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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!-- but may not be published or retransmitted without explicit permission.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 struct.sgml dated Wed Dec 21 16:15:32 2005 --><!-- corresponding to FAQ list version 4.0 --><html><!-- Mirrored from c-faq.com/struct/structhack.html by HTTrack Website Copier/3.x [XR&CO'2008], Sat, 14 Mar 2009 07:58:45 GMT --><head><meta name=GENERATOR content="faqproc"><title>Question 2.6</title><link href="fcnproto.html" rev=precedes><link href="firstclass.html" rel=precedes><link href="index.html" rev=subdocument></head><body bgcolor="#ffffff"><a href="fcnproto.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="firstclass.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 2.6</h1><p><font face=Helvetica size=8 color=blue><b>Q:</b></font>I came across some code that declared a structurelike this:<pre>struct name { int namelen; char namestr[1];};</pre>and then did some tricky allocation to makethe<TT>namestr</TT>array act like ithad several elements,with the numberrecorded by <TT>namelen</TT>.How does this work?Isitlegal or portable?</p><p><hr><p><font face=Helvetica size=8 color=blue><b>A:</b></font>It's not clear if it's legal or portable,but it is rather popular.An implementation of the techniquemight looksomething like this:<pre>#include <stdlib.h>#include <string.h>struct name *makename(char *newname){ struct name *ret = malloc(sizeof(struct name)-1 + strlen(newname)+1); /* -1 for initial [1]; +1 for \0 */ if(ret != NULL) { ret->namelen = strlen(newname); strcpy(ret->namestr, newname); } return ret;}</pre>This function allocatesan instance of the <TT>name</TT> structurewith the size adjustedso that the <TT>namestr</TT> field can hold the requested name(not just one character,as the structure declaration would suggest).</p><p>Despite its popularity,the technique is also somewhat notorious:Dennis Ritchie has called it``unwarranted chumminess with theC implementation,''and anofficial interpretationhas deemed that itis notstrictly conformingwith the C Standard,althoughit does seem to work underall known implementations.(Compilers which check array bounds carefullymight issue warnings.)</p><p>Another possibility isto declare the variable-size element verylarge, rather than very small.Theabove examplecould be rewritten like this:<pre>#include <stdlib.h>#include <string.h>#define MAXSIZE 100struct name { int namelen; char namestr[MAXSIZE];};struct name *makename(char *newname){ struct name *ret = malloc(sizeof(struct name)-MAXSIZE+strlen(newname)+1); /* +1 for \0 */ if(ret != NULL) { ret->namelen = strlen(newname); strcpy(ret->namestr, newname); } return ret;}</pre>where <TT>MAXSIZE</TT> is larger than any name which will be stored.However,it looks like this technique is disallowed bya strict interpretation of the Standardas well.Furthermore,either of these ``chummy'' structures must be used with care,since the programmer knows more about their size than the compiler does.</p><p>Of course, to be truly safe,the right thing to do is use a character pointer instead of an array:<pre>#include <stdlib.h>#include <string.h>struct name { int namelen; char *namep;};struct name *makename(char *newname){ struct name *ret = malloc(sizeof(struct name)); if(ret != NULL) { ret->namelen = strlen(newname); ret->namep = malloc(ret->namelen + 1); if(ret->namep == NULL) { free(ret); return NULL; } strcpy(ret->namep, newname); } return ret;}</pre>(Obviously, the ``convenience'' of havingthe length and the string stored in the same block of memoryhas now been lost,and freeing instances of this structure will require two calls to <TT>free</TT>;see question <a href="../malloc/freeforall.html">7.23</a>.)</p><p>When the data type being stored is characters,as in the above examples,it is straightforward to coalesce the two calls to <TT>malloc</TT> into one,to preserve contiguity(and therefore rescue the ability to use a single call to <TT>free</TT>):<pre>struct name *makename(char *newname){ char *buf = malloc(sizeof(struct name) + strlen(newname) + 1); struct name *ret = (struct name *)buf; ret->namelen = strlen(newname); ret->namep = buf + sizeof(struct name); strcpy(ret->namep, newname); return ret;}</pre></p><p>However,piggybacking a second regiononto a single <TT>malloc</TT> calllike thisis only portableif the second region is to be treated as an array of <TT>char</TT>.For any larger type,alignment(see questions <a href="padding.html">2.12</a> and <a href="../strangeprob/ptralign.html">16.7</a>)becomes significant and would have to be preserved.</p><p>C99 introducesthe concept of a <a href="../sx1/index.html#flexible array member"><dfn>flexible array member</dfn></a>,which allowsthe size of an array to be omitted if it is the last member in a structure,thusproviding a well-defined solution.</p><p>References:Rationale Sec. 3.5.4.2<br>C9X Sec. 6.5.2.1<br></p><!-- aend --><p><hr><a href="fcnproto.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="firstclass.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/struct/structhack.html by HTTrack Website Copier/3.x [XR&CO'2008], Sat, 14 Mar 2009 07:58:45 GMT --></html>⌨️ 快捷键说明
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