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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!-- but may not be published or retransmitted without explicit permission.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 aryptr.sgml dated Wed Dec 21 12:50:38 2005 --><!-- corresponding to FAQ list version 4.0 --><html><!-- Mirrored from c-faq.com/aryptr/pass2dary.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 6.18</title><link href="non0based.html" rev=precedes><link href="ary2dfunc2.html" rel=precedes><link href="index.html" rev=subdocument></head><body bgcolor="#ffffff"><a href="non0based.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="ary2dfunc2.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 6.18</h1><p><font face=Helvetica size=8 color=blue><b>Q:</b></font>My compiler complained when I passed a two-dimensional array to afunction expecting a pointer to a pointer.</p><p><hr><p><font face=Helvetica size=8 color=blue><b>A:</b></font>The rule(see question <a href="aryptrequiv.html">6.3</a>)by which arrays decay into pointers is<em>not</em>applied recursively.(Once the rule has been applied once,the result is a pointer to which the rule no longer applies.)An array of arrays (i.e. a two-dimensional array in C) decaysinto a pointer to an array, not a pointer to a pointer.Pointers to arrays can be confusing,and must be treated carefully;see also question <a href="ptrtoarray.html">6.13</a>.</p><p>If you are passing a two-dimensional array to a function:<pre> int array[NROWS][NCOLUMNS]; f(array);</pre>the function's declarationmust match:<pre> void f(int a[][NCOLUMNS]) { ... }</pre>or<pre> void f(int (*ap)[NCOLUMNS]) /* ap is a pointer to an array */ { ... }</pre>In the first declaration,the compiler performs the usualimplicit parameter rewritingof ``array of array''to ``pointer to array''(see questions<a href="aryptrequiv.html">6.3</a>and<a href="aryptrparam.html">6.4</a>);in the second form the pointer declaration is explicit.Since the called function does not allocate space for the array,it does not need to know the overall size,so the number ofrows,<TT>NROWS</TT>,can be omitted.Thewidthof the array is still important,so thecolumndimension<TT>NCOLUMNS</TT>(and, for three- or more dimensional arrays,the intervening ones)must beretained.</p><p>If a function is already declared as accepting apointer to a pointer,it isalmost certainlymeaninglessto pass a two-dimensional array directly to it.An intermediate pointer would have to be used when attempting to call itwith a two-dimensional array:<pre> extern g(int **ipp); int *ip = &array[0][0]; g(&ip); /* PROBABLY WRONG */</pre>butthis usage ismisleadingand almost certainlyincorrect,since the array has been ``flattened''(its shape has been lost).</p><p>See also questions <a href="aryvsadr.html">6.12</a> and <a href="dynlocarys.html">6.15</a>.</p><p>References:K&R1 Sec. 5.10 p. 110<br>K&R2 Sec. 5.9 p. 113<br>H&S Sec. 5.4.3 p. 126<br></p><!-- aend --><p><hr><a href="non0based.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="ary2dfunc2.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/aryptr/pass2dary.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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