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 Previous:&nbsp;<a rel="previous" accesskey="p" href="Socket-Option-Functions.html#Socket-Option-Functions">Socket Option Functions</a>,
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 <h4 class="subsection">16.12.2 Socket-Level Options</h4>

 <!-- sys/socket.h -->
 <!-- BSD -->
 <div class="defun">
 &mdash; Constant: int <b>SOL_SOCKET</b><var><a name="index-SOL_005fSOCKET-1831"></a></var><br>
 <blockquote><p>Use this constant as the <var>level</var> argument to <code>getsockopt</code> or
 <code>setsockopt</code> to manipulate the socket-level options described in
 this section.
 </p></blockquote></div>

    <p><a name="index-sys_002fsocket_002eh-1832"></a>Here is a table of socket-level option names; all are defined in the
 header file <samp><span class="file">sys/socket.h</span></samp>.

      <dl>
 <!-- sys/socket.h -->
 <!-- BSD -->
 <dt><code>SO_DEBUG</code><dd><!-- Extra blank line here makes the table look better. -->

      <p>This option toggles recording of debugging information in the underlying
 protocol modules.  The value has type <code>int</code>; a nonzero value means
 &ldquo;yes&rdquo;.
 <!-- !!! should say how this is used -->
 <!-- OK, anyone who knows, please explain. -->

      <!-- sys/socket.h -->
      <!-- BSD -->
      <br><dt><code>SO_REUSEADDR</code><dd>This option controls whether <code>bind</code> (see <a href="Setting-Address.html#Setting-Address">Setting Address</a>)
 should permit reuse of local addresses for this socket.  If you enable
 this option, you can actually have two sockets with the same Internet
 port number; but the system won't allow you to use the two
 identically-named sockets in a way that would confuse the Internet.  The
 reason for this option is that some higher-level Internet protocols,
 including FTP, require you to keep reusing the same port number.

      <p>The value has type <code>int</code>; a nonzero value means &ldquo;yes&rdquo;.

      <!-- sys/socket.h -->
      <!-- BSD -->
      <br><dt><code>SO_KEEPALIVE</code><dd>This option controls whether the underlying protocol should
 periodically transmit messages on a connected socket.  If the peer
 fails to respond to these messages, the connection is considered
 broken.  The value has type <code>int</code>; a nonzero value means
 &ldquo;yes&rdquo;.

      <!-- sys/socket.h -->
      <!-- BSD -->
      <br><dt><code>SO_DONTROUTE</code><dd>This option controls whether outgoing messages bypass the normal
 message routing facilities.  If set, messages are sent directly to the
 network interface instead.  The value has type <code>int</code>; a nonzero
 value means &ldquo;yes&rdquo;.

      <!-- sys/socket.h -->
      <!-- BSD -->
      <br><dt><code>SO_LINGER</code><dd>This option specifies what should happen when the socket of a type
 that promises reliable delivery still has untransmitted messages when
 it is closed; see <a href="Closing-a-Socket.html#Closing-a-Socket">Closing a Socket</a>.  The value has type
 <code>struct linger</code>.

      <!-- sys/socket.h -->
      <!-- BSD -->
      <div class="defun">
 &mdash; Data Type: <b>struct linger</b><var><a name="index-struct-linger-1833"></a></var><br>
 <blockquote> <p>This structure type has the following members:

                <dl>
 <dt><code>int l_onoff</code><dd>This field is interpreted as a boolean.  If nonzero, <code>close</code>
 blocks until the data are transmitted or the timeout period has expired.

                <br><dt><code>int l_linger</code><dd>This specifies the timeout period, in seconds.
 </dl>
              </p></blockquote></div>

      <!-- sys/socket.h -->
      <!-- BSD -->
      <br><dt><code>SO_BROADCAST</code><dd>This option controls whether datagrams may be broadcast from the socket.
 The value has type <code>int</code>; a nonzero value means &ldquo;yes&rdquo;.

      <!-- sys/socket.h -->
      <!-- BSD -->
      <br><dt><code>SO_OOBINLINE</code><dd>If this option is set, out-of-band data received on the socket is
 placed in the normal input queue.  This permits it to be read using
 <code>read</code> or <code>recv</code> without specifying the <code>MSG_OOB</code>
 flag.  See <a href="Out_002dof_002dBand-Data.html#Out_002dof_002dBand-Data">Out-of-Band Data</a>.  The value has type <code>int</code>; a
 nonzero value means &ldquo;yes&rdquo;.

      <!-- sys/socket.h -->
      <!-- BSD -->
      <br><dt><code>SO_SNDBUF</code><dd>This option gets or sets the size of the output buffer.  The value is a
 <code>size_t</code>, which is the size in bytes.

      <!-- sys/socket.h -->
      <!-- BSD -->
      <br><dt><code>SO_RCVBUF</code><dd>This option gets or sets the size of the input buffer.  The value is a
 <code>size_t</code>, which is the size in bytes.

      <!-- sys/socket.h -->
      <!-- GNU -->
      <br><dt><code>SO_STYLE</code><dd><!-- sys/socket.h -->
 <!-- BSD -->
 <dt><code>SO_TYPE</code><dd>This option can be used with <code>getsockopt</code> only.  It is used to
 get the socket's communication style.  <code>SO_TYPE</code> is the
 historical name, and <code>SO_STYLE</code> is the preferred name in GNU.
 The value has type <code>int</code> and its value designates a communication
 style; see <a href="Communication-Styles.html#Communication-Styles">Communication Styles</a>.

      <!-- sys/socket.h -->
      <!-- BSD -->
      <br><dt><code>SO_ERROR</code><dd><!-- Extra blank line here makes the table look better. -->

      <p>This option can be used with <code>getsockopt</code> only.  It is used to reset
 the error status of the socket.  The value is an <code>int</code>, which represents
 the previous error status.
 <!-- !!! what is "socket error status"?  this is never defined. -->
 </dl>

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	<p>
	Previous: <a rel="previous" accesskey="p" href="Socket-Option-Functions.html#Socket-Option-Functions">Socket Option Functions</a>,
	Up: <a rel="up" accesskey="u" href="Socket-Options.html#Socket-Options">Socket Options</a>
	<hr>
	</div>

	<h4 class="subsection">16.12.2 Socket-Level Options</h4>

	<!-- sys/socket.h -->
	<!-- BSD -->
	<div class="defun">
	— Constant: int <b>SOL_SOCKET</b><var><a name="index-SOL_005fSOCKET-1831"></a></var><br>
	<blockquote><p>Use this constant as the <var>level</var> argument to <code>getsockopt</code> or
	<code>setsockopt</code> to manipulate the socket-level options described in
	this section.
	</p></blockquote></div>

	<p><a name="index-sys_002fsocket_002eh-1832"></a>Here is a table of socket-level option names; all are defined in the
	header file <samp><span class="file">sys/socket.h</span></samp>.

	<dl>
	<!-- sys/socket.h -->
	<!-- BSD -->
	<dt><code>SO_DEBUG</code><dd><!-- Extra blank line here makes the table look better. -->

	<p>This option toggles recording of debugging information in the underlying
	protocol modules. The value has type <code>int</code>; a nonzero value means
	“yes”.
	<!-- !!! should say how this is used -->
	<!-- OK, anyone who knows, please explain. -->

	<!-- sys/socket.h -->
	<!-- BSD -->
	<br><dt><code>SO_REUSEADDR</code><dd>This option controls whether <code>bind</code> (see <a href="Setting-Address.html#Setting-Address">Setting Address</a>)
	should permit reuse of local addresses for this socket. If you enable
	this option, you can actually have two sockets with the same Internet
	port number; but the system won't allow you to use the two
	identically-named sockets in a way that would confuse the Internet. The
	reason for this option is that some higher-level Internet protocols,
	including FTP, require you to keep reusing the same port number.

	<p>The value has type <code>int</code>; a nonzero value means “yes”.

	<!-- sys/socket.h -->
	<!-- BSD -->
	<br><dt><code>SO_KEEPALIVE</code><dd>This option controls whether the underlying protocol should
	periodically transmit messages on a connected socket. If the peer
	fails to respond to these messages, the connection is considered
	broken. The value has type <code>int</code>; a nonzero value means
	“yes”.

	<!-- sys/socket.h -->
	<!-- BSD -->
	<br><dt><code>SO_DONTROUTE</code><dd>This option controls whether outgoing messages bypass the normal
	message routing facilities. If set, messages are sent directly to the
	network interface instead. The value has type <code>int</code>; a nonzero
	value means “yes”.

	<!-- sys/socket.h -->
	<!-- BSD -->
	<br><dt><code>SO_LINGER</code><dd>This option specifies what should happen when the socket of a type
	that promises reliable delivery still has untransmitted messages when
	it is closed; see <a href="Closing-a-Socket.html#Closing-a-Socket">Closing a Socket</a>. The value has type
	<code>struct linger</code>.

	<!-- sys/socket.h -->
	<!-- BSD -->
	<div class="defun">
	— Data Type: <b>struct linger</b><var><a name="index-struct-linger-1833"></a></var><br>
	<blockquote> <p>This structure type has the following members:

	<dl>
	<dt><code>int l_onoff</code><dd>This field is interpreted as a boolean. If nonzero, <code>close</code>
	blocks until the data are transmitted or the timeout period has expired.

	<br><dt><code>int l_linger</code><dd>This specifies the timeout period, in seconds.
	</dl>
	</p></blockquote></div>

	<!-- sys/socket.h -->
	<!-- BSD -->
	<br><dt><code>SO_BROADCAST</code><dd>This option controls whether datagrams may be broadcast from the socket.
	The value has type <code>int</code>; a nonzero value means “yes”.

	<!-- sys/socket.h -->
	<!-- BSD -->
	<br><dt><code>SO_OOBINLINE</code><dd>If this option is set, out-of-band data received on the socket is
	placed in the normal input queue. This permits it to be read using
	<code>read</code> or <code>recv</code> without specifying the <code>MSG_OOB</code>
	flag. See <a href="Out_002dof_002dBand-Data.html#Out_002dof_002dBand-Data">Out-of-Band Data</a>. The value has type <code>int</code>; a
	nonzero value means “yes”.

	<!-- sys/socket.h -->
	<!-- BSD -->
	<br><dt><code>SO_SNDBUF</code><dd>This option gets or sets the size of the output buffer. The value is a
	<code>size_t</code>, which is the size in bytes.

	<!-- sys/socket.h -->
	<!-- BSD -->
	<br><dt><code>SO_RCVBUF</code><dd>This option gets or sets the size of the input buffer. The value is a
	<code>size_t</code>, which is the size in bytes.

	<!-- sys/socket.h -->
	<!-- GNU -->
	<br><dt><code>SO_STYLE</code><dd><!-- sys/socket.h -->
	<!-- BSD -->
	<dt><code>SO_TYPE</code><dd>This option can be used with <code>getsockopt</code> only. It is used to
	get the socket's communication style. <code>SO_TYPE</code> is the
	historical name, and <code>SO_STYLE</code> is the preferred name in GNU.
	The value has type <code>int</code> and its value designates a communication
	style; see <a href="Communication-Styles.html#Communication-Styles">Communication Styles</a>.

	<!-- sys/socket.h -->
	<!-- BSD -->
	<br><dt><code>SO_ERROR</code><dd><!-- Extra blank line here makes the table look better. -->

	<p>This option can be used with <code>getsockopt</code> only. It is used to reset
	the error status of the socket. The value is an <code>int</code>, which represents
	the previous error status.
	<!-- !!! what is "socket error status"? this is never defined. -->
	</dl>

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