📄 byteswap.html
字号:
<!DOCTYPE HTML PUBLIC "-//W3O//DTD W3 HTML 2.0//EN"><!-- This collection of hypertext pages is Copyright 1995-2005 by Steve Summit. --><!-- Content from the book "C Programming FAQs: Frequently Asked Questions" --><!-- (Addison-Wesley, 1995, ISBN 0-201-84519-9) is made available here by --><!-- permission of the author and the publisher as a service to the community. --><!-- It is intended to complement the use of the published text --><!-- and is protected by international copyright laws. --><!-- The on-line content may be accessed freely for personal use --><!-- but may not be published or retransmitted without explicit permission. --><!-- --><!-- this page built Sat Dec 24 21:47:47 2005 by faqproc version 2.7 --><!-- from source file miscbits.sgml dated Wed Dec 21 13:02:12 2005 --><!-- corresponding to FAQ list version 4.0 --><html><!-- Mirrored from c-faq.com/misc/byteswap.html by HTTrack Website Copier/3.x [XR&CO'2008], Sat, 14 Mar 2009 07:59:04 GMT --><head><meta name=GENERATOR content="faqproc"><title>Question 20.9b</title><link href="endiantest.html" rev=precedes><link href="hexio.html" rel=precedes><link href="index.html" rev=subdocument></head><body bgcolor="#ffffff"><a href="endiantest.html" rev=precedes><img src="../images/buttonleft.gif" alt="prev"></a><a href="index.html" rev=subdocument><img src="../images/buttonup.gif" alt="up"></a><a href="hexio.html" rel=precedes><img src="../images/buttonright.gif" alt="next"></a> <a href="../index-2.html"><img src="../images/buttontop.gif" alt="top/contents"></a><a href="../search.html"><img src="../images/buttonsrch.gif" alt="search"></a><hr><p><!-- qbegin --><h1>comp.lang.c FAQ list<font color=blue>·</font><!-- qtag -->Question 20.9b</h1><p><font face=Helvetica size=8 color=blue><b>Q:</b></font>How do I swap bytes?</p><p><hr><p><font face=Helvetica size=8 color=blue><b>A:</b></font>V7 Unix had a <TT>swab</TT> function,but it seems to have been forgotten.</p><p>A problem withexplicit byte-swapping codeis that you haveto decide whether to call it or not,based on the byte order of the dataand the byte order of the machine in use.Question<a href="endiantest.html">20.9</a>shows how, but it's a nuisance.</p><p>A better solution is todefine functionswhich convert between the known byte order of the dataand the (unknown) byte order of the machinein use,and to arrange for these functions to be no-opson those machines which already match the desired byte order.A set of such functions,introduced with the BSD networking code but now in wide use,is <TT>ntohs</TT>, <TT>htons</TT>, <TT>ntohl</TT>, and <TT>htonl</TT>.These are intended to convert between``network'' and ``host'' byte orders,for ``short'' or ``long'' integers,where ``network'' order is always big-endian,and where ``short'' integers are always 16 bitsand ``long'' integers are 32 bits.(This is not the C definition, of course,but it's compatible with the C definition;see question <a href="../decl/inttypes.html">1.1</a>.)So if you know that the data you want to convert from or to is big-endian,you can use these functions.(The point is that you <em>always</em> call the functions,making your code much cleaner.Each function either swaps bytes if it has to, or does nothing.The decision to swap or not to swap gets made once,when the functions are implemented for a particular machine,rather than being made many times in many different calling programs.)</p><p>If you do have to write your own byte-swapping code,the two obvious approaches are again to use pointers or unions,as in question <a href="endiantest.html">20.9</a>.Here is an example using pointers:<pre>void byteswap(char *ptr, int nwords){ char *p = ptr; while(nwords-- > 0) { char tmp = *p; *p = *(p + 1); *(p + 1) = tmp; p += 2; }}</pre></p><p>And here is one using unions:<pre>union word { short int word; char halves[2]; };void byteswap(char *ptr, int nwords){ register union word *wp = (union word *)ptr; while(nwords-- > 0) { char tmp = wp->halves[0]; wp->halves[0] = wp->halves[1]; wp->halves[1] = tmp; wp++; }}</pre></p><p>These functions swap two-byte quantities;the extension to fouror morebytes should be obvious.The union-using code is imperfectin that it assumes that the passed-in pointer is word-aligned.It would also be possible to write functionsaccepting separate source and destination pointers,or accepting single words and returning the swapped values.</p><p>References:PCS Sec. 11 p. 179<br></p><!-- aend --><p><hr><a href="endiantest.html" rev=precedes><img src="../images/buttonleft.gif" alt="prev"></a><a href="index.html" rev=subdocument><img src="../images/buttonup.gif" alt="up"></a><a href="hexio.html" rel=precedes><img src="../images/buttonright.gif" alt="next"></a> <a href="../questions.html"><img src="../images/buttontop.gif" alt="contents"></a><a href="../search.html"><img src="../images/buttonsrch.gif" alt="search"></a><br><!-- lastfooter --><a href="../about.html">about this FAQ list</a> <a href="../eskimo.html">about eskimo</a> <a href="../search.html">search</a> <a href="../feedback.html">feedback</a> <a href="copyright.html">copyright</a><p>Hosted by<a href="http://www.eskimo.com/"><img src="../../www.eskimo.com/img/link/eskitiny.gif" alt="Eskimo North"></a></body><!-- Mirrored from c-faq.com/misc/byteswap.html by HTTrack Website Copier/3.x [XR&CO'2008], Sat, 14 Mar 2009 07:59:04 GMT --></html>⌨️ 快捷键说明
复制代码
Ctrl + C
搜索代码
Ctrl + F
全屏模式
F11
切换主题
Ctrl + Shift + D
显示快捷键
?
增大字号
Ctrl + =
减小字号
Ctrl + -