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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!-- but may not be published or retransmitted without explicit permission.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 malloc.sgml dated Wed Dec 21 13:50:44 2005 --><!-- corresponding to FAQ list version 4.0 --><html><!-- Mirrored from c-faq.com/malloc/malloc1.html by HTTrack Website Copier/3.x [XR&CO'2008], Sat, 14 Mar 2009 07:58:49 GMT --><head><meta name=GENERATOR content="faqproc"><title>Question 7.1</title><link href="../aryptr/arybits.html" rev=precedes><link href="malloc2.html" rel=precedes><link href="index.html" rev=subdocument></head><body bgcolor="#ffffff"><a href="../aryptr/arybits.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="malloc2.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 7.1</h1><p><font face=Helvetica size=8 color=blue><b>Q:</b></font>Why doesn'tthis fragmentwork?<pre> char *answer; printf("Type something:\n"); gets(answer); printf("You typed \"%s\"\n", answer);</pre></p><p><hr><p><font face=Helvetica size=8 color=blue><b>A:</b></font>The pointer variable <TT>answer</TT>,which is handed to<TT>gets()</TT>as the location into which the response should bestored,has not been set topoint to any valid storage.It isan uninitialized variable, just as is the variable <TT>i</TT> in<pre> int i; printf("i = %d\n", i);</pre>That is,in the first piece of code,we cannot say where the pointer <TT>answer</TT>points,just as we cannot say what value <TT>i</TT> will have in the second.(Since local variables are not initialized,and typically contain garbage,it is not even guaranteed that <TT>answer</TT> starts out as anull pointer.See questions<a href="../decl/initval.html">1.30</a>and<a href="../null/null1.html">5.1</a>.)</p><p>The simplest way to correctthe question-asking programis touse a local array, instead of apointer, and let the compilerworry aboutallocation:<pre>#include <stdio.h>#include <string.h>char answer[100], *p;printf("Type something:\n");fgets(answer, sizeof answer, stdin);if((p = strchr(answer, '\n')) != NULL) *p = '\0';printf("You typed \"%s\"\n", answer);</pre></p>This example also uses <TT>fgets()</TT>instead of <TT>gets()</TT>,so thatthe end of the arraycannot be overwritten.(See question<a href="../stdio/getsvsfgets.html">12.23</a>.Unfortunately for this example,<TT>fgets()</TT> does not automatically delete the trailing <TT>\n</TT>,as<TT>gets()</TT> would.)It would also be possible to use <TT>malloc()</TT> to allocate the<TT>answer</TT>buffer,and to parameterize the buffer size(with something like <TT>#define ANSWERSIZE 100</TT> ).<!-- aend --><p><hr><a href="../aryptr/arybits.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="malloc2.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/malloc/malloc1.html by HTTrack Website Copier/3.x [XR&CO'2008], Sat, 14 Mar 2009 07:58:49 GMT --></html>⌨️ 快捷键说明
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