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termios(4) termios(4)
termios
NAME
termios - general terminal line discipline
SYNOPSIS
#include
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. In
practice, applications and become an application's standard input, output,
and error files.
Job Control Overview
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 affect 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 an open(2) on a terminal that is not already
a controlling terminal for another session. 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 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 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:
1. If there is enough data available to satisfy the entire request, and
the read completes successfully the number of bytes read is
returned.
2. 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.
3. 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 undelimited
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 MIN
and TIME members of the c_cc array are used to determine how to process
the bytes received.
MIN represents the minimum number of bytes that should be received when
the read(2) function successfully returns. TIME is a timer of 0.1 second
granularity that is used to time out bursty and short term data
transmissions. If MIN is greater than {MAX_INPUT} the response to the
request is undefined. The four possible values for MIN and TIME and their
interactions are described below.
Case A: MIN > 0, TIME > 0
In this case TIME serves as an interbyte timer and is activated after the
first byte is received. Since it is an interbyte timer, it is reset after
a byte is received. The interaction between MIN and TIME is as follows: as
soon as one byte is received, the interbyte timer is started. If MIN bytes
are received before the interbyte timer expires (remember that the timer
is reset upon receipt of each byte), the read is satisfied. If the timer
expires before MIN bytes are received, the characters received to that
point are returned to the user. Note that if TIME expires at least one
byte is returned because the timer would not have been enabled unless a
byte was received. In this case ( MIN > 0, TIME > 0) the read blocks until
the MIN and TIME 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: MIN > 0, TIME = 0
In this case, since the value of TIME is zero, the timer plays no role and
only MIN is significant. A pending read is not satisfied until MIN bytes
are received (that is, the pending read blocks until MIN 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: MIN = 0, TIME > 0
In this case, since MIN = 0, TIME no longer represents interbyte 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
TIME*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: MIN = 0, TIME = 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 Standard. 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.
VBELTIME
The value of VBELTIME specifies the minimum period of silence between
bell sounds from the terminal, in tenths of a second. After a bell
sounds at the terminal, the system ignores any other bell signals
delivered to the terminal for the next VBELTIME tenths of seconds. All
other characters are delivered as they would normally be.
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 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 SIGINTR.
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
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 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 (a la CRMOD).
ONLRET
NL performs the CR function.
ONOCR
No CR output in column 0.
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.
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.
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(4) 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 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.
ECHOK
Visual erase for line kill.
ECHOE
Visually erase characters.
ECHO
Enable echoing.
ECHONL
Echo NL even if ECHO is off.
ISIG
Enable signals INTR, QUIT, [D]SUSP. */
ICANON
Canonicalize input lines.
IEXTEN
Enable DISCARD and LNEXT.
EXTPROC
External processing; in external processing mode, characters are never
echoed, special characters are never processed, and no characters are
erased. This mode is used in programs such as telnetd(1).
TOSTOP
Stop background jobs from output.
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 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 MIN bytes have been received
or the time-out value TIME 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.
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.
Index Name Special Character Default Value
VEOF EOF ^D
VEOL EOL _POSIX_VDISABLE
VERASE ERASE ^H
VKILL KILL ^U
VINTR INTR ^C
VQUIT QUIT ^\
VSUSP SUSP ^Z
VSTART START ^Q
VSTOP STOP ^S
VMIN --- 1
VTIME --- 0
VBELTIME --- 3
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.