termios

TERMIOS(4)                BSD Kernel Interfaces Manual                TERMIOS(4)

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), 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 setup 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 from 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 does not 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 does not 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(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 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.

     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 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 MIN and TIME is as follows: as
     soon as one byte is received, the inter-byte timer is started.  If MIN
     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 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(), the result is as if data had been received immediately after the
     read().

   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 (i.e., 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 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 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() 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(), representing an end-of-
             file indication.  If ICANON is set, the EOF character is discarded
             when processed.

     NL      Special character on input and is recognized if the ICANON flag is
             set.  It is the line delimiter ‘\n’.

     EOL     Special character on input and is recognized if the ICANON flag is
             set.  Is an additional line delimiter, like NL.

     SUSP    If the ISIG flag is enabled, receipt of the SUSP character causes a
             SIGTSTP signal to be sent to all processes in the foreground
             process group for which the terminal is the controlling terminal,
             and the SUSP character is discarded when processed.

     STOP    Special character on both input and output and is recognized if the
             IXON (output control) or IXOFF (input control) flag is set.  Can be
             used to temporarily suspend output.  It is useful with fast
             terminals to prevent output from disappearing before it can be
             read.  If IXON is set, the STOP character is discarded when
             processed.

     START   Special character on both input and output and is recognized if the
             IXON (output control) or IXOFF (input control) flag is set.  Can be
             used to resume output that has been suspended by a STOP character.
             If IXON is set, the START character is discarded when processed.

     CR      Special character on input and is recognized if the ICANON flag is
             set; it is the ‘\r’, as denoted in the C Standard {2}.  When ICANON
             and ICRNL are set and IGNCR is not set, this character is
             translated into a NL, and has the same effect as a NL character.

     The following special characters are extensions defined by this system and
     are not a part of IEEE Std 1003.1 (“POSIX.1”) termios.

     EOL2    Secondary EOL character.  Same function as EOL.

     WERASE  Special character on input and is recognized if the ICANON flag is
             set.  Erases the last word in the current line according to one of
             two algorithms.  If the ALTWERASE flag is not set, first any
             preceding whitespace is erased, and then the maximal sequence of
             non-whitespace characters.  If ALTWERASE is set, first any
             preceding whitespace is erased, and then the maximal sequence of
             alphabetic/underscores or non alphabetic/underscores.  As a special
             case in this second algorithm, the first previous non-whitespace
             character is skipped in determining whether the preceding word is a
             sequence of alphabetic/underscores.  This sounds confusing but
             turns out to be quite practical.

     REPRINT
             Special character on input and is recognized if the ICANON flag is
             set.  Causes the current input edit line to be retyped.

     DSUSP   Has similar actions to the SUSP character, except that the SIGTSTP
             signal is delivered when one of the processes in the foreground
             process group issues a read() to the controlling terminal.

     LNEXT   Special character on input and is recognized if the IEXTEN flag is
             set.  Receipt of this character causes the next character to be
             taken literally.

     DISCARD
             Special character on input and is recognized if the IEXTEN flag is
             set.  Receipt of this character toggles the flushing of terminal
             output.

     STATUS  Special character on input and is recognized if the ICANON flag is
             set.  Receipt of this character causes a SIGINFO signal to be sent
             to the foreground process group of the terminal.  Also, if the
             NOKERNINFO flag is not set, it causes the kernel to write a status
             message to the terminal that displays the current load average, the
             name of the command in the foreground, its process ID, the symbolic
             wait channel, the number of user and system seconds used, the
             percentage of cpu the process is getting, and the resident set size
             of the process.

     The NL and CR characters cannot be changed.  The values for all the
     remaining characters can be set and are described later in the document
     under Special Control Characters.

     Special character functions associated with changeable special control
     characters can be disabled individually by setting their value to
     {_POSIX_VDISABLE}; see Special Control Characters.

     If two or more special characters have the same value, the function
     performed when that character is received is undefined.

   Modem Disconnect
     If a modem disconnect is detected by the terminal interface for a
     controlling terminal, and if CLOCAL is not set in the c_cflag field for the
     terminal, the SIGHUP signal is sent to the controlling process associated
     with the terminal.  Unless other arrangements have been made, this causes
     the controlling process to terminate.  Any subsequent call to the read()
     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 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 */
           IXANY    /* any char will restart after stop */
           IMAXBEL  /* ring bell on input queue full */

     In the context of asynchronous serial data transmission, a break condition
     is defined as a sequence of zero-valued bits that continues for more than
     the time to send one byte.  The entire sequence of zero-valued bits is
     interpreted as a single break condition, even if it continues for a time
     equivalent to more than one byte.  In contexts other than asynchronous
     serial data transmission the definition of a break condition is
     implementation defined.

     If IGNBRK is set, a break condition detected on input is ignored, that is,
     not put on the input queue and therefore not read by any process.  If
     IGNBRK is not set and BRKINT is set, the break condition flushes the input
     and output queues and if the terminal is the controlling terminal of a
     foreground process group, the break condition generates a single SIGINT
     signal to that foreground process group.  If neither IGNBRK nor BRKINT is
     set, a break condition is read as a single ‘\0’, or if PARMRK is set, as
     ‘\377’, ‘\0’, ‘\0’.

     If IGNPAR is set, a byte with a framing or parity error (other than break)
     is ignored.

     If PARMRK is set, and IGNPAR is not set, a byte with a framing or parity
     error (other than break) is given to the application as the three-character
     sequence ‘\377’, ‘\0’, X, where ‘\377’, ‘\0’ is a two-character flag
     preceding each sequence and X is the data of the character received in
     error.  To avoid ambiguity in this case, if ISTRIP is not set, a valid
     character of ‘\377’ is given to the application as ‘\377’, ‘\377’.  If
     neither PARMRK nor IGNPAR is set, a framing or parity error (other than
     break) is given to the application as a single character ‘\0’.

     If INPCK is set, input parity checking is enabled.  If INPCK is not set,
     input parity checking is disabled, allowing output parity generation
     without input parity errors.  Note that whether input parity checking is
     enabled or disabled is independent of whether parity detection is enabled
     or disabled (see Control Modes).  If parity detection is enabled but input
     parity checking is disabled, the hardware to which the terminal is
     connected recognizes the parity bit, but the terminal special file does not
     check whether this bit is set correctly or not.

     If ISTRIP is set, valid input bytes are first stripped to seven bits,
     otherwise all eight bits are processed.

     If INLCR is set, a received NL character is translated into a CR character.
     If IGNCR is set, a received CR character is ignored (not read).  If IGNCR
     is not set and ICRNL is set, a received CR character is translated into a
     NL character.

     If IXON is set, start/stop output control is enabled.  A received STOP
     character suspends output and a received START character restarts output.
     If IXANY is also set, then any character may restart output.  When IXON is
     set, START and STOP characters are not read, but merely perform flow
     control functions.  When IXON is not set, the START and STOP characters are
     read.

     If IXOFF is set, start/stop input control is enabled.  The system shall
     transmit one or more STOP characters, which are intended to cause the
     terminal device to stop transmitting data, as needed to prevent the input
     queue from overflowing and causing the undefined behavior described in
     Input Processing and Reading Data, and shall transmit one or more START
     characters, which are intended to cause the terminal device to resume
     transmitting data, as soon as the device can continue transmitting data
     without risk of overflowing the input queue.  The precise conditions under
     which STOP and START characters are transmitted are implementation defined.

     If IMAXBEL is set and the input queue is full, subsequent input shall cause
     an ASCII BEL character to be transmitted to the output queue.

     The initial input control value after open() is implementation defined.

   Output Modes
     Values of the c_oflag field describe the basic terminal output control, and
     are composed of the following masks:

           OPOST   /* enable following output processing */
           ONLCR   /* map NL to CR-NL (ala CRMOD) */
           OCRNL   /* map CR to NL */
           TABDLY  /* tab delay mask */
           TAB0    /* no tab delay and expansion */
           TAB3    /* expand tabs to spaces */
           ONOEOT  /* discard EOT's ‘^D’ on output) */
           ONOCR   /* do not transmit CRs on column 0 */
           ONLRET  /* on the terminal NL performs the CR function */

     If OPOST is set, the remaining flag masks are interpreted as follows;
     otherwise characters are transmitted without change.

     If ONLCR is set, newlines are translated to carriage return, linefeeds.

     If OCRNL is set, carriage returns are translated to newlines.

     The TABDLY bits specify the tab delay.  The c_oflag is masked with TABDLY
     and compared with the values TAB0 or TAB3.  If TAB3 is set, tabs are
     expanded to the appropriate number of spaces (assuming 8 column tab stops).

     If ONOEOT is set, ASCII EOT's are discarded on output.

     If ONOCR is set, no CR character is transmitted when at column 0 (first
     position).

     If ONLRET is set, the NL character is assumed to do the carriage-return
     function; the column pointer will be set to 0.

   Control Modes
     Values of the c_cflag field describe the basic terminal hardware control,
     and are composed of the following masks.  Not all values specified are
     supported by all hardware.

           CSIZE       /* character size mask */
           CS5         /* 5 bits (pseudo) */
           CS6         /* 6 bits */
           CS7         /* 7 bits */
           CS8         /* 8 bits */
           CSTOPB      /* send 2 stop bits */
           CREAD       /* enable receiver */
           PARENB      /* parity enable */
           PARODD      /* odd parity, else even */
           HUPCL       /* hang up on last close */
           CLOCAL      /* ignore modem status lines */
           CCTS_OFLOW  /* CTS flow control of output */
           CRTSCTS     /* 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.

     The CCTS_OFLOW (CRTSCTS) flag is currently unused.

     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 */
           ECHO        /* enable echoing */
           ECHONL      /* echo NL even if ECHO is off */
           ECHOPRT     /* visual erase mode for hardcopy */
           ECHOCTL     /* echo control chars as ^(Char) */
           ISIG        /* enable signals INTR, QUIT, [D]SUSP */
           ICANON      /* canonicalize input lines */
           ALTWERASE   /* use alternate WERASE algorithm */
           IEXTEN      /* enable DISCARD and LNEXT */
           EXTPROC     /* external processing */
           TOSTOP      /* stop background jobs from output */
           FLUSHO      /* output being flushed (state) */
           NOKERNINFO  /* no kernel output from VSTATUS */
           PENDIN      /* XXX retype pending input (state) */
           NOFLSH      /* don't flush after interrupt */

     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 MIN bytes have been received
     or the timeout 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.

     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>.

SEE ALSO
     stty(1), tcgetsid(3), tcsendbreak(3), tcsetattr(3), tcsetsid(3), tty(4)

BSD                             December 26, 2009                            BSD