189 lines
15 KiB
HTML
189 lines
15 KiB
HTML
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<title>realloc</title>
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<body class="mediawiki ltr sitedir-ltr ns-0 ns-subject page-c_memory_realloc skin-cppreference2 action-view cpp-navbar">
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<div id="cpp-content-base">
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<h1 id="firstHeading" class="firstHeading">realloc</h1>
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<div id="bodyContent">
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<div id="contentSub"><span class="subpages">< <a href="c.html">c</a>‎ | <a href="c-memory.html">memory</a></span></div>
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<div id="mw-content-text" lang="zh" dir="ltr" class="mw-content-ltr" xml:lang="zh">
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<div>定义于头文件 <code><stdlib.h></code></div>
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<td></td>
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<td></td>
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<div><span class="mw-geshi c source-c"><span class="kw4">void</span> <span class="sy2">*</span>realloc<span class="br0">(</span> <span class="kw4">void</span> <span class="sy2">*</span>ptr, <a href="c-types-size_t.html"><span class="kw100">size_t</span></a> new_size <span class="br0">)</span><span class="sy4">;</span></span></div>
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<td class="t-dcl-nopad"></td>
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<tr class="t-dcl-sep">
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<td></td>
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<td></td>
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<p>重新分配给定的内存区域。它必须是之前为 <span class="t-lc"><a href="c-memory-malloc.html">malloc()</a></span> 、 <span class="t-lc"><a href="c-memory-calloc.html">calloc()</a></span> 或 <code>realloc()</code> 所分配，并且仍未被 <span class="t-lc"><a href="c-memory-free.html">free</a></span> 或 <code>realloc</code> 的调用所释放。否则，结果未定义。</p>
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<p>重新分配按以下二者之一执行：</p>
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<div class="t-li1"><span class="t-li">a)</span> 可能的话，扩张或收缩 <code>ptr</code> 所指向的已存在内存。内容在新旧大小中的较小者范围内保持不变。若扩张范围，则数组新增部分的内容是未定义的。</div>
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<div class="t-li1"><span class="t-li">b)</span> 分配一个大小为 <code>new_size</code> 字节的新内存块，并复制大小等于新旧大小中较小者的内存区域，然后释放旧内存块。</div>
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<p>若无足够内存，则不释放旧内存块，并返回空指针。</p>
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<p>若 <code>ptr</code> 为 <span class="t-lc"><a href="c-types-NULL.html">NULL</a></span> ，则行为与调用 <span class="t-c"><span class="mw-geshi c source-c"><a href="c-memory-malloc.html"><span class="kw403">malloc</span></a><span class="br0">(</span>new_size<span class="br0">)</span></span></span> 相同。</p>
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<p>若 <code>new_size</code> 为零，则行为是实现定义的（可返回空指针，此情况下可能或可能不释放旧内存，或返回不会用于访问存储的非空指针）。</p>
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<table class="t-rev-begin">
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<tr class="t-rev t-until-c23">
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<td>
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<p>若 <code>new_size</code> 为零，则行为是实现定义的（可能返回空指针，该情况下可能或可能不释放旧内存，或可能返回某个不能用于访问存储的非空指针）。<span class="t-rev-inl t-since-c17"><span>这种用法被弃用（经由 <a rel="nofollow" class="external text" href="http://www.open-std.org/jtc1/sc22/wg14/www/docs/n2396.htm#dr_400">DR 400</a> ）。</span> <span><span class="t-mark-rev t-since-c17">(C17 起)</span></span></span></p>
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</td>
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<td><span class="t-mark-rev t-until-c23">(C23 前)</span></td>
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<tr class="t-rev t-since-c23">
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<td>
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<p>若 <code>new_size</code> 为零，则行为未定义。</p>
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</td>
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<td><span class="t-mark-rev t-since-c23">(C23 起)</span></td>
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</table>
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<table class="t-rev-begin">
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<tr class="t-rev t-since-c11">
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<td>
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<p><code>realloc</code> 是线程安全的：它表现得如同只访问通过其参数可见的内存区域，而非任何静态存储。</p>
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<p>先前令 <span class="t-lc"><a href="c-memory-free.html">free</a></span> 或 <code>realloc</code> 归还一块内存区域的调用，<i>同步于</i>任何分配函数的调用，包括分配相同或部分相同内存区域的 <code>realloc</code> 。这种同步出现于任何解分配函数所做的内存访问后，和任何 <code>realloc</code> 所做内存访问前。所有操作一块特定内存区域的分配及解分配函数拥有单独全序。</p>
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</td>
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<td><span class="t-mark-rev t-since-c11">(C11 起)</span></td>
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</tr>
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</table>
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<h3><span class="mw-headline" id=".E5.8F.82.E6.95.B0">参数</span></h3>
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<table class="t-par-begin">
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<tr class="t-par">
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<td>ptr</td>
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<td>-</td>
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<td>指向需要重新分配的内存区域的指针</td>
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</tr>
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<tr class="t-par">
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<td>new_size</td>
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<td>-</td>
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<td>数组的新大小（字节数）</td>
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</tr>
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</table>
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<h3><span class="mw-headline" id=".E8.BF.94.E5.9B.9E.E5.80.BC">返回值</span></h3>
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<p>成功时，返回指向新分配内存的指针。返回的指针必须用 <span class="t-lc"><a href="c-memory-free.html">free()</a></span> 或 <code>realloc()</code> 归还。原指针 <code>ptr</code> 被非法化，而且任何通过它的访问是未定义行为（即使重分配是就地的）。</p>
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<p>失败时，返回空指针。原指针 <code>ptr</code> 保持有效，并需要通过 <span class="t-lc"><a href="c-memory-free.html">free()</a></span> 或 <code>realloc()</code> 归还。</p>
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<h3><span class="mw-headline" id=".E6.B3.A8.E8.A7.A3">注解</span></h3>
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<p>本来（ C89 中），增加对零大小的支持是为了容纳这种代码：</p>
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<div dir="ltr" class="mw-geshi" style="text-align: left;">
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<div class="c source-c">
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<pre class="de1">OBJ <span class="sy2">*</span>p <span class="sy1">=</span> <a href="c-memory-calloc.html"><span class="kw404">calloc</span></a><span class="br0">(</span><span class="nu0">0</span>, <span class="kw4">sizeof</span><span class="br0">(</span>OBJ<span class="br0">)</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// “零长度”占位</span>
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...
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<span class="kw1">while</span><span class="br0">(</span><span class="nu0">1</span><span class="br0">)</span> <span class="br0">{</span>
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p <span class="sy1">=</span> realloc<span class="br0">(</span>p, c <span class="sy2">*</span> <span class="kw4">sizeof</span><span class="br0">(</span>OBJ<span class="br0">)</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// 重分配，直至大小稳定</span>
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... <span class="co1">// 可能会修改 c 或跳出循环的代码</span>
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<span class="br0">}</span></pre></div>
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</div>
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<h3><span class="mw-headline" id=".E7.A4.BA.E4.BE.8B">示例</span></h3>
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<div class="t-example">
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<div class="t-example-live-link"></div>
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<div dir="ltr" class="mw-geshi" style="text-align: left;">
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<div class="c source-c">
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<pre class="de1"><span class="co2">#include <stdio.h></span>
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<span class="co2">#include <stdlib.h></span>
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<span class="kw4">int</span> main<span class="br0">(</span><span class="kw4">void</span><span class="br0">)</span>
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<span class="br0">{</span>
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<span class="kw4">int</span> <span class="sy2">*</span>pa <span class="sy1">=</span> <a href="c-memory-malloc.html"><span class="kw403">malloc</span></a><span class="br0">(</span><span class="nu0">10</span> <span class="sy2">*</span> <span class="kw4">sizeof</span> <span class="sy2">*</span>pa<span class="br0">)</span><span class="sy4">;</span> <span class="co1">// 分配 10 个 int 的数组</span>
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<span class="kw1">if</span><span class="br0">(</span>pa<span class="br0">)</span> <span class="br0">{</span>
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<a href="c-io-fprintf.html"><span class="kw850">printf</span></a><span class="br0">(</span><span class="st0">"%zu bytes allocated. Storing ints: "</span>, <span class="nu0">10</span><span class="sy2">*</span><span class="kw4">sizeof</span><span class="br0">(</span><span class="kw4">int</span><span class="br0">)</span><span class="br0">)</span><span class="sy4">;</span>
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<span class="kw1">for</span><span class="br0">(</span><span class="kw4">int</span> n <span class="sy1">=</span> <span class="nu0">0</span><span class="sy4">;</span> n <span class="sy1"><</span> <span class="nu0">10</span><span class="sy4">;</span> <span class="sy2">++</span>n<span class="br0">)</span>
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<a href="c-io-fprintf.html"><span class="kw850">printf</span></a><span class="br0">(</span><span class="st0">"%d "</span>, pa<span class="br0">[</span>n<span class="br0">]</span> <span class="sy1">=</span> n<span class="br0">)</span><span class="sy4">;</span>
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<span class="br0">}</span>
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<span class="kw4">int</span> <span class="sy2">*</span>pb <span class="sy1">=</span> realloc<span class="br0">(</span>pa, <span class="nu0">1000000</span> <span class="sy2">*</span> <span class="kw4">sizeof</span> <span class="sy2">*</span>pb<span class="br0">)</span><span class="sy4">;</span> <span class="co1">// 重分配数组更大的大小</span>
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<span class="kw1">if</span><span class="br0">(</span>pb<span class="br0">)</span> <span class="br0">{</span>
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<a href="c-io-fprintf.html"><span class="kw850">printf</span></a><span class="br0">(</span><span class="st0">"<span class="es1">\n</span>%zu bytes allocated, first 10 ints are: "</span>, <span class="nu0">1000000</span><span class="sy2">*</span><span class="kw4">sizeof</span><span class="br0">(</span><span class="kw4">int</span><span class="br0">)</span><span class="br0">)</span><span class="sy4">;</span>
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<span class="kw1">for</span><span class="br0">(</span><span class="kw4">int</span> n <span class="sy1">=</span> <span class="nu0">0</span><span class="sy4">;</span> n <span class="sy1"><</span> <span class="nu0">10</span><span class="sy4">;</span> <span class="sy2">++</span>n<span class="br0">)</span>
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<a href="c-io-fprintf.html"><span class="kw850">printf</span></a><span class="br0">(</span><span class="st0">"%d "</span>, pb<span class="br0">[</span>n<span class="br0">]</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// 展示数组</span>
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<a href="c-memory-free.html"><span class="kw405">free</span></a><span class="br0">(</span>pb<span class="br0">)</span><span class="sy4">;</span>
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<span class="br0">}</span> <span class="kw1">else</span> <span class="br0">{</span> <span class="co1">// 若 realloc 失败，则需要返回原指针</span>
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<a href="c-memory-free.html"><span class="kw405">free</span></a><span class="br0">(</span>pa<span class="br0">)</span><span class="sy4">;</span>
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<span class="br0">}</span>
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<span class="br0">}</span></pre></div>
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</div>
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<p>输出：</p>
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<div dir="ltr" class="mw-geshi" style="text-align: left;">
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<div class="text source-text">
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<pre class="de1">40 bytes allocated. Storing ints: 0 1 2 3 4 5 6 7 8 9
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4000000 bytes allocated, first 10 ints are: 0 1 2 3 4 5 6 7 8 9</pre></div>
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</div>
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</div>
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<h3><span class="mw-headline" id=".E5.BC.95.E7.94.A8">引用</span></h3>
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<div class="t-ref-std-17">
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<ul>
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<li>C17 标准（ISO/IEC 9899:2018）：</li>
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</ul>
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<dl>
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<dd>
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<ul>
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<li>7.22.3.5 The realloc function (p: 254)</li>
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</ul>
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</dd>
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</dl>
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</div>
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<div class="t-ref-std-11">
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<ul>
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<li>C11 标准（ISO/IEC 9899:2011）：</li>
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</ul>
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<dl>
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<dd>
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<ul>
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<li>7.22.3.5 The realloc function (p: 349)</li>
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</ul>
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</dd>
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</dl>
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</div>
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<div class="t-ref-std-c99">
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<ul>
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<li>C99 标准（ISO/IEC 9899:1999）：</li>
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</ul>
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<dl>
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<dd>
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<ul>
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<li>7.20.3.4 The realloc function (p: 314)</li>
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</ul>
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</dd>
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</dl>
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</div>
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<div class="t-ref-std-c89">
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<ul>
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<li>C89/C90 标准（ISO/IEC 9899:1990）：</li>
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</ul>
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<dl>
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<dd>
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<ul>
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<li>4.10.3.4 The realloc function</li>
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</ul>
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</dd>
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</dl>
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</div>
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<h3><span class="mw-headline" id=".E5.8F.82.E9.98.85">参阅</span></h3>
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<table class="t-dsc-begin"></table>
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