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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!-- but may not be published or retransmitted without explicit permission.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 stdio.sgml dated Wed Dec 21 13:07:57 2005 --><!-- corresponding to FAQ list version 4.0 --><html><!-- Mirrored from c-faq.com/stdio/getcharc.html by HTTrack Website Copier/3.x [XR&CO'2008], Sat, 14 Mar 2009 07:58:57 GMT --><head><meta name=GENERATOR content="faqproc"><title>Question 12.1</title><link href="../ansi/experiment.html" rev=precedes><link href="eofval.html" rel=precedes><link href="index.html" rev=subdocument></head><body bgcolor="#ffffff"><a href="../ansi/experiment.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="eofval.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 12.1</h1><p><font face=Helvetica size=8 color=blue><b>Q:</b></font>What's wrong withthis code?<pre>char c;while((c = getchar()) != EOF) ...</pre></p><p><hr><p><font face=Helvetica size=8 color=blue><b>A:</b></font>For one thing,thevariable to hold <TT>getchar</TT>'s return valuemust be an <TT>int</TT>.<TT>EOF</TT> is an``out of band'' return value from <TT>getchar</TT>:it is distinct from all possible <TT>char</TT> valueswhich <TT>getchar</TT> can return.(On modern systems,it does not reflect any actual end-of-file characterstored in a file;it is a signal that no more characters are available.)<TT>getchar</TT>'s return valuemust be stored in a variable larger than <TT>char</TT>so that it can hold all possible <TT>char</TT> values,<em>and</em> <TT>EOF</TT>.</p><p>Two failure modes are possibleif,as in the fragment above,<TT>getchar</TT>'s return valueis assigned toa<TT>char</TT>.</p><OL><li>Iftype<TT>char</TT> is signed,and if <TT>EOF</TT> is defined(as is usual)as -1,the character with the decimal value 255(<TT>'\377'</TT> or <TT>'\xff'</TT> in C)willbe sign-extended andwillcompare equal to <TT>EOF</TT>,prematurely terminating the input.<a href="char8b.html" rel=subdocument>[footnote]</a><li>Iftype <TT>char</TT> is unsigned,anactual<TT>EOF</TT>valuewill betruncated(by having its higher-order bits discarded,probably resulting in 255 or <TT>0xff</TT>)and will <em>not</em> be recognized as <TT>EOF</TT>,resulting in effectively infinite input.</OL><p>The bug can go undetected for a long time, however,if <TT>char</TT>s are signed and if the input is all 7-bit characters.(Whetherplain <TT>char</TT> is signed or unsignedis implementation-defined.)</p><p>References:K&R1 Sec. 1.5 p. 14<br>K&R2 Sec. 1.5.1 p. 16<br>ISO Sec. 6.1.2.5, Sec. 7.9.1, Sec. 7.9.7.5<br>H&S Sec. 5.1.3 p. 116, Sec. 15.1, Sec. 15.6<br>CT&P Sec. 5.1 p. 70<br>PCS Sec. 11 p. 157<br></p><!-- aend --><p><hr><a href="../ansi/experiment.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="eofval.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/stdio/getcharc.html by HTTrack Website Copier/3.x [XR&CO'2008], Sat, 14 Mar 2009 07:58:57 GMT --></html>⌨️ 快捷键说明
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