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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!-- but may not be published or retransmitted without explicit permission.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 strangeprob.sgml dated Tue Jun 29 21:44:40 2004 --><!-- corresponding to FAQ list version 4.0 --><html><!-- Mirrored from c-faq.com/strangeprob/ptralign.html by HTTrack Website Copier/3.x [XR&CO'2008], Sat, 14 Mar 2009 07:59:01 GMT --><head><meta name=GENERATOR content="faqproc"><title>Question 16.7</title><link href="strlitnomod.html" rev=precedes><link href="segv.html" rel=precedes><link href="index.html" rev=subdocument></head><body bgcolor="#ffffff"><a href="strlitnomod.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="segv.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 16.7</h1><p><font face=Helvetica size=8 color=blue><b>Q:</b></font>I've got some code that's trying to unpack external structures, but it's crashingwith a message about an ``unaligned access.''What does this mean?The code looks like this:<pre>struct mystruct { char c; long int i32; int i16;} s;char buf[7], *p;fread(buf, 7, 1, fp);p = buf;s.c = *p++;s.i32 = *(long int *)p;p += 4;s.i16 = *(int *)p;</pre></p><p><hr><p><font face=Helvetica size=8 color=blue><b>A:</b></font>The problem is that you're playing too fast and loose with yourpointers.Some machines require that data values be stored at appropriately aligned addresses.For instance,two-byte <TT>short int</TT>smight be constrained to sit at even addresses,and four-byte <TT>long int</TT>s at multiples of four.(See also question <a href="../struct/padding.html">2.12</a>.)By converting a <TT>char *</TT>(which can point to any byte)to an <TT>int *</TT> or <TT>long int *</TT>,and then indirectingon it,youcan end upasking theprocessor tofetch amultibytevalue from an unalignedaddress,which it isn't willing to do.</p><p>A better way to unpackexternal structuresis with code like<pre> unsigned char *p = buf; s.c = *p++; s.i32 = (long)*p++ << 24; s.i32 |= (long)*p++ << 16; s.i32 |= (unsigned)(*p++ << 8); s.i32 |= *p++; s.i16 = *p++ << 8; s.i16 |= *p++;</pre></p><p>This codealso gives you control over byte order.(This example,though,assumesthat a <TT>char</TT> is 8 bits,and that the <TT>long int</TT> and <TT>int</TT>being unpacked from the ``external structure''are 32 and 16 bits, respectively.)See question <a href="../stdio/extconform.html">12.42</a>(which contains some similar code)for a few explanationsand caveats.</p><p>See also question <a href="../ptrs/castincr.html">4.5</a>.</p><p>References:ISO Sec. 6.3.3.2, Sec. 6.3.4<br>H&S Sec. 6.1.3 pp. 164-5<br></p><!-- aend --><p><hr><a href="strlitnomod.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="segv.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/strangeprob/ptralign.html by HTTrack Website Copier/3.x [XR&CO'2008], Sat, 14 Mar 2009 07:59:01 GMT --></html>⌨️ 快捷键说明
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