| TERMIOS(4) | Device Drivers Manual | TERMIOS(4) | 
termios —
#include <termios.h>
termios structure associated with
  the terminal file has the CLOCAL flag set in the
  cflag, or if the O_NONBLOCK flag is set in the
  open(2) call, then the open will
  succeed even without a connection being present.
In practice, applications seldom open these files; they are opened by special programs, such as getty(8) or rlogind(8), and become an application's standard input, output, and error files.
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.
\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.
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.
INTRISIG 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.QUITISIG 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.ERASEICANON 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.KILLICANON 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.EOFICANON 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.NLICANON flag is set. It is the line delimiter
      ‘\n’.EOLICANON flag is set. Is an additional line
      delimiter, like NL.SUSPISIG 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.STOPIXON (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.STARTIXON (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.CRICANON flag is set; it is the
      ‘\r’, as denoted in the C Standard
      {2}. When ICANON and ICRNL
      are set and IGNCR is not set, this character is
      translated into a NL, and has the same effect as a
      NL character.The following special characters are extensions defined by this
    system and are not a part of IEEE Std 1003.1
    (“POSIX.1”) termios.
EOL2EOL character. Same function as
      EOL.WERASEICANON 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.REPRINTICANON flag is set. Causes the current input edit
      line to be retyped.DSUSPSUSP 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.LNEXTIEXTEN flag is set. Receipt of this character
      causes the next character to be taken literally.DISCARDIEXTEN flag is set. Receipt of this character
      toggles the flushing of terminal output.STATUSICANON 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 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.
IGNBRKBRKINTIGNPARPARMRKINPCKISTRIPINLCRIGNCRICRNLIXONIXOFFIXANYIMAXBELIn 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.
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.
CSIZECS5CS6CS7CS8CSTOPBCREADPARENBPARODDHUPCLCLOCALCCTS_OFLOWCTS flow control of output */CRTSCTSCCTS_OFLOW |
      CCTS_IFLOW */CRTS_IFLOWMDMBUFThe 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.
ECHOKEECHOEECHOECHONLNL even if ECHO is
      off */ECHOPRTECHOCTLISIGINTR,
      QUIT, [D]SUSP */ICANONALTWERASEWERASE algorithm */IEXTENDISCARD and
      LNEXT */EXTPROCTOSTOPFLUSHONOKERNINFOVSTATUS */PENDINNOFLSHIf 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).
<ttydefaults.h>.
| Index Name | Special Character | Default Value | 
| VEOF | EOF | ^D | 
| VEOL | EOL | _POSIX_VDISABLE | 
| VEOL2 | EOL2 | _POSIX_VDISABLE | 
| VERASE | ERASE | ^? ‘ \177’ | 
| VWERASE | WERASE | ^W | 
| VKILL | KILL | ^U | 
| VREPRINT | REPRINT | ^R | 
| VINTR | INTR | ^C | 
| VQUIT | QUIT | ^\\ ‘ \34’ | 
| VSUSP | SUSP | ^Z | 
| VDSUSP | DSUSP | ^Y | 
| VSTART | START | ^Q | 
| VSTOP | STOP | ^S | 
| VLNEXT | LNEXT | ^V | 
| VDISCARD | DISCARD | ^O | 
| VMIN | --- | 1 | 
| VTIME | --- | 0 | 
| VSTATUS | STATUS | ^T | 
If the value of one of the changeable special control characters
    (see Special Characters) is
    {_POSIX_VDISABLE}, that function is disabled; that
    is, no input data is recognized as the disabled special character. If
    ICANON is not set, the value of
    {_POSIX_VDISABLE} has no special meaning for the
    VMIN and VTIME entries of
    the c_cc array.
The initial values of the flags and control characters after
    open(2) is set according to the
    values in the header
    <sys/ttydefaults.h>.
| October 7, 2006 | NetBSD 9.4 |