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<h4 class="subsection">16.8.3 Socket Pairs</h4>

<p><a name="index-creating-a-socket-pair-1783"></a><a name="index-socket-pair-1784"></a><a name="index-opening-a-socket-pair-1785"></a>
<a name="index-sys_002fsocket_002eh-1786"></a>A <dfn>socket pair</dfn> consists of a pair of connected (but unnamed)
sockets.  It is very similar to a pipe and is used in much the same
way.  Socket pairs are created with the <code>socketpair</code> function,
declared in <samp><span class="file">sys/socket.h</span></samp>.  A socket pair is much like a pipe; the
main difference is that the socket pair is bidirectional, whereas the
pipe has one input-only end and one output-only end (see <a href="Pipes-and-FIFOs.html#Pipes-and-FIFOs">Pipes and FIFOs</a>).

<!-- sys/socket.h -->
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<div class="defun">
&mdash; Function: int <b>socketpair</b> (<var>int namespace, int style, int protocol, int filedes</var><tt>[2]</tt>)<var><a name="index-socketpair-1787"></a></var><br>
<blockquote><p>This function creates a socket pair, returning the file descriptors in
<var>filedes</var><code>[0]</code> and <var>filedes</var><code>[1]</code>.  The socket pair
is a full-duplex communications channel, so that both reading and writing
may be performed at either end.

        <p>The <var>namespace</var>, <var>style</var> and <var>protocol</var> arguments are
interpreted as for the <code>socket</code> function.  <var>style</var> should be
one of the communication styles listed in <a href="Communication-Styles.html#Communication-Styles">Communication Styles</a>. 
The <var>namespace</var> argument specifies the namespace, which must be
<code>AF_LOCAL</code> (see <a href="Local-Namespace.html#Local-Namespace">Local Namespace</a>); <var>protocol</var> specifies the
communications protocol, but zero is the only meaningful value.

        <p>If <var>style</var> specifies a connectionless communication style, then
the two sockets you get are not <em>connected</em>, strictly speaking,
but each of them knows the other as the default destination address,
so they can send packets to each other.

        <p>The <code>socketpair</code> function returns <code>0</code> on success and <code>-1</code>
on failure.  The following <code>errno</code> error conditions are defined
for this function:

          <dl>
<dt><code>EMFILE</code><dd>The process has too many file descriptors open.

          <br><dt><code>EAFNOSUPPORT</code><dd>The specified namespace is not supported.

          <br><dt><code>EPROTONOSUPPORT</code><dd>The specified protocol is not supported.

          <br><dt><code>EOPNOTSUPP</code><dd>The specified protocol does not support the creation of socket pairs. 
</dl>
        </p></blockquote></div>

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