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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!-- but may not be published or retransmitted without explicit permission.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 21:47:47 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/bitmanip.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 20.7</title><link href="intovf.html" rev=precedes><link href="bitsets.html" rel=precedes><link href="index.html" rev=subdocument></head><body bgcolor="#ffffff"><a href="intovf.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="bitsets.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 20.7</h1><p><font face=Helvetica size=8 color=blue><b>Q:</b></font>How can I manipulate individual bits?</p><p><hr><p><font face=Helvetica size=8 color=blue><b>A:</b></font>Bit manipulation isstraightforwardin C,and commonly done.To extract(test)a bit,use the bitwise AND (<TT>&</TT>)operator,along with a bit maskrepresenting the bit(s) you're interested in:<pre> value & 0x04</pre>To set a bit,use the bitwise OR (<TT>|</TT> or <TT>|=</TT>) operator:<pre> value |= 0x04</pre>To clear a bit,use the bitwise complement (<TT>~</TT>)and the AND (<TT>&</TT> or <TT>&=</TT>) operators:<pre> value &= ~0x04</pre>(The preceding three examples all manipulatethe third-least significant,or2**2,bit,expressed as the constant bitmask <TT>0x04</TT>.)</p><p>To manipulate an arbitrary bit, use the shift-left operator(<TT><<</TT>) to generate the mask you need:<pre> value & (1 << bitnumber) value |= (1 << bitnumber) value &= ~(1 << bitnumber)</pre>Alternatively,you may wish to precomputean array of masks:<pre> unsigned int masks[] = {0x01, 0x02, 0x04, 0x08, 0x10, 0x20, 0x40, 0x80}; value & masks[bitnumber] value |= masks[bitnumber] value &= ~masks[bitnumber]</pre></p><p>To avoid surprises involving the sign bit,it is often a good idea to use unsigned integral typesin codewhich manipulatesbits and bytes.</p><p>See also questions <a href="../bool/bool2.html">9.2</a> and <a href="bitsets.html">20.8</a>.</p><p>References:K&R1 Sec. 2.9 pp. 44-45<br>K&R2 Sec. 2.9 pp. 48-49<br>ISO Sec. 6.3.3.3, Sec. 6.3.7, Sec. 6.3.10, Sec. 6.3.12<br>H&S Sec. 7.5.5 p. 197, Sec. 7.6.3 pp. 205-6, Sec. 7.6.6 p. 210<br></p><!-- aend --><p><hr><a href="intovf.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="bitsets.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/bitmanip.html by HTTrack Website Copier/3.x [XR&CO'2008], Sat, 14 Mar 2009 07:59:04 GMT --></html>⌨️ 快捷键说明
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