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<h5 class="subsubsection">17.4.9.3 Special Characters for Flow Control</h5>

<p>These special characters may be active in either canonical or noncanonical
input mode, but their use is controlled by the flags <code>IXON</code> and
<code>IXOFF</code> (see <a href="Input-Modes.html#Input-Modes">Input Modes</a>).

<!-- termios.h -->
<!-- POSIX.1 -->
<div class="defun">
&mdash; Macro: int <b>VSTART</b><var><a name="index-VSTART-1982"></a></var><br>
<blockquote><p><a name="index-START-character-1983"></a>This is the subscript for the START character in the special control
character array.  <var>termios</var><code>.c_cc[VSTART]</code> holds the
character itself.

        <p>The START character is used to support the <code>IXON</code> and <code>IXOFF</code>
input modes.  If <code>IXON</code> is set, receiving a START character resumes
suspended output; the START character itself is discarded.  If
<code>IXANY</code> is set, receiving any character at all resumes suspended
output; the resuming character is not discarded unless it is the START
character.  <code>IXOFF</code> is set, the system may also transmit START
characters to the terminal.

        <p>The usual value for the START character is <kbd>C-q</kbd>.  You may not be
able to change this value&mdash;the hardware may insist on using <kbd>C-q</kbd>
regardless of what you specify. 
</p></blockquote></div>

<!-- termios.h -->
<!-- POSIX.1 -->
<div class="defun">
&mdash; Macro: int <b>VSTOP</b><var><a name="index-VSTOP-1984"></a></var><br>
<blockquote><p><a name="index-STOP-character-1985"></a>This is the subscript for the STOP character in the special control
character array.  <var>termios</var><code>.c_cc[VSTOP]</code> holds the character
itself.

        <p>The STOP character is used to support the <code>IXON</code> and <code>IXOFF</code>
input modes.  If <code>IXON</code> is set, receiving a STOP character causes
output to be suspended; the STOP character itself is discarded.  If
<code>IXOFF</code> is set, the system may also transmit STOP characters to the
terminal, to prevent the input queue from overflowing.

        <p>The usual value for the STOP character is <kbd>C-s</kbd>.  You may not be
able to change this value&mdash;the hardware may insist on using <kbd>C-s</kbd>
regardless of what you specify. 
</p></blockquote></div>

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