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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!-- but may not be published or retransmitted without explicit permission.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/calloc.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 7.31</title><link href="reallocnull.html" rev=precedes><link href="alloca.html" rel=precedes><link href="index.html" rev=subdocument></head><body bgcolor="#ffffff"><a href="reallocnull.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="alloca.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 7.31</h1><p><font face=Helvetica size=8 color=blue><b>Q:</b></font>What's the difference between <TT>calloc</TT> and <TT>malloc</TT>?Which should Iuse?Is it safe to take advantage of <TT>calloc</TT>'szero-filling?Does <TT>free</TT> workon memory allocated with <TT>calloc</TT>,or do youneed a <TT>cfree</TT>?</p><p><hr><p><font face=Helvetica size=8 color=blue><b>A:</b></font><TT>calloc(m, n)</TT> is essentially equivalent to<pre>p = malloc(m * n);memset(p, 0, m * n);</pre></p><p>There is no important difference betweenthe twoother than the number of argumentsand the zero fill.<a href="callocvslazy.html" rel=subdocument>[footnote]</a></p><p>Use whichever function is convenient.Don't rely on <TT>calloc</TT>'s zero fill too much(see below);usually,it's best to initialize data structures yourself,on a field-by-field basis,especially if there are pointer fields.</p><p><TT>calloc</TT>'szero fill is all-bits-zero,andis therefore guaranteed to yield the value 0for all integral types(including<TT>'\0'</TT>forcharacter types).Butitdoes <em>not</em>guarantee useful null pointer values(seesection<a href="../null/index.html">5</a> of this list)or floating-point zero values.</p><p><TT>free</TT>isproperlyused to free the memory allocatedby <TT>calloc</TT>;there is no Standard <TT>cfree</TT> function.</p><p>One imagined distinction that is <em>not</em> significantbetween <TT>malloc</TT> and <TT>calloc</TT>is whethera single elementor an array of elementsis being allocated.Though <TT>calloc</TT>'s two-argument calling convention suggests that itis supposed tobe used to allocate an array of <TT>m</TT> items of size <TT>n</TT>,there is no suchrequirement;it is perfectlypermissibleto allocate one item with <TT>calloc</TT>(by passing one argument as 1)or to allocate an array with <TT>malloc</TT>(by doing the multiplication yourself;see for examplethe code fragment inquestion <a href="../aryptr/dynarray.html">6.14</a>).(Nor does structure padding enter into the question;any padding necessaryto make arrays of structures work correctlyis always handled by the compiler,and reflected by <TT>sizeof</TT>.See question <a href="../struct/endpad.html">2.13</a>.)</p><p>References:ISO Sec. 7.10.3 to 7.10.3.2<br>H&S Sec. 16.1 p. 386, Sec. 16.2 p. 386<br>PCS Sec. 11 pp. 141,142<br></p><!-- aend --><p><hr><a href="reallocnull.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="alloca.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/calloc.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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