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<!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!-- but may not be published or retransmitted without explicit permission.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 21:47:46 2005 by faqproc version 2.7 --><!-- from source file ansi.sgml dated Wed Aug 11 00:43:45 2004 --><!-- corresponding to FAQ list version 4.0 --><html><!-- Mirrored from c-faq.com/ansi/memmove.html by HTTrack Website Copier/3.x [XR&CO'2008], Sat, 14 Mar 2009 07:58:56 GMT --><head><meta name=GENERATOR content="faqproc"><title>Question 11.25</title><link href="voidparith.html" rev=precedes><link href="malloc0.html" rel=precedes><link href="index.html" rev=subdocument></head><body bgcolor="#ffffff"><a href="voidparith.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="malloc0.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 11.25</h1><p><font face=Helvetica size=8 color=blue><b>Q:</b></font>What's the difference between<TT>memcpy</TT> and<TT>memmove</TT>?</p><p><hr><p><font face=Helvetica size=8 color=blue><b>A:</b></font><TT>memmove</TT> offers guaranteed behaviorif thememoryregionspointed to by thesource and destination argumentsoverlap.<TT>memcpy</TT> makes no such guarantee,and may therefore be more efficiently implementable.When in doubt, it's safer to use <TT>memmove</TT>.</p><p>It seemssimple enough to implement <TT>memmove</TT>;the overlap guaranteeapparently requiresonly an additional test:<pre>void *memmove(void *dest, void const *src, size_t n){ register char *dp = dest; register char const *sp = src; if(dp < sp) { while(n-- > 0) *dp++ = *sp++; } else { dp += n; sp += n; while(n-- > 0) *--dp = *--sp; } return dest;}</pre>The problemwith this codeis in that additional test:the comparison<TT>(dp < sp)</TT>is not quite portable(it compares two pointerswhich do not necessarily point within the same object)and may not be as cheap as it looks.On some machines(particularly segmented architectures),it may be tricky and significantly less efficient<a href="norml.html" rel=subdocument>[footnote]</a>to implement.</p><p>References:K&R2 Sec. B3 p. 250<br>ISO Sec. 7.11.2.1, Sec. 7.11.2.2<br>Rationale Sec. 4.11.2<br>H&S Sec. 14.3 pp. 341-2<br>PCS Sec. 11 pp. 165-6<br></p><!-- aend --><p><hr><a href="voidparith.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="malloc0.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/ansi/memmove.html by HTTrack Website Copier/3.x [XR&CO'2008], Sat, 14 Mar 2009 07:58:56 GMT --></html>⌨️ 快捷键说明
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