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<HTML><!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"><!-- Created on September, 12 2004 by texi2html 1.64 --><!-- Written by: Lionel Cons <Lionel.Cons@cern.ch> (original author) Karl Berry <karl@freefriends.org> Olaf Bachmann <obachman@mathematik.uni-kl.de> and many others.Maintained by: Olaf Bachmann <obachman@mathematik.uni-kl.de>Send bugs and suggestions to <texi2html@mathematik.uni-kl.de> --><HEAD><TITLE>Autoconf, Automake, and Libtool: C Endianness</TITLE><META NAME="description" CONTENT="Autoconf, Automake, and Libtool: C Endianness"><META NAME="keywords" CONTENT="Autoconf, Automake, and Libtool: C Endianness"><META NAME="resource-type" CONTENT="document"><META NAME="distribution" CONTENT="global"><META NAME="Generator" CONTENT="texi2html 1.64"><script language="Javascript"><!-- // Check the browser version. function checkVersion() { if (navigator.appVersion.charAt(0)>=3) return true; if (navigator.appVersion.charAt(0)>=4) return true; else return false; } if (checkVersion()) { homeon = new Image(); 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Modern systems do this by storing thenumber byte by byte such that the bytes can simply be concatenated intothe final number. However, the order of storage varies: some systemsstore the least significant byte at the lowest address in memory, whilesome store the most significant byte there. These are referred to as<EM>little-endian</EM> and <EM>big-endian</EM> systems,respectively.<A NAME="DOCF32" HREF="autobook_fot.html#FOOT32">(32)</A></P><P>This difference means that portable code may not make any assumptionsabout the order of storage of a number. For example, code like thiswill act differently on different systems:<TABLE><tr><td> </td><td class=smallexample><FONT SIZE=-1><pre> /* Example of non-portable code; don't do this */ int i = 4; char c = *(char *) i;</FONT></pre></td></tr></table></P><P>Although that was a contrived example, real problems arise when writingnumeric data in a file or across a network connection. If the file ornetwork connection may be read on a different type of system, numericdata must be written in a format which can be unambiguously recovered.It is not portable to simply do something like<TABLE><tr><td> </td><td class=smallexample><FONT SIZE=-1><pre> /* Example of non-portable code; don't do this */ write (fd, &i, sizeof i);</FONT></pre></td></tr></table>This example is non-portable both because of endianness and because itassumes that the size of the type of <CODE>i</CODE> are the same on bothsystems.</P><P>Instead, do something like this:<TABLE><tr><td> </td><td class=smallexample><FONT SIZE=-1><pre> int j; char buf[4]; for (j = 0; j < 4; ++j) buf[j] = (i >> (j * 8)) & 0xff; write (fd, buf, 4); /* In real code, check the return value */</FONT></pre></td></tr></table>This unambiguously writes out a little endian 4 byte value. The codewill work on any system, and the result can be read unambiguously on anysystem.</P><P>Another approach to handling endianness is to use the <CODE>hton<VAR>s</VAR></CODE>and <CODE>ntoh<VAR>s</VAR></CODE> functions available on most systems. Thesefunctions convert between <EM>network endianness</EM> and host endianness.Network endianness is big-endian; it has that name because the standardTCP/IP network protocols use big-endian ordering.</P><P>These functions come in two sizes: <CODE>htonl</CODE> and <CODE>ntohl</CODE> operateon 4-byte quantities, and <CODE>htons</CODE> and <CODE>ntohs</CODE> operate on2-byte quantities. The <CODE>hton</CODE> functions convert host endianness tonetwork endianness. The <CODE>ntoh</CODE> functions convert networkendianness to host endianness. On big-endian systems, these functionssimply return their arguments; on little-endian systems, they returntheir arguments after swapping the bytes.</P><P>Although these functions are used in a lot of existing code, they can bedifficult to use in highly portable code, because they require knowingthe exact size of your data types. If you know that the type <CODE>int</CODE>is exactly 4 bytes long, then it is possible to write code like thefollowing:<TABLE><tr><td> </td><td class=smallexample><FONT SIZE=-1><pre> int j; j = htonl (i); write (fd, &j, 4);</FONT></pre></td></tr></table>However, if <CODE>int</CODE> is not exactly 4 bytes long, this example willnot work correctly on all systems.</P><P><A NAME="C Structure Layout"></A></TR></TABLE><BR> <FONT SIZE="-1">This document was generatedby <I>Gary V. Vaughan</I> on <I>September, 12 2004</I>using <A HREF="http://www.mathematik.uni-kl.de/~obachman/Texi2html"><I>texi2html</I></A></BODY></HTML>⌨️ 快捷键说明
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