159 lines
11 KiB
HTML
159 lines
11 KiB
HTML
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<html lang="zh" dir="ltr" class="client-nojs" xmlns="http://www.w3.org/1999/xhtml" xml:lang="zh">
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<title>calloc</title>
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<body class="mediawiki ltr sitedir-ltr ns-0 ns-subject page-c_memory_calloc skin-cppreference2 action-view cpp-navbar">
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<div id="cpp-content-base">
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<h1 id="firstHeading" class="firstHeading">calloc</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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<div><span class="mw-geshi c source-c"><span class="kw4">void</span><span class="sy2">*</span> calloc<span class="br0">(</span> <a href="c-types-size_t.html"><span class="kw100">size_t</span></a> num, <a href="c-types-size_t.html"><span class="kw100">size_t</span></a> 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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<p>为 <code>num</code> 个对象的数组分配内存，并初始化所有分配存储中的字节为零。</p>
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<p>若分配成功，会返回指向分配内存块最低位（首位）字节的指针，它为任何类型适当地对齐。</p>
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<p>若 <code>size</code> 为零，则行为是实现定义的（可返回空指针，或返回不可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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<p><code>calloc</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>令 <span class="t-lc"><a href="c-memory-free.html">free</a></span> 或 <span class="t-lc"><a href="c-memory-realloc.html">realloc</a></span> 解分配一块内存区域的先前调用，<i>同步于</i>令 <span class="t-lc"><strong class="selflink">calloc</strong></span> 分配相同或部分相同的内存区域的调用。这种同步出现于任何解分配函数所做的内存访问之后，和任何 <code>calloc</code> 所做的内存访问之前。所有操作每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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</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>num</td>
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<td>-</td>
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<td>对象数目</td>
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</tr>
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<tr class="t-par">
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<td>size</td>
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<td>-</td>
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<td>每个对象的大小</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。为避免内存泄漏，必须用 <span class="t-lc"><a href="c-memory-free.html">free()</a></span> 或 <code>realloc()</code> 解分配返回的指针。</p>
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<p>失败时，返回空指针。</p>
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<h3><span class="mw-headline" id=".E6.B3.A8.E6.84.8F">注意</span></h3>
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<p>因为对齐需求的缘故，分配的字节数不必等于 <code>num*size</code> 。</p>
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<p>初始化所有位为零不保证浮点数或指针被各种初始化为 0.0 或空指针（尽管这在所有常见平台上为真）。</p>
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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> calloc<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”占位符</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> <a href="c-memory-realloc.html"><span class="kw404">realloc</span></a><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>p1 <span class="sy1">=</span> calloc<span class="br0">(</span><span class="nu0">4</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> <span class="co1">// 分配并清零 4 个 int 的数组</span>
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<span class="kw4">int</span> <span class="sy2">*</span>p2 <span class="sy1">=</span> calloc<span class="br0">(</span><span class="nu0">1</span>, <span class="kw4">sizeof</span><span class="br0">(</span><span class="kw4">int</span><span class="br0">[</span><span class="nu0">4</span><span class="br0">]</span><span class="br0">)</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// 等价，直接命名数组类型</span>
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<span class="kw4">int</span> <span class="sy2">*</span>p3 <span class="sy1">=</span> calloc<span class="br0">(</span><span class="nu0">4</span>, <span class="kw4">sizeof</span> <span class="sy2">*</span>p3<span class="br0">)</span><span class="sy4">;</span> <span class="co1">// 等价，免去重复类型名</span>
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<span class="kw1">if</span><span class="br0">(</span>p2<span class="br0">)</span> <span class="br0">{</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">4</span><span class="sy4">;</span> <span class="sy2">++</span>n<span class="br0">)</span> <span class="co1">// 打印数组</span>
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<a href="c-io-fprintf.html"><span class="kw850">printf</span></a><span class="br0">(</span><span class="st0">"p2[%d] == %d<span class="es1">\n</span>"</span>, n, p2<span class="br0">[</span>n<span class="br0">]</span><span class="br0">)</span><span class="sy4">;</span>
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<span class="br0">}</span>
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<a href="c-memory-free.html"><span class="kw405">free</span></a><span class="br0">(</span>p1<span class="br0">)</span><span class="sy4">;</span>
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<a href="c-memory-free.html"><span class="kw405">free</span></a><span class="br0">(</span>p2<span class="br0">)</span><span class="sy4">;</span>
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<a href="c-memory-free.html"><span class="kw405">free</span></a><span class="br0">(</span>p3<span class="br0">)</span><span class="sy4">;</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">p2[0] == 0
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p2[1] == 0
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p2[2] == 0
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p2[3] == 0</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-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>7.22.3.2 The calloc function (p: 348)</li>
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</ul>
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</dd>
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</dl>
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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>7.20.3.1 The calloc function (p: 313)</li>
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</ul>
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</dd>
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</dl>
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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>4.10.3.1 The calloc 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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