📄 realloc.html
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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!-- but may not be published or retransmitted without explicit permission.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/realloc.html by HTTrack Website Copier/3.x [XR&CO'2008], Sat, 14 Mar 2009 07:58:52 GMT --><head><meta name=GENERATOR content="faqproc"><title>Question 7.29</title><link href="sizeof.html" rev=precedes><link href="reallocnull.html" rel=precedes><link href="index.html" rev=subdocument></head><body bgcolor="#ffffff"><a href="sizeof.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="reallocnull.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 7.29</h1><p><font face=Helvetica size=8 color=blue><b>Q:</b></font>Having dynamically allocated an array(as in question <a href="../aryptr/dynarray.html">6.14</a>),can I change its size?</p><p><hr><p><font face=Helvetica size=8 color=blue><b>A:</b></font>Yes.Thisis exactly what <TT>realloc</TT> is for.Given a region of <TT>malloc</TT>ed memory(such as <TT>dynarray</TT> from question <a href="../aryptr/dynarray.html">6.14</a>),its size can be changed using code like:<pre> dynarray = realloc(dynarray, 20 * sizeof(int));</pre>Note that <TT>realloc</TT> may not always be able to enlarge<a href="shrink.html" rel=subdocument>[footnote]</a>memory regionsin-place.Whenit is able to,it simply gives you back the same pointer you handed it,butif it must go to some other part of memoryto find enough contiguous space,it will return adifferent pointer(and the previous pointer value will become unusable).</p><p>If<TT>realloc</TT>cannot find enoughspace at all,it returns a null pointer,and leaves the previous region allocated.<a href="realloctrash.html" rel=subdocument>[footnote]</a>Therefore,you usually don't want to immediately assign the new pointerto the old variable.Instead,use a temporary pointer:<pre> #include <stdio.h> #include <stdlib.h> int *newarray = (int *)realloc((void *)dynarray, 20 * sizeof(int)); if(newarray != NULL) dynarray = newarray; else { fprintf(stderr, "Can't reallocate memory\n"); /* dynarray remains allocated */ }</pre></p><p>When reallocating memory,be careful ifthere are any other pointerslying aroundwhich point into(``alias'')that memory:if <TT>realloc</TT>must locate the new regionsomewhere else,those other pointers must also be adjusted.Here is a (contrived,and careless of <TT>malloc</TT>'s return values) example:<pre>#include <stdio.h>#include <stdlib.h>#include <string.h>char *p, *p2, *newp;int tmpoffset;p = malloc(10);strcpy(p, "Hello,"); /* p is a string */p2 = strchr(p, ','); /* p2 points into that string */tmpoffset = p2 - p;newp = realloc(p, 20);if(newp != NULL) { p = newp; /* p may have moved */ p2 = p + tmpoffset; /* relocate p2 as well */ strcpy(p2, ", world!");}printf("%s\n", p);</pre>(It is safest to recompute pointers based on offsets,as in the code fragment above.The alternative--relocating pointersbased on the difference,<TT>newp - p</TT>,between the base pointer's valuebefore and after the <TT>realloc</TT>--is not guaranteed to work,because pointer subtraction is only definedwhen performed on pointers into the same object.See also question <a href="ptrafterfree.html">7.21</a>.)</p><p>See also questions<a href="cast.html">7.7</a>and<a href="reallocnull.html">7.30</a>.</p><p>References:K&R2 Sec. B5 p. 252<br>ISO Sec. 7.10.3.4<br>H&S Sec. 16.3 pp. 387-8<br></p><!-- aend --><p><hr><a href="sizeof.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="reallocnull.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/realloc.html by HTTrack Website Copier/3.x [XR&CO'2008], Sat, 14 Mar 2009 07:58:52 GMT --></html>⌨️ 快捷键说明
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