
c_lflag
field describe the control of
various functions, and are composed of the following
masks.
ECHOKE
ECHOE
ECHO
ECHONL
NL
even if
ECHO
is off */
ECHOPRT
ECHOCTL
ISIG
INTR
,
QUIT
,
[D]SUSP
*/
ICANON
ALTWERASE
WERASE
algorithm */
IEXTEN
DISCARD
and
LNEXT
*/
EXTPROC
TOSTOP
FLUSHO
NOKERNINFO
VSTATUS
*/
PENDIN
NOFLSH
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
).
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 |
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
>.