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 <h4 class="subsection">16.6.5 Byte Order Conversion</h4>

 <p><a name="index-byte-order-conversion_002c-for-socket-1747"></a><a name="index-converting-byte-order-1748"></a>
 <a name="index-big_002dendian-1749"></a><a name="index-little_002dendian-1750"></a>Different kinds of computers use different conventions for the
 ordering of bytes within a word.  Some computers put the most
 significant byte within a word first (this is called &ldquo;big-endian&rdquo;
 order), and others put it last (&ldquo;little-endian&rdquo; order).

    <p><a name="index-network-byte-order-1751"></a>So that machines with different byte order conventions can
 communicate, the Internet protocols specify a canonical byte order
 convention for data transmitted over the network.  This is known
 as <dfn>network byte order</dfn>.

    <p>When establishing an Internet socket connection, you must make sure that
 the data in the <code>sin_port</code> and <code>sin_addr</code> members of the
 <code>sockaddr_in</code> structure are represented in network byte order.
 If you are encoding integer data in the messages sent through the
 socket, you should convert this to network byte order too.  If you don't
 do this, your program may fail when running on or talking to other kinds
 of machines.

    <p>If you use <code>getservbyname</code> and <code>gethostbyname</code> or
 <code>inet_addr</code> to get the port number and host address, the values are
 already in network byte order, and you can copy them directly into
 the <code>sockaddr_in</code> structure.

    <p>Otherwise, you have to convert the values explicitly.  Use <code>htons</code>
 and <code>ntohs</code> to convert values for the <code>sin_port</code> member.  Use
 <code>htonl</code> and <code>ntohl</code> to convert IPv4 addresses for the
 <code>sin_addr</code> member.  (Remember, <code>struct in_addr</code> is equivalent
 to <code>uint32_t</code>.)  These functions are declared in
 <samp><span class="file">netinet/in.h</span></samp>.
 <a name="index-netinet_002fin_002eh-1752"></a>
 <!-- netinet/in.h -->
 <!-- BSD -->

 <div class="defun">
 &mdash; Function: uint16_t <b>htons</b> (<var>uint16_t hostshort</var>)<var><a name="index-htons-1753"></a></var><br>
 <blockquote><p>This function converts the <code>uint16_t</code> integer <var>hostshort</var> from
 host byte order to network byte order.
 </p></blockquote></div>

 <!-- netinet/in.h -->
 <!-- BSD -->
 <div class="defun">
 &mdash; Function: uint16_t <b>ntohs</b> (<var>uint16_t netshort</var>)<var><a name="index-ntohs-1754"></a></var><br>
 <blockquote><p>This function converts the <code>uint16_t</code> integer <var>netshort</var> from
 network byte order to host byte order.
 </p></blockquote></div>

 <!-- netinet/in.h -->
 <!-- BSD -->
 <div class="defun">
 &mdash; Function: uint32_t <b>htonl</b> (<var>uint32_t hostlong</var>)<var><a name="index-htonl-1755"></a></var><br>
 <blockquote><p>This function converts the <code>uint32_t</code> integer <var>hostlong</var> from
 host byte order to network byte order.

         <p>This is used for IPv4 Internet addresses.
 </p></blockquote></div>

 <!-- netinet/in.h -->
 <!-- BSD -->
 <div class="defun">
 &mdash; Function: uint32_t <b>ntohl</b> (<var>uint32_t netlong</var>)<var><a name="index-ntohl-1756"></a></var><br>
 <blockquote><p>This function converts the <code>uint32_t</code> integer <var>netlong</var> from
 network byte order to host byte order.

         <p>This is used for IPv4 Internet addresses.
 </p></blockquote></div>

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	Copyright (C) 1993, 1994, 1995, 1996, 1997, 1998, 1999, 2001, 2002,
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	<h4 class="subsection">16.6.5 Byte Order Conversion</h4>

	<p><a name="index-byte-order-conversion_002c-for-socket-1747"></a><a name="index-converting-byte-order-1748"></a>
	<a name="index-big_002dendian-1749"></a><a name="index-little_002dendian-1750"></a>Different kinds of computers use different conventions for the
	ordering of bytes within a word. Some computers put the most
	significant byte within a word first (this is called “big-endian”
	order), and others put it last (“little-endian” order).

	<p><a name="index-network-byte-order-1751"></a>So that machines with different byte order conventions can
	communicate, the Internet protocols specify a canonical byte order
	convention for data transmitted over the network. This is known
	as <dfn>network byte order</dfn>.

	<p>When establishing an Internet socket connection, you must make sure that
	the data in the <code>sin_port</code> and <code>sin_addr</code> members of the
	<code>sockaddr_in</code> structure are represented in network byte order.
	If you are encoding integer data in the messages sent through the
	socket, you should convert this to network byte order too. If you don't
	do this, your program may fail when running on or talking to other kinds
	of machines.

	<p>If you use <code>getservbyname</code> and <code>gethostbyname</code> or
	<code>inet_addr</code> to get the port number and host address, the values are
	already in network byte order, and you can copy them directly into
	the <code>sockaddr_in</code> structure.

	<p>Otherwise, you have to convert the values explicitly. Use <code>htons</code>
	and <code>ntohs</code> to convert values for the <code>sin_port</code> member. Use
	<code>htonl</code> and <code>ntohl</code> to convert IPv4 addresses for the
	<code>sin_addr</code> member. (Remember, <code>struct in_addr</code> is equivalent
	to <code>uint32_t</code>.) These functions are declared in
	<samp><span class="file">netinet/in.h</span></samp>.
	<a name="index-netinet_002fin_002eh-1752"></a>
	<!-- netinet/in.h -->
	<!-- BSD -->

	<div class="defun">
	— Function: uint16_t <b>htons</b> (<var>uint16_t hostshort</var>)<var><a name="index-htons-1753"></a></var><br>
	<blockquote><p>This function converts the <code>uint16_t</code> integer <var>hostshort</var> from
	host byte order to network byte order.
	</p></blockquote></div>

	<!-- netinet/in.h -->
	<!-- BSD -->
	<div class="defun">
	— Function: uint16_t <b>ntohs</b> (<var>uint16_t netshort</var>)<var><a name="index-ntohs-1754"></a></var><br>
	<blockquote><p>This function converts the <code>uint16_t</code> integer <var>netshort</var> from
	network byte order to host byte order.
	</p></blockquote></div>

	<!-- netinet/in.h -->
	<!-- BSD -->
	<div class="defun">
	— Function: uint32_t <b>htonl</b> (<var>uint32_t hostlong</var>)<var><a name="index-htonl-1755"></a></var><br>
	<blockquote><p>This function converts the <code>uint32_t</code> integer <var>hostlong</var> from
	host byte order to network byte order.

	<p>This is used for IPv4 Internet addresses.
	</p></blockquote></div>

	<!-- netinet/in.h -->
	<!-- BSD -->
	<div class="defun">
	— Function: uint32_t <b>ntohl</b> (<var>uint32_t netlong</var>)<var><a name="index-ntohl-1756"></a></var><br>
	<blockquote><p>This function converts the <code>uint32_t</code> integer <var>netlong</var> from
	network byte order to host byte order.

	<p>This is used for IPv4 Internet addresses.
	</p></blockquote></div>

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