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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!-- but may not be published or retransmitted without explicit permission.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/index.html by HTTrack Website Copier/3.x [XR&CO'2008], Sat, 14 Mar 2009 07:54:48 GMT --><head><meta name=GENERATOR content="faqproc"><title>Memory Allocation</title></head><body bgcolor="#ffffff"> <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><H1>7. Memory Allocation</H1><p><a href="malloc1.html" rel=subdocument>7.1</a>Why doesn'tthis fragmentwork?<pre> char *answer; printf("Type something:\n"); gets(answer); printf("You typed \"%s\"\n", answer);</pre></p><p><a href="malloc2.html" rel=subdocument>7.2</a>I can't get <TT>strcat</TT> to work.I tried<pre> char *s1 = "Hello, "; char *s2 = "world!"; char *s3 = strcat(s1, s2);</pre>but I got strange results.</p><p><a href="attitude.html" rel=subdocument>7.3</a>But theman pagefor <TT>strcat</TT> says thatit takes two <TT>char *</TT>'s as arguments.How amI supposed to know to allocate things?</p><p><a href="lucky.html" rel=subdocument>7.3b</a>I just tried the code<pre>char *p;strcpy(p, "abc");</pre>and it worked.How?Why didn't it crash?</p><p><a href="ptrvaralloc.html" rel=subdocument>7.3c</a>How much memory does a pointer variable allocate?</p><p><a href="linebfdur.html" rel=subdocument>7.4</a>I'mreadinglines from a file into anarray,with this code:<pre> char linebuf[80]; char *lines[100]; int i; for(i = 0; i < 100; i++) { char *p = fgets(linebuf, 80, fp); if(p == NULL) break; lines[i] = p; }</pre>Why do all the lines end up containing copies of the last line?</p><p><a href="retaggr.html" rel=subdocument>7.5a</a>I have a function that is supposed to return a string,but when it returns to its caller,the returned string is garbage.</p><p><a href="retaggr2.html" rel=subdocument>7.5b</a>So what's the right way to return a stringor other aggregate?</p><p><a href="decl.html" rel=subdocument>7.6</a>Why am I getting``warning: assignment of pointer from integer lacks a cast''for calls to <TT>malloc</TT>?</p><p><a href="cast.html" rel=subdocument>7.7</a>Why does some code carefully cast the values returned by <TT>malloc</TT>to the pointer type being allocated?</p><p><a href="mallocnocast.html" rel=subdocument>7.7b</a>What's wrong with casting <TT>malloc</TT>'s return value?</p><p><a href="mallocnocast2.html" rel=subdocument>7.7c</a>In a call to <TT>malloc</TT>,what does an error like``Cannot convert `<TT>void *</TT>' to `<TT>int *'''mean?</TT></p><p><a href="sizeofchar.html" rel=subdocument>7.8</a>I see code like<pre> char *p = malloc(strlen(s) + 1); strcpy(p, s);</pre>Shouldn't that be <TT>malloc((strlen(s) + 1) * sizeof(char))</TT>?</p><p><a href="mymallocretp.html" rel=subdocument>7.9</a>I wrote a little wrapper around <TT>malloc</TT>,but it doesn't work:<pre> #include <stdio.h> #include <stdlib.h> mymalloc(void *retp, size_t size) { retp = malloc(size); if(retp == NULL) { fprintf(stderr, "out of memory\n"); exit(EXIT_FAILURE); } }</pre></p><p><a href="mimic3.html" rel=subdocument>7.10</a>I'm trying to declare a pointer and allocate some space for it,but it'snot working.What's wrong withthis code?<pre>char *p;*p = malloc(10);</pre></p><p><a href="fcninit2.html" rel=subdocument>7.10a</a>What's wrong withthis initialization?<pre>char *p = malloc(10);</pre>My compiler is complaining aboutan ``invalid initializer'',or something.</p><p><a href="lintpalign2.html" rel=subdocument>7.10b</a>How can I shut off the``warning: possible pointer alignment problem''messagewhich<TT>lint</TT> gives me for each call to <TT>malloc</TT>?</p><p><a href="dynarray2.html" rel=subdocument>7.11</a>How can I dynamically allocate arrays?</p><p><a href="memavail2.html" rel=subdocument>7.12</a>How can I find out how much memory is available?</p><p><a href="malloc0xref.html" rel=subdocument>7.13</a>What should <TT>malloc(0)</TT> do?Return a null pointer or a pointer to 0 bytes?</p><p><a href="lazyalloc.html" rel=subdocument>7.14</a>I've heard that some operating systemsdon't actually allocate <TT>malloc</TT>'ed memoryuntiltheprogram tries touseit.Is this legal?</p><p><a href="sizetlong.html" rel=subdocument>7.15</a><TT>malloc</TT>isreturning crazy pointer values,but I <em>did</em> read question <a href="decl.html">7.6</a>and I have included the line<pre> extern void *malloc();</pre>before I call it.</p><p><a href="sizewrap.html" rel=subdocument>7.16</a>I'm allocating a large array for some numeric work,using the line<pre> double *array = malloc(300 * 300 * sizeof(double));</pre><TT>malloc</TT> isn't returning null,butthe program is acting strangely,as if it's overwriting memory,or <TT>malloc</TT> isn't allocating as much as I asked for,or something.</p><p><a href="segment.html" rel=subdocument>7.17</a>I've got 8 meg of memory in my PC.Why can I only seem to <TT>malloc</TT> 640K or so?</p><p><a href="efficiency.html" rel=subdocument>7.18</a>My application depends heavily ondynamic allocation of nodes for data structures,and <TT>malloc</TT>/<TT>free</TT> overheadisbecominga bottleneck.What canI do?</p><p><a href="crash.html" rel=subdocument>7.19</a>My program is crashing, apparently somewhere down inside<TT>malloc</TT>,but I can't see anything wrong with it.Is there a bug in <TT>malloc</TT>?</p><p><a href="noscale.html" rel=subdocument>7.19b</a></p><p>I'm dynamically allocating an array, like this:<pre> int *iarray = (int *)malloc(nints);</pre><TT>malloc</TT> isn't returning NULL,but the code isn't working.</p><p><a href="useafterfree.html" rel=subdocument>7.20</a>You can't use dynamically-allocated memory after you free it, can you?</p><p><a href="ptrafterfree.html" rel=subdocument>7.21</a>Why isn't a pointernullafter calling<TT>free</TT>?<br>How unsafe is it to use(assign, compare)a pointer value after it's been freed?</p><p><a href="local.html" rel=subdocument>7.22</a>When I call <TT>malloc</TT> to allocate memory for apointer which is local to a function,do I have to explicitly <TT>free</TT> it?</p><p><a href="freeforall.html" rel=subdocument>7.23</a>I'm allocating structures which contain pointers to otherdynamically-allocated objects.When I freea structure,do Ialsohave to free each subsidiary pointer?</p><p><a href="freeb4exit.html" rel=subdocument>7.24</a>Must I free allocated memory before the program exits?</p><p><a href="freetoOS.html" rel=subdocument>7.25</a>I have a program which <TT>malloc</TT>sand later<TT>free</TT>s a lotof memory,butI can see from the operating systemthatmemory usagedoesn'tactuallygo back down.</p><p><a href="freesize.html" rel=subdocument>7.26</a>How does <TT>free</TT> know how many bytes to free?</p><p><a href="querysize.html" rel=subdocument>7.27</a>So can I query the malloc package to find out how big anallocated block is?</p><p><a href="sizeof.html" rel=subdocument>7.28</a>Why doesn't <TT>sizeof</TT> tell me the size ofthe block of memory pointed to by a pointer?</p><p><a href="realloc.html" rel=subdocument>7.29</a>Having dynamically allocated an array(as in question <a href="../aryptr/dynarray.html">6.14</a>),can I change its size?</p><p><a href="reallocnull.html" rel=subdocument>7.30</a>Is it legal to pass a null pointer as the first argument to <TT>realloc</TT>?Why would you want to?</p><p><a href="calloc.html" rel=subdocument>7.31</a>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><a href="alloca.html" rel=subdocument>7.32</a>What is <TT>alloca</TT> and why is its use discouraged?</p><hr><p><a href="../index-2.html">top</a></p><p><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><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></p></body><!-- Mirrored from c-faq.com/malloc/index.html by HTTrack Website Copier/3.x [XR&CO'2008], Sat, 14 Mar 2009 07:55:04 GMT --></html>⌨️ 快捷键说明
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