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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!-- but may not be published or retransmitted without explicit permission.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 21:47:45 2005 by faqproc version 2.7 --><!-- from source file struct.sgml dated Wed Dec 21 16:15:32 2005 --><!-- corresponding to FAQ list version 4.0 --><html><!-- Mirrored from c-faq.com/struct/padding.html by HTTrack Website Copier/3.x [XR&CO'2008], Sat, 14 Mar 2009 07:58:46 GMT --><head><meta name=GENERATOR content="faqproc"><title>Question 2.12</title><link href="io.html" rev=precedes><link href="endpad.html" rel=precedes><link href="index.html" rev=subdocument></head><body bgcolor="#ffffff"><a href="io.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="endpad.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 2.12</h1><p><font face=Helvetica size=8 color=blue><b>Q:</b></font>Why is mycompilerleaving holes in structures,wasting spaceand preventing``binary'' I/O to external datafiles?CanI turnthisoff,or otherwise control the alignment of structure fields?</p><p><hr><p><font face=Helvetica size=8 color=blue><b>A:</b></font>Many machinesaccess values in memorymost efficientlywhen the values are appropriately aligned.(For example,on a byte-addressed machine,<TT>short int</TT>s of size 2might best be placed at even addresses,and<TT>long int</TT>s of size 4at addresses which are a multiple of 4.)Some machines cannot perform unaligned accessesat all,and<em>require</em> thatall databe appropriately aligned.</p><p>Therefore, if you declare a structure like<pre> struct { char c; int i; };</pre>the compiler will usuallyleave an unnamed, unused holebetween the <TT>char</TT> and <TT>int</TT> fields,to ensure that the<TT>int</TT> fieldis properly aligned.(This incremental alignment of the second fieldbased on the firstrelies on the factthat the structure itselfis always properly aligned,with the most conservativealignment requirement.The compiler guarantees this alignment for structures it allocates,as does <TT>malloc</TT>.)</p><p>Your compiler may provide an extension to give youcontrolover the packing of structures(i.e. whether they are padded or not),perhapswitha <TT>#pragma</TT> (seequestion <a href="../ansi/pragma.html">11.20</a>),but thereis no standard method.</p><p>If you're worried about wasted space,you can minimize the effects of paddingby ordering the members ofastructurebased on their base types,from largest to smallest.You can sometimes get more control over size and alignmentby using bit-fields,although theyhave their own drawbacks.(See question <a href="bitfields.html">2.26</a>.)</p><p>See alsoquestions <a href="endpad.html">2.13</a>, <a href="../strangeprob/ptralign.html">16.7</a>, and <a href="../misc/binaryfiles.html">20.5</a>.</p><p>Additional links:<br><br>A bit more<a href="align.html">explanation of ``alignment''</a>and why it requires paddding<br><br>Additional<a href="align.esr.html">ideas on working with alignment and padding</a>by Eric Raymond,couched in the form of six new FAQ list questions<br><br>Corrections to the above from<a href="align.esr2.html">Norm Diamond</a>and<a href="align.esr3.html">Clive Feather</a></p><p>References:K&R2 Sec. 6.4 p. 138<br>H&S Sec. 5.6.4 p. 135<br></p><!-- aend --><p><hr><a href="io.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="endpad.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/struct/padding.html by HTTrack Website Copier/3.x [XR&CO'2008], Sat, 14 Mar 2009 07:58:46 GMT --></html>⌨️ 快捷键说明
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