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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!-- but may not be published or retransmitted without explicit permission.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/bitfield0.html by HTTrack Website Copier/3.x [XR&CO'2008], Sat, 14 Mar 2009 07:58:47 GMT --><head><meta name=GENERATOR content="faqproc"><title>Question 2.25</title><link href="enumprint.html" rev=precedes><link href="bitfields.html" rel=precedes><link href="index.html" rev=subdocument></head><body bgcolor="#ffffff"><a href="enumprint.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="bitfields.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 2.25</h1><p><font face=Helvetica size=8 color=blue><b>Q:</b></font>I came across some structure declarations with colons and numbers next to certainfields,like this:<pre>struct record { char *name; int refcount : 4; unsigned dirty : 1;};</pre>What gives?</p><p><hr><p><font face=Helvetica size=8 color=blue><b>A:</b></font>Those are <a href="../sx1/index.html#bit-fields"><dfn>bit-fields</dfn></a>;the number gives the exact size of the field,in bits.(See any complete book on C forthe details.)Bit-fields can be used to save spacein structures having several binary flags or othersmall fields,and they can also be usedin an attempt to conform to externally-imposed storage layouts.(Their success at the latter task is mitigatedby the fact that bit-fields are assigned left-to-right on some machines and right-to-left on others).</p><p>Note that the colon notationfor specifying the size of a field in bitsis <em>only</em> valid in structures(and in unions);you cannot use this mechanismto specify the size of arbitraryvariables.(See questions <a href="../decl/exactsizes.html">1.2</a> and <a href="../decl/int16.html">1.3</a>.)</p><p>References:K&R1 Sec. 6.7 pp. 136-8<br>K&R2 Sec. 6.9 pp. 149-50<br>ISO Sec. 6.5.2.1<br>H&S Sec. 5.6.5 pp. 136-8<br></p><!-- aend --><p><hr><a href="enumprint.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="bitfields.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/bitfield0.html by HTTrack Website Copier/3.x [XR&CO'2008], Sat, 14 Mar 2009 07:58:47 GMT --></html>⌨️ 快捷键说明
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