📄 reallocnull.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!-- but may not be published or retransmitted without explicit permission.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/reallocnull.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 7.30</title><link href="realloc.html" rev=precedes><link href="calloc.html" rel=precedes><link href="index.html" rev=subdocument></head><body bgcolor="#ffffff"><a href="realloc.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="calloc.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 7.30</h1><p><font face=Helvetica size=8 color=blue><b>Q:</b></font>Is it legal to pass a null pointer as the first argument to <TT>realloc</TT>?Why would you want to?</p><p><hr><p><font face=Helvetica size=8 color=blue><b>A:</b></font>ANSI C sanctionsthis usage(and the related <TT>realloc(</TT>...<TT>, 0)</TT>, which frees),although several earlier implementations do not support it,so itmay not befully portable.Passing an initially-null pointer to <TT>realloc</TT> can make iteasier to write a self-starting incremental allocation algorithm.</p><p>Here is an example--thisfunction readsan arbitrarily-long lineinto dynamically-allocated memory,reallocating the input buffer as necessary.(The caller must <TT>free</TT> the returned pointerwhen it is no longer needed.)<pre>#include <stdio.h>#include <stdlib.h>/* read a line from fp into malloc'ed memory *//* returns NULL on EOF or error *//* (use feof or ferror to distinguish) */char *agetline(FILE *fp){ char *retbuf = NULL; size_t nchmax = 0; register int c; size_t nchread = 0; char *newbuf; while((c = getc(fp)) != EOF) { if(nchread >= nchmax) { nchmax += 20; if(nchread >= nchmax) { /* in case nchmax overflowed */ free(retbuf); return NULL; }#ifdef SAFEREALLOC newbuf = realloc(retbuf, nchmax + 1);#else if(retbuf == NULL) /* in case pre-ANSI realloc */ newbuf = malloc(nchmax + 1); else newbuf = realloc(retbuf, nchmax + 1);#endif /* +1 for \0 */ if(newbuf == NULL) { free(retbuf); return NULL; } retbuf = newbuf; } if(c == '\n') break; retbuf[nchread++] = c; } if(retbuf != NULL) { retbuf[nchread] = '\0'; newbuf = realloc(retbuf, nchread + 1); if(newbuf != NULL) retbuf = newbuf; } return retbuf;}</pre>(In production code,a line like <TT>nchmax += 20</TT>can prove troublesome,as the function may do lots of reallocating.Many programmers favor multiplicative reallocation,e.g. <TT>nchmax *= 2</TT>,although it obviously isn'tquite asself-starting,and can run into problems if ithas to allocate a huge arraybut memory is limited.)</p><p>References:ISO Sec. 7.10.3.4<br>H&S Sec. 16.3 p. 388<br></p><!-- aend --><p><hr><a href="realloc.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="calloc.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/reallocnull.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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