path: root/static/netbsd/man4/termios.4 4.html

<table class="head">
  <tr>
    <td class="head-ltitle">TERMIOS(4)</td>
    <td class="head-vol">Device Drivers Manual</td>
    <td class="head-rtitle">TERMIOS(4)</td>
  </tr>
</table>
<div class="manual-text">
<section class="Sh">
<h1 class="Sh" id="NAME"><a class="permalink" href="#NAME">NAME</a></h1>
<p class="Pp"><code class="Nm">termios</code> &#x2014; <span class="Nd">general
    terminal line discipline</span></p>
</section>
<section class="Sh">
<h1 class="Sh" id="SYNOPSIS"><a class="permalink" href="#SYNOPSIS">SYNOPSIS</a></h1>
<p class="Pp"><code class="In">#include
    &lt;<a class="In">termios.h</a>&gt;</code></p>
</section>
<section class="Sh">
<h1 class="Sh" id="DESCRIPTION"><a class="permalink" href="#DESCRIPTION">DESCRIPTION</a></h1>
<p class="Pp">This describes a general terminal line discipline that is
    supported on tty asynchronous communication ports.</p>
<section class="Ss">
<h2 class="Ss" id="Opening_a_Terminal_Device_File"><a class="permalink" href="#Opening_a_Terminal_Device_File">Opening
  a Terminal Device File</a></h2>
<p class="Pp">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
    <a class="permalink" href="#CARRIER"><i class="Em" id="CARRIER">CARRIER
    DETECT</i></a> (CD) line. If the <code class="Nm">termios</code> structure
    associated with the terminal file has the <code class="Dv">CLOCAL</code>
    flag set in the cflag, or if the <code class="Dv">O_NONBLOCK</code> flag is
    set in the <a class="Xr">open(2)</a> call, then the open will succeed even
    without a connection being present.</p>
<p class="Pp">In practice, applications seldom open these files; they are opened
    by special programs, such as <a class="Xr">getty(8)</a> or
    <a class="Xr">rlogind(8)</a>, and become an application's standard input,
    output, and error files.</p>
</section>
<section class="Ss">
<h2 class="Ss" id="Job_Control_in_a_Nutshell"><a class="permalink" href="#Job_Control_in_a_Nutshell">Job
  Control in a Nutshell</a></h2>
<p class="Pp">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
    <a class="permalink" href="#job"><i class="Em" id="job">job control</i></a>;
    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).</p>
<p class="Pp">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 &quot;login&quot;.</p>
<p class="Pp">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 &quot;job&quot;.</p>
<p class="Pp" id="foreground">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
    <a class="permalink" href="#foreground"><i class="Em">foreground</i></a>.
    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
    <a class="permalink" href="#background"><i class="Em" id="background">background</i></a>.</p>
<p class="Pp">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).</p>
<p class="Pp">If the job is to be started in the background (as denoted by the
    shell operator &quot;&amp;&quot;), 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).</p>
<p class="Pp">If the job is started in the foreground, the user may type a
    character (usually &#x2018;<code class="Li">^Z</code>&#x2019;) which
    generates the terminal stop signal (<code class="Dv">SIGTSTP</code>) and has
    the affect of stopping the entire job. The shell will notice that the job
    stopped (see <a class="Xr">wait(2)</a>), and will resume running after
    placing itself in the foreground.</p>
<p class="Pp">The shell also has commands for placing stopped jobs in the
    background, and for placing stopped or background jobs into the
  foreground.</p>
</section>
<section class="Ss">
<h2 class="Ss" id="Orphaned_Process_Groups"><a class="permalink" href="#Orphaned_Process_Groups">Orphaned
  Process Groups</a></h2>
<p class="Pp">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.</p>
</section>
<section class="Ss">
<h2 class="Ss" id="The_Controlling_Terminal"><a class="permalink" href="#The_Controlling_Terminal">The
  Controlling Terminal</a></h2>
<p class="Pp">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 <code class="Dv">TIOCSCTTY</code> 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.</p>
<p class="Pp">The controlling terminal is inherited by a child process during a
    <a class="Xr">fork(2)</a> function call. A process relinquishes its
    controlling terminal when it creates a new session with the
    <a class="Xr">setsid(2)</a> 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.</p>
<p class="Pp">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.</p>
</section>
<section class="Ss">
<h2 class="Ss" id="Terminal_Access_Control"><a class="permalink" href="#Terminal_Access_Control">Terminal
  Access Control</a></h2>
<p class="Pp">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
    <code class="Dv">SIGTTIN</code> 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 <code class="Dv">SIGTTIN</code> signal, or if the
    process group of the reading process is orphaned, the
    <a class="Xr">read(2)</a> returns -1 with <var class="Va">errno set to</var>
    <code class="Er">EIO</code> and no signal is sent. The default action of the
    <code class="Dv">SIGTTIN</code> signal is to stop the process to which it is
    sent.</p>
<p class="Pp">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 <code class="Dv">SIGTTOU</code> signal unless one
    of the following special cases apply: If <code class="Dv">TOSTOP</code> is
    not set, or if <code class="Dv">TOSTOP</code> is set and the process is
    ignoring or blocking the <code class="Dv">SIGTTOU</code> signal, the process
    is allowed to write to the terminal and the <code class="Dv">SIGTTOU</code>
    signal is not sent. If <code class="Dv">TOSTOP</code> is set, and the
    process group of the writing process is orphaned, and the writing process is
    not ignoring or blocking <code class="Dv">SIGTTOU</code>, the
    <a class="Xr">write(2)</a> returns -1 with errno set to
    <code class="Er">EIO</code> and no signal is sent.</p>
<p class="Pp">Certain calls that set terminal parameters are treated in the same
    fashion as write, except that <code class="Dv">TOSTOP</code> is ignored;
    that is, the effect is identical to that of terminal writes when
    <code class="Dv">TOSTOP</code> is set.</p>
</section>
<section class="Ss">
<h2 class="Ss" id="Input_Processing_and_Reading_Data"><a class="permalink" href="#Input_Processing_and_Reading_Data">Input
  Processing and Reading Data</a></h2>
<p class="Pp">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, {<code class="Dv">MAX_INPUT</code>}, 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
    <code class="Dv">IMAXBEL</code> flag in the <code class="Nm">termios</code>
    <var class="Fa">c_iflag</var>. If this flag is set, the terminal is sent an
    ASCII <code class="Dv">BEL</code> character each time a character is
    received while the input queue is full. Otherwise, the input queue is
    flushed upon receiving the character.</p>
<p class="Pp">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
    <var class="Fa">c_iflag</var> and <var class="Fa">c_lflag</var> 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.</p>
<p class="Pp">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.</p>
<p class="Pp">Another dependency is whether the
    <code class="Dv">O_NONBLOCK</code> flag is set by <a class="Xr">open(2)</a>
    or <a class="Xr">fcntl(2)</a>. If the <code class="Dv">O_NONBLOCK</code>
    flag is clear, then the read request is blocked until data is available or a
    signal has been received. If the <code class="Dv">O_NONBLOCK</code> flag is
    set, then the read request is completed, without blocking, in one of three
    ways:</p>
<ol class="Bl-enum Bd-indent">
  <li>If there is enough data available to satisfy the entire request, and the
      read completes successfully the number of bytes read is returned.</li>
  <li>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.</li>
  <li>If there is no data available, the read returns -1, with errno set to
      <code class="Er">EAGAIN</code>.</li>
</ol>
<p class="Pp">When data is available depends on whether the input processing
    mode is canonical or noncanonical.</p>
</section>
<section class="Ss">
<h2 class="Ss" id="Canonical_Mode_Input_Processing"><a class="permalink" href="#Canonical_Mode_Input_Processing">Canonical
  Mode Input Processing</a></h2>
<p class="Pp">In canonical mode input processing, terminal input is processed in
    units of lines. A line is delimited by a newline
    &#x2018;<code class="Li">\n</code>&#x2019; character, an end-of-file
    (<code class="Dv">EOF</code>) character, or an end-of-line
    (<code class="Dv">EOL</code>) character. See the
    <a class="Sx" href="#Special_Characters">Special Characters</a> section for
    more information on <code class="Dv">EOF</code> and
    <code class="Dv">EOL</code>. 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.</p>
<p class="Pp">{<code class="Dv">MAX_CANON</code>} 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 {<code class="Dv">MAX_INPUT</code>},
    is exceeded.</p>
<p class="Pp">Erase and kill processing occur when either of two special
    characters, the <code class="Dv">ERASE</code> and
    <code class="Dv">KILL</code> characters (see the
    <a class="Sx" href="#Special_Characters">Special Characters</a> section), is
    received. This processing affects data in the input queue that has not yet
    been delimited by a newline <code class="Dv">NL</code>,
    <code class="Dv">EOF</code>, or <code class="Dv">EOL</code> character. This
    un-delimited data makes up the current line. The
    <code class="Dv">ERASE</code> character deletes the last character in the
    current line, if there is any. The <code class="Dv">KILL</code> character
    deletes all data in the current line, if there is any. The
    <code class="Dv">ERASE</code> and <code class="Dv">KILL</code> characters
    have no effect if there is no data in the current line. The
    <code class="Dv">ERASE</code> and <code class="Dv">KILL</code> characters
    themselves are not placed in the input queue.</p>
</section>
<section class="Ss">
<h2 class="Ss" id="Noncanonical_Mode_Input_Processing"><a class="permalink" href="#Noncanonical_Mode_Input_Processing">Noncanonical
  Mode Input Processing</a></h2>
<p class="Pp">In noncanonical mode input processing, input bytes are not
    assembled into lines, and erase and kill processing does not occur. The
    values of the <code class="Dv">VMIN</code> and <code class="Dv">VTIME</code>
    members of the <var class="Fa">c_cc</var> array are used to determine how to
    process the bytes received.</p>
<p class="Pp"><code class="Dv">VMIN</code> represents the minimum number of
    bytes that should be received when the <a class="Xr">read(2)</a> system call
    successfully returns. <code class="Dv">VTIME</code> is a timer of 0.1 second
    granularity that is used to time out bursty and short term data
    transmissions. If <code class="Dv">VMIN</code> is greater than
    {<code class="Dv">MAX_INPUT</code>}, the response to the request is
    undefined. The four possible values for <code class="Dv">VMIN</code> and
    <code class="Dv">VTIME</code> and their interactions are described
  below.</p>
</section>
<section class="Ss">
<h2 class="Ss" id="Case_A:_VMIN___0,_VTIME___0"><a class="permalink" href="#Case_A:_VMIN___0,_VTIME___0">Case
  A: VMIN &gt; 0, VTIME &gt; 0</a></h2>
<p class="Pp">In this case <code class="Dv">VTIME</code> 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 <code class="Dv">VMIN</code> and <code class="Dv">VTIME</code> is as
    follows: as soon as one byte is received, the inter-byte timer is started.
    If <code class="Dv">VMIN</code> 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
    <code class="Dv">VMIN</code> bytes are received, the characters received to
    that point are returned to the user. Note that if
    <code class="Dv">VTIME</code> expires at least one byte is returned because
    the timer would not have been enabled unless a byte was received. In this
    case (<code class="Dv">VMIN</code> &gt; 0, <code class="Dv">VTIME</code>
    &gt; 0) the read blocks until the <code class="Dv">VMIN</code> and
    <code class="Dv">VTIME</code> 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 <a class="Xr">read(2)</a>, the result is as if data had been received
    immediately after the <a class="Xr">read(2)</a>.</p>
</section>
<section class="Ss">
<h2 class="Ss" id="Case_B:_VMIN___0,_VTIME_=_0"><a class="permalink" href="#Case_B:_VMIN___0,_VTIME_=_0">Case
  B: VMIN &gt; 0, VTIME = 0</a></h2>
<p class="Pp">In this case, since the value of <code class="Dv">VTIME</code> is
    zero, the timer plays no role and only <code class="Dv">VMIN</code> is
    significant. A pending read is not satisfied until
    <code class="Dv">VMIN</code> bytes are received (i.e., the pending read
    blocks until <code class="Dv">VMIN</code> bytes are received), or a signal
    is received. A program that uses this case to read record-based terminal
    <code class="Dv">I/O</code> may block indefinitely in the read
  operation.</p>
</section>
<section class="Ss">
<h2 class="Ss" id="Case_C:_VMIN_=_0,_VTIME___0"><a class="permalink" href="#Case_C:_VMIN_=_0,_VTIME___0">Case
  C: VMIN = 0, VTIME &gt; 0</a></h2>
<p class="Pp">In this case, since <code class="Dv">VMIN</code> = 0,
    <code class="Dv">VTIME</code> 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
    <code class="Dv">VTIME</code>*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.</p>
</section>
<section class="Ss">
<h2 class="Ss" id="Case_D:_VMIN_=_0,_VTIME_=_0"><a class="permalink" href="#Case_D:_VMIN_=_0,_VTIME_=_0">Case
  D: VMIN = 0, VTIME = 0</a></h2>
<p class="Pp">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.</p>
</section>
<section class="Ss">
<h2 class="Ss" id="Writing_Data_and_Output_Processing"><a class="permalink" href="#Writing_Data_and_Output_Processing">Writing
  Data and Output Processing</a></h2>
<p class="Pp">When a process writes one or more bytes to a terminal device file,
    they are processed according to the <var class="Fa">c_oflag</var> field (see
    the <a class="Sx" href="#Output_Modes">Output Modes</a> section). The
    implementation may provide a buffering mechanism; as such, when a call to
    <a class="Xr">write(2)</a> completes, all of the bytes written have been
    scheduled for transmission to the device, but the transmission will not
    necessarily have been completed.</p>
</section>
<section class="Ss">
<h2 class="Ss" id="Special_Characters"><a class="permalink" href="#Special_Characters">Special
  Characters</a></h2>
<p class="Pp">Certain characters have special functions on input or output or
    both. These functions are summarized as follows:</p>
<dl class="Bl-tag">
  <dt id="INTR"><a class="permalink" href="#INTR"><code class="Dv">INTR</code></a></dt>
  <dd>Special character on input and is recognized if the
      <code class="Dv">ISIG</code> flag (see the
      <a class="Sx" href="#Local_Modes">Local Modes</a> section) is enabled.
      Generates a <code class="Dv">SIGINT</code> signal which is sent to all
      processes in the foreground process group for which the terminal is the
      controlling terminal. If <code class="Dv">ISIG</code> is set, the
      <code class="Dv">INTR</code> character is discarded when processed.</dd>
  <dt id="QUIT"><a class="permalink" href="#QUIT"><code class="Dv">QUIT</code></a></dt>
  <dd>Special character on input and is recognized if the
      <code class="Dv">ISIG</code> flag is enabled. Generates a
      <code class="Dv">SIGQUIT</code> signal which is sent to all processes in
      the foreground process group for which the terminal is the controlling
      terminal. If <code class="Dv">ISIG</code> is set, the
      <code class="Dv">QUIT</code> character is discarded when processed.</dd>
  <dt id="ERASE"><a class="permalink" href="#ERASE"><code class="Dv">ERASE</code></a></dt>
  <dd>Special character on input and is recognized if the
      <code class="Dv">ICANON</code> flag is set. Erases the last character in
      the current line; see
      <a class="Sx" href="#Canonical_Mode_Input_Processing">Canonical Mode Input
      Processing</a>. It does not erase beyond the start of a line, as delimited
      by an <code class="Dv">NL</code>, <code class="Dv">EOF</code>, or
      <code class="Dv">EOL</code> character. If <code class="Dv">ICANON</code>
      is set, the <code class="Dv">ERASE</code> character is discarded when
      processed.</dd>
  <dt id="KILL"><a class="permalink" href="#KILL"><code class="Dv">KILL</code></a></dt>
  <dd>Special character on input and is recognized if the
      <code class="Dv">ICANON</code> flag is set. Deletes the entire line, as
      delimited by a <code class="Dv">NL</code>, <code class="Dv">EOF</code>, or
      <code class="Dv">EOL</code> character. If <code class="Dv">ICANON</code>
      is set, the <code class="Dv">KILL</code> character is discarded when
      processed.</dd>
  <dt id="EOF"><a class="permalink" href="#EOF"><code class="Dv">EOF</code></a></dt>
  <dd>Special character on input and is recognized if the
      <code class="Dv">ICANON</code> 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 <code class="Dv">EOF</code> is discarded. Thus, if
      there are no bytes waiting (that is, the <code class="Dv">EOF</code>
      occurred at the beginning of a line), a byte count of zero is returned
      from the <a class="Xr">read(2)</a>, representing an end-of-file
      indication. If <code class="Dv">ICANON</code> is set, the
      <code class="Dv">EOF</code> character is discarded when processed.</dd>
  <dt id="NL"><a class="permalink" href="#NL"><code class="Dv">NL</code></a></dt>
  <dd>Special character on input and is recognized if the
      <code class="Dv">ICANON</code> flag is set. It is the line delimiter
      &#x2018;<code class="Li">\n</code>&#x2019;.</dd>
  <dt id="EOL"><a class="permalink" href="#EOL"><code class="Dv">EOL</code></a></dt>
  <dd>Special character on input and is recognized if the
      <code class="Dv">ICANON</code> flag is set. Is an additional line
      delimiter, like <code class="Dv">NL</code>.</dd>
  <dt id="SUSP"><a class="permalink" href="#SUSP"><code class="Dv">SUSP</code></a></dt>
  <dd>If the <code class="Dv">ISIG</code> flag is enabled, receipt of the
      <code class="Dv">SUSP</code> character causes a
      <code class="Dv">SIGTSTP</code> signal to be sent to all processes in the
      foreground process group for which the terminal is the controlling
      terminal, and the <code class="Dv">SUSP</code> character is discarded when
      processed.</dd>
  <dt id="STOP"><a class="permalink" href="#STOP"><code class="Dv">STOP</code></a></dt>
  <dd>Special character on both input and output and is recognized if the
      <code class="Dv">IXON</code> (output control) or
      <code class="Dv">IXOFF</code> (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
      <code class="Dv">IXON</code> is set, the <code class="Dv">STOP</code>
      character is discarded when processed.</dd>
  <dt id="START"><a class="permalink" href="#START"><code class="Dv">START</code></a></dt>
  <dd>Special character on both input and output and is recognized if the
      <code class="Dv">IXON</code> (output control) or
      <code class="Dv">IXOFF</code> (input control) flag is set. Can be used to
      resume output that has been suspended by a <code class="Dv">STOP</code>
      character. If <code class="Dv">IXON</code> is set, the
      <code class="Dv">START</code> character is discarded when processed.</dd>
  <dt id="CR"><a class="permalink" href="#CR"><code class="Dv">CR</code></a></dt>
  <dd>Special character on input and is recognized if the
      <code class="Dv">ICANON</code> flag is set; it is the
      &#x2018;<code class="Li">\r</code>&#x2019;, as denoted in the C Standard
      {2}. When <code class="Dv">ICANON</code> and <code class="Dv">ICRNL</code>
      are set and <code class="Dv">IGNCR</code> is not set, this character is
      translated into a <code class="Dv">NL</code>, and has the same effect as a
      <code class="Dv">NL</code> character.</dd>
</dl>
<p class="Pp">The following special characters are extensions defined by this
    system and are not a part of <span class="St">IEEE Std 1003.1
    (&#x201C;POSIX.1&#x201D;)</span> <code class="Nm">termios</code>.</p>
<dl class="Bl-tag">
  <dt id="EOL2"><a class="permalink" href="#EOL2"><code class="Dv">EOL2</code></a></dt>
  <dd>Secondary <code class="Dv">EOL</code> character. Same function as
      <code class="Dv">EOL</code>.</dd>
  <dt id="WERASE"><a class="permalink" href="#WERASE"><code class="Dv">WERASE</code></a></dt>
  <dd>Special character on input and is recognized if the
      <code class="Dv">ICANON</code> flag is set. Erases the last word in the
      current line according to one of two algorithms. If the
      <code class="Dv">ALTWERASE</code> flag is not set, first any preceding
      whitespace is erased, and then the maximal sequence of non-whitespace
      characters. If <code class="Dv">ALTWERASE</code> 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.</dd>
  <dt id="REPRINT"><a class="permalink" href="#REPRINT"><code class="Dv">REPRINT</code></a></dt>
  <dd>Special character on input and is recognized if the
      <code class="Dv">ICANON</code> flag is set. Causes the current input edit
      line to be retyped.</dd>
  <dt id="DSUSP"><a class="permalink" href="#DSUSP"><code class="Dv">DSUSP</code></a></dt>
  <dd>Has similar actions to the <code class="Dv">SUSP</code> character, except
      that the <code class="Dv">SIGTSTP</code> signal is delivered when one of
      the processes in the foreground process group issues a
      <a class="Xr">read(2)</a> to the controlling terminal.</dd>
  <dt id="LNEXT"><a class="permalink" href="#LNEXT"><code class="Dv">LNEXT</code></a></dt>
  <dd>Special character on input and is recognized if the
      <code class="Dv">IEXTEN</code> flag is set. Receipt of this character
      causes the next character to be taken literally.</dd>
  <dt id="DISCARD"><a class="permalink" href="#DISCARD"><code class="Dv">DISCARD</code></a></dt>
  <dd>Special character on input and is recognized if the
      <code class="Dv">IEXTEN</code> flag is set. Receipt of this character
      toggles the flushing of terminal output.</dd>
  <dt id="STATUS"><a class="permalink" href="#STATUS"><code class="Dv">STATUS</code></a></dt>
  <dd>Special character on input and is recognized if the
      <code class="Dv">ICANON</code> flag is set. Receipt of this character
      causes a <code class="Dv">SIGINFO</code> signal to be sent to the
      foreground process group of the terminal. Also, if the
      <code class="Dv">NOKERNINFO</code> 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.</dd>
</dl>
<p class="Pp">The <code class="Dv">NL</code> and <code class="Dv">CR</code>
    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.</p>
<p class="Pp">Special character functions associated with changeable special
    control characters can be disabled individually by setting their value to
    {<code class="Dv">_POSIX_VDISABLE</code>}; see
    <a class="Sx" href="#Special_Control_Characters">Special Control
    Characters</a>.</p>
<p class="Pp">If two or more special characters have the same value, the
    function performed when that character is received is undefined.</p>
</section>
<section class="Ss">
<h2 class="Ss" id="Modem_Disconnect"><a class="permalink" href="#Modem_Disconnect">Modem
  Disconnect</a></h2>
<p class="Pp">If a modem disconnect is detected by the terminal interface for a
    controlling terminal, and if <code class="Dv">CLOCAL</code> is not set in
    the <var class="Fa">c_cflag</var> field for the terminal, the
    <code class="Dv">SIGHUP</code> 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
    <a class="Xr">read(2)</a> 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
    <a class="Xr">write(2)</a> to the terminal device returns -1, with
    <var class="Va">errno</var> set to <code class="Er">EIO</code>, until the
    device is closed.</p>
</section>
</section>
<section class="Sh">
<h1 class="Sh" id="GENERAL_TERMINAL_INTERFACE"><a class="permalink" href="#GENERAL_TERMINAL_INTERFACE">GENERAL
  TERMINAL INTERFACE</a></h1>
<section class="Ss">
<h2 class="Ss" id="Closing_a_Terminal_Device_File"><a class="permalink" href="#Closing_a_Terminal_Device_File">Closing
  a Terminal Device File</a></h2>
<p class="Pp">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
    <code class="Dv">HUPCL</code> is set in the control modes, and the
    communications port supports a disconnect function, the terminal device
    performs a disconnect.</p>
</section>
<section class="Ss">
<h2 class="Ss" id="Parameters_That_Can_Be_Set"><a class="permalink" href="#Parameters_That_Can_Be_Set">Parameters
  That Can Be Set</a></h2>
<p class="Pp">Routines that need to control certain terminal I/O characteristics
    do so by using the <code class="Nm">termios</code> structure as defined in
    the header <code class="In">&lt;<a class="In">termios.h</a>&gt;</code>. This
    structure contains minimally four scalar elements of bit flags and one array
    of special characters. The scalar flag elements are named:
    <var class="Fa">c_iflag</var>, <var class="Fa">c_oflag</var>,
    <var class="Fa">c_cflag</var>, and <var class="Fa">c_lflag</var>. The
    character array is named <var class="Fa">c_cc</var>, and its maximum index
    is <code class="Dv">NCCS</code>.</p>
</section>
<section class="Ss">
<h2 class="Ss" id="Input_Modes"><a class="permalink" href="#Input_Modes">Input
  Modes</a></h2>
<p class="Pp">Values of the <var class="Fa">c_iflag</var> field describe the
    basic terminal input control, and are composed of following masks:</p>
<p class="Pp"></p>
<div class="Bd-indent">
<dl class="Bl-tag Bl-compact">
  <dt id="IGNBRK"><a class="permalink" href="#IGNBRK"><code class="Dv">IGNBRK</code></a></dt>
  <dd>/* ignore BREAK condition */</dd>
  <dt id="BRKINT"><a class="permalink" href="#BRKINT"><code class="Dv">BRKINT</code></a></dt>
  <dd>/* map BREAK to SIGINT */</dd>
  <dt id="IGNPAR"><a class="permalink" href="#IGNPAR"><code class="Dv">IGNPAR</code></a></dt>
  <dd>/* ignore (discard) parity errors */</dd>
  <dt id="PARMRK"><a class="permalink" href="#PARMRK"><code class="Dv">PARMRK</code></a></dt>
  <dd>/* mark parity and framing errors */</dd>
  <dt id="INPCK"><a class="permalink" href="#INPCK"><code class="Dv">INPCK</code></a></dt>
  <dd>/* enable checking of parity errors */</dd>
  <dt id="ISTRIP"><a class="permalink" href="#ISTRIP"><code class="Dv">ISTRIP</code></a></dt>
  <dd>/* strip 8th bit off chars */</dd>
  <dt id="INLCR"><a class="permalink" href="#INLCR"><code class="Dv">INLCR</code></a></dt>
  <dd>/* map NL into CR */</dd>
  <dt id="IGNCR"><a class="permalink" href="#IGNCR"><code class="Dv">IGNCR</code></a></dt>
  <dd>/* ignore CR */</dd>
  <dt id="ICRNL"><a class="permalink" href="#ICRNL"><code class="Dv">ICRNL</code></a></dt>
  <dd>/* map CR to NL (ala CRMOD) */</dd>
  <dt id="IXON"><a class="permalink" href="#IXON"><code class="Dv">IXON</code></a></dt>
  <dd>/* enable output flow control */</dd>
  <dt id="IXOFF"><a class="permalink" href="#IXOFF"><code class="Dv">IXOFF</code></a></dt>
  <dd>/* enable input flow control */</dd>
  <dt id="IXANY"><a class="permalink" href="#IXANY"><code class="Dv">IXANY</code></a></dt>
  <dd>/* any char will restart after stop */</dd>
  <dt id="IMAXBEL"><a class="permalink" href="#IMAXBEL"><code class="Dv">IMAXBEL</code></a></dt>
  <dd>/* ring bell on input queue full */</dd>
</dl>
</div>
<p class="Pp">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.</p>
<p class="Pp">If <code class="Dv">IGNBRK</code> 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 <code class="Dv">IGNBRK</code> is not
    set and <code class="Dv">BRKINT</code> 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
    <code class="Dv">SIGINT</code> signal to that foreground process group. If
    neither <code class="Dv">IGNBRK</code> nor <code class="Dv">BRKINT</code> is
    set, a break condition is read as a single
    &#x2018;<code class="Li">\0</code>&#x2019;, or if
    <code class="Dv">PARMRK</code> is set, as
    &#x2018;<code class="Li">\377</code>&#x2019;,
    &#x2018;<code class="Li">\0</code>&#x2019;,
    &#x2018;<code class="Li">\0</code>&#x2019;.</p>
<p class="Pp">If <code class="Dv">IGNPAR</code> is set, a byte with a framing or
    parity error (other than break) is ignored.</p>
<p class="Pp">If <code class="Dv">PARMRK</code> is set, and
    <code class="Dv">IGNPAR</code> is not set, a byte with a framing or parity
    error (other than break) is given to the application as the three-character
    sequence &#x2018;<code class="Li">\377</code>&#x2019;,
    &#x2018;<code class="Li">\0</code>&#x2019;, X, where
    &#x2018;<code class="Li">\377</code>&#x2019;,
    &#x2018;<code class="Li">\0</code>&#x2019; 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 <code class="Dv">ISTRIP</code> is not set, a
    valid character of &#x2018;<code class="Li">\377</code>&#x2019; is given to
    the application as &#x2018;<code class="Li">\377</code>&#x2019;,
    &#x2018;<code class="Li">\377</code>&#x2019;. If neither
    <code class="Dv">PARMRK</code> nor <code class="Dv">IGNPAR</code> is set, a
    framing or parity error (other than break) is given to the application as a
    single character &#x2018;<code class="Li">\0</code>&#x2019;.</p>
<p class="Pp">If <code class="Dv">INPCK</code> is set, input parity checking is
    enabled. If <code class="Dv">INPCK</code> 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
    <a class="Sx" href="#Control_Modes">Control Modes</a>). 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.</p>
<p class="Pp">If <code class="Dv">ISTRIP</code> is set, valid input bytes are
    first stripped to seven bits, otherwise all eight bits are processed.</p>
<p class="Pp">If <code class="Dv">INLCR</code> is set, a received
    <code class="Dv">NL</code> character is translated into a
    <code class="Dv">CR</code> character. If <code class="Dv">IGNCR</code> is
    set, a received <code class="Dv">CR</code> character is ignored (not read).
    If <code class="Dv">IGNCR</code> is not set and
    <code class="Dv">ICRNL</code> is set, a received <code class="Dv">CR</code>
    character is translated into a <code class="Dv">NL</code> character.</p>
<p class="Pp">If <code class="Dv">IXON</code> is set, start/stop output control
    is enabled. A received <code class="Dv">STOP</code> character suspends
    output and a received <code class="Dv">START</code> character restarts
    output. If <code class="Dv">IXANY</code> is also set, then any character may
    restart output. When <code class="Dv">IXON</code> is set,
    <code class="Dv">START</code> and <code class="Dv">STOP</code> characters
    are not read, but merely perform flow control functions. When
    <code class="Dv">IXON</code> is not set, the <code class="Dv">START</code>
    and <code class="Dv">STOP</code> characters are read.</p>
<p class="Pp">If <code class="Dv">IXOFF</code> is set, start/stop input control
    is enabled. The system shall transmit one or more
    <code class="Dv">STOP</code> 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
    <a class="Sx" href="#Input_Processing_and_Reading_Data">Input Processing and
    Reading Data</a>, and shall transmit one or more
    <code class="Dv">START</code> 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 <code class="Dv">STOP</code> and START
    characters are transmitted are implementation defined.</p>
<p class="Pp">If <code class="Dv">IMAXBEL</code> is set and the input queue is
    full, subsequent input shall cause an ASCII <code class="Dv">BEL</code>
    character to be transmitted to the output queue.</p>
<p class="Pp">The initial input control value after <a class="Xr">open(2)</a> is
    implementation defined.</p>
</section>
<section class="Ss">
<h2 class="Ss" id="Output_Modes"><a class="permalink" href="#Output_Modes">Output
  Modes</a></h2>
<p class="Pp">Values of the <var class="Fa">c_oflag</var> field describe the
    basic terminal output control, and are composed of the following masks:</p>
<p class="Pp"></p>
<div class="Bd-indent">
<dl class="Bl-tag Bl-compact">
  <dt id="OPOST"><a class="permalink" href="#OPOST"><code class="Dv">OPOST</code></a></dt>
  <dd>/* enable following output processing */</dd>
  <dt id="ONLCR"><a class="permalink" href="#ONLCR"><code class="Dv">ONLCR</code></a></dt>
  <dd>/* map NL to CR-NL (ala <code class="Dv">CRMOD</code>) */</dd>
  <dt id="OCRNL"><a class="permalink" href="#OCRNL"><code class="Dv">OCRNL</code></a></dt>
  <dd>/* map CR to NL */</dd>
  <dt id="OXTABS"><a class="permalink" href="#OXTABS"><code class="Dv">OXTABS</code></a></dt>
  <dd>/* expand tabs to spaces */</dd>
  <dt id="ONOEOT"><a class="permalink" href="#ONOEOT"><code class="Dv">ONOEOT</code></a></dt>
  <dd>/* discard <code class="Dv">EOT</code>'s (^D) on output */</dd>
  <dt id="ONOCR"><a class="permalink" href="#ONOCR"><code class="Dv">ONOCR</code></a></dt>
  <dd>/* do not transmit CRs on column 0 */</dd>
  <dt id="ONLRET"><a class="permalink" href="#ONLRET"><code class="Dv">ONLRET</code></a></dt>
  <dd>/* on the terminal NL performs the CR function */</dd>
</dl>
</div>
<p class="Pp">If <code class="Dv">OPOST</code> is set, the remaining flag masks
    are interpreted as follows; otherwise characters are transmitted without
    change.</p>
<p class="Pp">If <code class="Dv">ONLCR</code> is set, newlines are translated
    to carriage return, linefeeds.</p>
<p class="Pp">If <code class="Dv">OCRNL</code> is set, carriage returns are
    translated to newlines.</p>
<p class="Pp">If <code class="Dv">OXTABS</code> is set, tabs are expanded to the
    appropriate number of spaces (assuming 8 column tab stops).</p>
<p class="Pp">If <code class="Dv">ONOEOT</code> is set, ASCII
    <code class="Dv">EOT</code>'s are discarded on output.</p>
<p class="Pp">If <code class="Dv">ONOCR</code> is set, no CR character is
    transmitted when at column 0 (first position).</p>
<p class="Pp">If <code class="Dv">ONLRET</code> is set, the NL character is
    assumed to do the carriage-return function; the column pointer will be set
    to 0.</p>
</section>
<section class="Ss">
<h2 class="Ss" id="Control_Modes"><a class="permalink" href="#Control_Modes">Control
  Modes</a></h2>
<p class="Pp">Values of the <var class="Fa">c_cflag</var> field describe the
    basic terminal hardware control, and are composed of the following masks.
    Not all values specified are supported by all hardware.</p>
<p class="Pp"></p>
<div class="Bd-indent">
<dl class="Bl-tag Bl-compact">
  <dt id="CSIZE"><a class="permalink" href="#CSIZE"><code class="Dv">CSIZE</code></a></dt>
  <dd>/* character size mask */</dd>
  <dt id="CS5"><a class="permalink" href="#CS5"><code class="Dv">CS5</code></a></dt>
  <dd>/* 5 bits (pseudo) */</dd>
  <dt id="CS6"><a class="permalink" href="#CS6"><code class="Dv">CS6</code></a></dt>
  <dd>/* 6 bits */</dd>
  <dt id="CS7"><a class="permalink" href="#CS7"><code class="Dv">CS7</code></a></dt>
  <dd>/* 7 bits */</dd>
  <dt id="CS8"><a class="permalink" href="#CS8"><code class="Dv">CS8</code></a></dt>
  <dd>/* 8 bits */</dd>
  <dt id="CSTOPB"><a class="permalink" href="#CSTOPB"><code class="Dv">CSTOPB</code></a></dt>
  <dd>/* send 2 stop bits */</dd>
  <dt id="CREAD"><a class="permalink" href="#CREAD"><code class="Dv">CREAD</code></a></dt>
  <dd>/* enable receiver */</dd>
  <dt id="PARENB"><a class="permalink" href="#PARENB"><code class="Dv">PARENB</code></a></dt>
  <dd>/* parity enable */</dd>
  <dt id="PARODD"><a class="permalink" href="#PARODD"><code class="Dv">PARODD</code></a></dt>
  <dd>/* odd parity, else even */</dd>
  <dt id="HUPCL"><a class="permalink" href="#HUPCL"><code class="Dv">HUPCL</code></a></dt>
  <dd>/* hang up on last close */</dd>
  <dt id="CLOCAL"><a class="permalink" href="#CLOCAL"><code class="Dv">CLOCAL</code></a></dt>
  <dd>/* ignore modem status lines */</dd>
  <dt id="CCTS_OFLOW"><a class="permalink" href="#CCTS_OFLOW"><code class="Dv">CCTS_OFLOW</code></a></dt>
  <dd>/* <code class="Dv">CTS</code> flow control of output */</dd>
  <dt id="CRTSCTS"><a class="permalink" href="#CRTSCTS"><code class="Dv">CRTSCTS</code></a></dt>
  <dd>/* logically the same as <code class="Dv">CCTS_OFLOW |</code>
      <code class="Dv">CCTS_IFLOW</code> */</dd>
  <dt id="CRTS_IFLOW"><a class="permalink" href="#CRTS_IFLOW"><code class="Dv">CRTS_IFLOW</code></a></dt>
  <dd>/* RTS flow control of input */</dd>
  <dt id="MDMBUF"><a class="permalink" href="#MDMBUF"><code class="Dv">MDMBUF</code></a></dt>
  <dd>/* flow control output via Carrier */</dd>
</dl>
</div>
<p class="Pp">The <code class="Dv">CSIZE</code> bits specify the byte size in
    bits for both transmission and reception. The <var class="Fa">c_cflag</var>
    is masked with <code class="Dv">CSIZE</code> and compared with the values
    <code class="Dv">CS5</code>, <code class="Dv">CS6</code>,
    <code class="Dv">CS7</code>, or <code class="Dv">CS8</code>. This size does
    not include the parity bit, if any. If <code class="Dv">CSTOPB</code> is
    set, two stop bits are used, otherwise one stop bit. For example, at 110
    baud, two stop bits are normally used.</p>
<p class="Pp">If <code class="Dv">CREAD</code> 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
    <code class="Nm">termios</code> specification it would be omitted.</p>
<p class="Pp">If <code class="Dv">PARENB</code> is set, parity generation and
    detection are enabled and a parity bit is added to each character. If parity
    is enabled, <code class="Dv">PARODD</code> specifies odd parity if set,
    otherwise even parity is used.</p>
<p class="Pp">If <code class="Dv">HUPCL</code> 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.</p>
<p class="Pp">If <code class="Dv">CLOCAL</code> is set, a connection does not
    depend on the state of the modem status lines. If
    <code class="Dv">CLOCAL</code> is clear, the modem status lines are
    monitored.</p>
<p class="Pp">Under normal circumstances, a call to the
    <a class="Xr">open(2)</a> function waits for the modem connection to
    complete. However, if the <code class="Dv">O_NONBLOCK</code> flag is set or
    if <code class="Dv">CLOCAL</code> has been set, the
    <a class="Xr">open(2)</a> function returns immediately without waiting for
    the connection.</p>
<p class="Pp">If the <a class="Xr">tty(4)</a>
    <code class="Dv">TIOCFLAG_CLOCAL</code> flag has been set on the port then
    the <code class="Dv">CLOCAL</code> flag will automatically be set on every
    open.</p>
<p class="Pp">The <code class="Dv">CCTS_OFLOW</code> and
    <code class="Dv">CRTS_IFLOW</code> flags are currently unused. Only
    <code class="Dv">CRTSCTS</code>, which has the combined effect, is
    implemented. Note that <code class="Dv">CRTSCTS</code> 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.</p>
<p class="Pp">If the <a class="Xr">tty(4)</a>
    <code class="Dv">TIOCFLAG_CRTSCTS</code> flag has been set on the port then
    the <code class="Dv">CRTSCTS</code> flag will automatically be set on every
    open.</p>
<p class="Pp">If <code class="Dv">MDMBUF</code> is set then output flow control
    is controlled by the state of Carrier Detect.</p>
<p class="Pp">If the <a class="Xr">tty(4)</a>
    <code class="Dv">TIOCFLAG_MDMBUF</code> flag has been set on the port then
    the <code class="Dv">MDMBUF</code> flag will automatically be set on every
    open.</p>
<p class="Pp">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.</p>
</section>
<section class="Ss">
<h2 class="Ss" id="Local_Modes"><a class="permalink" href="#Local_Modes">Local
  Modes</a></h2>
<p class="Pp">Values of the <var class="Fa">c_lflag</var> field describe the
    control of various functions, and are composed of the following masks.</p>
<p class="Pp"></p>
<div class="Bd-indent">
<dl class="Bl-tag Bl-compact">
  <dt id="ECHOKE"><a class="permalink" href="#ECHOKE"><code class="Dv">ECHOKE</code></a></dt>
  <dd>/* visual erase for line kill */</dd>
  <dt id="ECHOE"><a class="permalink" href="#ECHOE"><code class="Dv">ECHOE</code></a></dt>
  <dd>/* visually erase chars */</dd>
  <dt id="ECHO"><a class="permalink" href="#ECHO"><code class="Dv">ECHO</code></a></dt>
  <dd>/* enable echoing */</dd>
  <dt id="ECHONL"><a class="permalink" href="#ECHONL"><code class="Dv">ECHONL</code></a></dt>
  <dd>/* echo <code class="Dv">NL</code> even if <code class="Dv">ECHO</code> is
      off */</dd>
  <dt id="ECHOPRT"><a class="permalink" href="#ECHOPRT"><code class="Dv">ECHOPRT</code></a></dt>
  <dd>/* visual erase mode for hardcopy */</dd>
  <dt id="ECHOCTL"><a class="permalink" href="#ECHOCTL"><code class="Dv">ECHOCTL</code></a></dt>
  <dd>/* echo control chars as ^(Char) */</dd>
  <dt id="ISIG"><a class="permalink" href="#ISIG"><code class="Dv">ISIG</code></a></dt>
  <dd>/* enable signals <code class="Dv">INTR</code>,
      <code class="Dv">QUIT</code>, <code class="Dv">[D]SUSP</code> */</dd>
  <dt id="ICANON"><a class="permalink" href="#ICANON"><code class="Dv">ICANON</code></a></dt>
  <dd>/* canonicalize input lines */</dd>
  <dt id="ALTWERASE"><a class="permalink" href="#ALTWERASE"><code class="Dv">ALTWERASE</code></a></dt>
  <dd>/* use alternative <code class="Dv">WERASE</code> algorithm */</dd>
  <dt id="IEXTEN"><a class="permalink" href="#IEXTEN"><code class="Dv">IEXTEN</code></a></dt>
  <dd>/* enable <code class="Dv">DISCARD</code> and
      <code class="Dv">LNEXT</code> */</dd>
  <dt id="EXTPROC"><a class="permalink" href="#EXTPROC"><code class="Dv">EXTPROC</code></a></dt>
  <dd>/* external processing */</dd>
  <dt id="TOSTOP"><a class="permalink" href="#TOSTOP"><code class="Dv">TOSTOP</code></a></dt>
  <dd>/* stop background jobs from output */</dd>
  <dt id="FLUSHO"><a class="permalink" href="#FLUSHO"><code class="Dv">FLUSHO</code></a></dt>
  <dd>/* output being flushed (state) */</dd>
  <dt id="NOKERNINFO"><a class="permalink" href="#NOKERNINFO"><code class="Dv">NOKERNINFO</code></a></dt>
  <dd>/* no kernel output from <code class="Dv">VSTATUS</code> */</dd>
  <dt id="PENDIN"><a class="permalink" href="#PENDIN"><code class="Dv">PENDIN</code></a></dt>
  <dd>/* re-echo input buffer at next read */</dd>
  <dt id="NOFLSH"><a class="permalink" href="#NOFLSH"><code class="Dv">NOFLSH</code></a></dt>
  <dd>/* don't flush output on signal */</dd>
</dl>
</div>
<p class="Pp">If <code class="Dv">ECHO</code> is set, input characters are
    echoed back to the terminal. If <code class="Dv">ECHO</code> is not set,
    input characters are not echoed.</p>
<p class="Pp">If <code class="Dv">ECHOE</code> and
    <code class="Dv">ICANON</code> are set, the <code class="Dv">ERASE</code>
    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.</p>
<p class="Pp">If <code class="Dv">ECHOK</code> and
    <code class="Dv">ICANON</code> are set, the <code class="Dv">KILL</code>
    character causes the current line to be discarded and the system echoes the
    &#x2018;<code class="Li">\n</code>&#x2019; character after the
    <code class="Dv">KILL</code> character.</p>
<p class="Pp">If <code class="Dv">ECHOKE</code> and
    <code class="Dv">ICANON</code> are set, the <code class="Dv">KILL</code>
    character causes the current line to be discarded and the system causes the
    terminal to erase the line from the display.</p>
<p class="Pp">If <code class="Dv">ECHOPRT</code> and
    <code class="Dv">ICANON</code> are set, the system assumes that the display
    is a printing device and prints a backslash and the erased characters when
    processing <code class="Dv">ERASE</code> characters, followed by a forward
    slash.</p>
<p class="Pp">If <code class="Dv">ECHOCTL</code> is set, the system echoes
    control characters in a visible fashion using a caret followed by the
    control character.</p>
<p class="Pp">If <code class="Dv">ALTWERASE</code> is set, the system uses an
    alternative algorithm for determining what constitutes a word when
    processing <code class="Dv">WERASE</code> characters (see
    <code class="Dv">WERASE</code>).</p>
<p class="Pp">If <code class="Dv">ECHONL</code> and
    <code class="Dv">ICANON</code> are set, the
    &#x2018;<code class="Li">\n</code>&#x2019; character echoes even if
    <code class="Dv">ECHO</code> is not set.</p>
<p class="Pp">If <code class="Dv">ICANON</code> is set, canonical processing is
    enabled. This enables the erase and kill edit functions, and the assembly of
    input characters into lines delimited by <code class="Dv">NL</code>,
    <code class="Dv">EOF</code>, and <code class="Dv">EOL</code>, as described
    in <a class="Sx" href="#Canonical_Mode_Input_Processing">Canonical Mode
    Input Processing</a>.</p>
<p class="Pp">If <code class="Dv">ICANON</code> is not set, read requests are
    satisfied directly from the input queue. A read is not satisfied until at
    least <code class="Dv">VMIN</code> bytes have been received or the timeout
    value <code class="Dv">VTIME</code> expired between bytes. The time value
    represents tenths of seconds. See
    <a class="Sx" href="#Noncanonical_Mode_Input_Processing">Noncanonical Mode
    Input Processing</a> for more details.</p>
<p class="Pp">If <code class="Dv">ISIG</code> is set, each input character is
    checked against the special control characters <code class="Dv">INTR</code>,
    <code class="Dv">QUIT</code>, and <code class="Dv">SUSP</code> (job control
    only). If an input character matches one of these control characters, the
    function associated with that character is performed. If
    <code class="Dv">ISIG</code> is not set, no checking is done. Thus these
    special input functions are possible only if <code class="Dv">ISIG</code> is
    set.</p>
<p class="Pp">If <code class="Dv">IEXTEN</code> is set, implementation-defined
    functions are recognized from the input data. How
    <code class="Dv">IEXTEN</code> being set interacts with
    <code class="Dv">ICANON</code>, <code class="Dv">ISIG</code>,
    <code class="Dv">IXON</code>, or <code class="Dv">IXOFF</code> is
    implementation defined. If <code class="Dv">IEXTEN</code> is not set, then
    implementation-defined functions are not recognized, and the corresponding
    input characters are not processed as described for
    <code class="Dv">ICANON</code>, <code class="Dv">ISIG</code>,
    <code class="Dv">IXON</code>, and <code class="Dv">IXOFF</code>.</p>
<p class="Pp">If <code class="Dv">NOFLSH</code> is set, the normal flush of the
    input and output queues associated with the <code class="Dv">INTR</code>,
    <code class="Dv">QUIT</code>, and <code class="Dv">SUSP</code> characters
    are not be done.</p>
<p class="Pp">If <code class="Dv">TOSTOP</code> is set, the signal
    <code class="Dv">SIGTTOU</code> 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 <code class="Dv">SIGTTOU</code> signals are excepted and allowed
    to produce output and the <code class="Dv">SIGTTOU</code> signal is not
    sent.</p>
<p class="Pp">If <code class="Dv">NOKERNINFO</code> is set, the kernel does not
    produce a status message when processing <code class="Dv">STATUS</code>
    characters (see <code class="Dv">STATUS</code>).</p>
</section>
<section class="Ss">
<h2 class="Ss" id="Special_Control_Characters"><a class="permalink" href="#Special_Control_Characters">Special
  Control Characters</a></h2>
<p class="Pp">The special control characters values are defined by the array
    <var class="Fa">c_cc</var>. 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
    <code class="In">&lt;<a class="In">ttydefaults.h</a>&gt;</code>.</p>
<p class="Pp"></p>
<table class="Bl-column Bd-indent Bl-compact">
  <tr id="Index">
    <td><a class="permalink" href="#Index"><i class="Em">Index Name</i></a></td>
    <td>Special Character</td>
    <td>Default Value</td>
  </tr>
  <tr id="VEOF">
    <td><a class="permalink" href="#VEOF"><code class="Dv">VEOF</code></a></td>
    <td>EOF</td>
    <td>^D</td>
  </tr>
  <tr id="VEOL">
    <td><a class="permalink" href="#VEOL"><code class="Dv">VEOL</code></a></td>
    <td>EOL</td>
    <td>_POSIX_VDISABLE</td>
  </tr>
  <tr id="VEOL2">
    <td><a class="permalink" href="#VEOL2"><code class="Dv">VEOL2</code></a></td>
    <td>EOL2</td>
    <td>_POSIX_VDISABLE</td>
  </tr>
  <tr id="VERASE">
    <td><a class="permalink" href="#VERASE"><code class="Dv">VERASE</code></a></td>
    <td>ERASE</td>
    <td>^? &#x2018;<code class="Li">\177</code>&#x2019;</td>
  </tr>
  <tr id="VWERASE">
    <td><a class="permalink" href="#VWERASE"><code class="Dv">VWERASE</code></a></td>
    <td>WERASE</td>
    <td>^W</td>
  </tr>
  <tr id="VKILL">
    <td><a class="permalink" href="#VKILL"><code class="Dv">VKILL</code></a></td>
    <td>KILL</td>
    <td>^U</td>
  </tr>
  <tr id="VREPRINT">
    <td><a class="permalink" href="#VREPRINT"><code class="Dv">VREPRINT</code></a></td>
    <td>REPRINT</td>
    <td>^R</td>
  </tr>
  <tr id="VINTR">
    <td><a class="permalink" href="#VINTR"><code class="Dv">VINTR</code></a></td>
    <td>INTR</td>
    <td>^C</td>
  </tr>
  <tr id="VQUIT">
    <td><a class="permalink" href="#VQUIT"><code class="Dv">VQUIT</code></a></td>
    <td>QUIT</td>
    <td>^\\ &#x2018;<code class="Li">\34</code>&#x2019;</td>
  </tr>
  <tr id="VSUSP">
    <td><a class="permalink" href="#VSUSP"><code class="Dv">VSUSP</code></a></td>
    <td>SUSP</td>
    <td>^Z</td>
  </tr>
  <tr id="VDSUSP">
    <td><a class="permalink" href="#VDSUSP"><code class="Dv">VDSUSP</code></a></td>
    <td>DSUSP</td>
    <td>^Y</td>
  </tr>
  <tr id="VSTART">
    <td><a class="permalink" href="#VSTART"><code class="Dv">VSTART</code></a></td>
    <td>START</td>
    <td>^Q</td>
  </tr>
  <tr id="VSTOP">
    <td><a class="permalink" href="#VSTOP"><code class="Dv">VSTOP</code></a></td>
    <td>STOP</td>
    <td>^S</td>
  </tr>
  <tr id="VLNEXT">
    <td><a class="permalink" href="#VLNEXT"><code class="Dv">VLNEXT</code></a></td>
    <td>LNEXT</td>
    <td>^V</td>
  </tr>
  <tr id="VDISCARD">
    <td><a class="permalink" href="#VDISCARD"><code class="Dv">VDISCARD</code></a></td>
    <td>DISCARD</td>
    <td>^O</td>
  </tr>
  <tr id="VMIN">
    <td><a class="permalink" href="#VMIN"><code class="Dv">VMIN</code></a></td>
    <td>---</td>
    <td>1</td>
  </tr>
  <tr id="VTIME">
    <td><a class="permalink" href="#VTIME"><code class="Dv">VTIME</code></a></td>
    <td>---</td>
    <td>0</td>
  </tr>
  <tr id="VSTATUS">
    <td><a class="permalink" href="#VSTATUS"><code class="Dv">VSTATUS</code></a></td>
    <td>STATUS</td>
    <td>^T</td>
  </tr>
</table>
<p class="Pp">If the value of one of the changeable special control characters
    (see <a class="Sx" href="#Special_Characters">Special Characters</a>) is
    {<code class="Dv">_POSIX_VDISABLE</code>}, that function is disabled; that
    is, no input data is recognized as the disabled special character. If
    <code class="Dv">ICANON</code> is not set, the value of
    {<code class="Dv">_POSIX_VDISABLE</code>} has no special meaning for the
    <code class="Dv">VMIN</code> and <code class="Dv">VTIME</code> entries of
    the <var class="Fa">c_cc</var> array.</p>
<p class="Pp">The initial values of the flags and control characters after
    <a class="Xr">open(2)</a> is set according to the values in the header
    <code class="In">&lt;<a class="In">sys/ttydefaults.h</a>&gt;</code>.</p>
</section>
</section>
<section class="Sh">
<h1 class="Sh" id="SEE_ALSO"><a class="permalink" href="#SEE_ALSO">SEE
  ALSO</a></h1>
<p class="Pp"><a class="Xr">tcsendbreak(3)</a>,
  <a class="Xr">tcsetattr(3)</a></p>
</section>
</div>
<table class="foot">
  <tr>
    <td class="foot-date">October 7, 2006</td>
    <td class="foot-os">NetBSD 10.1</td>
  </tr>
</table>