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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!-- but may not be published or retransmitted without explicit permission.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 expr.sgml dated Sat Jul 3 17:10:33 2004 --><!-- corresponding to FAQ list version 4.0 --><html><!-- Mirrored from c-faq.com/expr/expec0.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 3.10b</title><link href="experiment.html" rev=precedes><link href="confused.html" rel=precedes><link href="index.html" rev=subdocument></head><body bgcolor="#ffffff"><a href="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="confused.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 3.10b</h1><p><font face=Helvetica size=8 color=blue><b>Q:</b></font>People told me that if I evaluated an undefined expression,or accessed an uninitialized variable,I'd get a random, garbage value.But I tried it, and got <em>zero</em>.What's up with that?</p><p><hr><p><font face=Helvetica size=8 color=blue><b>A:</b></font>It's hard to answer this question,because it's hard to seewhat thecitation ofthe ``unexpected'' value of 0 is supposed to prove.C does guarantee thatcertainvalues will be initialized to 0(see question <a href="../decl/initval.html">1.30</a>),but for the rest(and certainly for the results of those undefined expressions),it <em>is</em> true that you might get garbage.The fact that you happened to get 0one timedoes not meanyou were wrong to have expected garbage,nor does it mean that you can depend on this happening next time(much lessthat you shouldwrite code which depends on it!).</p><p>Most memory blocks newly delivered by the operating system,and most as-yet-untouched stack frames,do tend to be zeroed,sothe first time you access them,they may happen to contain 0,but after a program has run for a while,these regularities rapidly disappear.(And programs which unwittingly depend ona circumstantial initial value of an uninitialized variablecan be <em>very</em> difficult to debug,because the ``expected'' values may coincidentally arisein all the small, easy test cases,while the unexpeccted values and the attendant crashes happenonly in the larger, longer-running, much-harder-to-trace-through invocations.)</p><!-- aend --><p><hr><a href="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="confused.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/expr/expec0.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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