126 lines
11 KiB
HTML
126 lines
11 KiB
HTML
<!DOCTYPE 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>std::calloc</title>
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<body class="mediawiki ltr sitedir-ltr ns-0 ns-subject page-cpp_memory_c_calloc skin-cppreference2 action-view cpp-navbar">
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<h1 id="firstHeading" class="firstHeading"><span style="font-size:0.7em; line-height:130%">std::</span>calloc</h1>
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<div id="bodyContent">
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<div id="contentSub"><span class="subpages">< <a href="cpp.html">cpp</a>‎ | <a href="cpp-memory.html">memory</a>‎ | <a href="cpp-memory-c.html">c</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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<table class="t-dcl-begin">
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<td>
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<div>定义于头文件 <code><a href="cpp-header-cstdlib.html"><cstdlib></a></code></div>
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<td></td>
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<td></td>
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<div><span class="mw-geshi cpp source-cpp"><span class="kw4">void</span><span class="sy2">*</span> calloc<span class="br0">(</span> <a href="cpp-types-size_t.html"><span class="kw100">std::<span class="me2">size_t</span></span></a> num, <a href="cpp-types-size_t.html"><span class="kw100">std::<span class="me2">size_t</span></span></a> size <span class="br0">)</span><span class="sy4">;</span></span></div>
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<td></td>
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<p>分配 <code>num</code> 个大小为 <code>size</code> 的对象的数组，并初始化所有位为零。</p>
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<p>若分配成功，则返回指向为任何对象类型适当对齐的，被分配内存块最低（首）字节的指针。</p>
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<p>若 <code>size</code> 为零，则行为是实现定义的（可以返回空指针，亦可返回某个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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<tr class="t-rev t-since-cxx11">
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<td>
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<p>要求下列函数是线程安全的：</p>
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<ul>
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<li><a href="cpp-memory-new-operator_new.html"><tt>operator new</tt></a> 及 <a href="cpp-memory-new-operator_delete.html"><tt>operator delete</tt></a> 的库版本</li>
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<li>全局 <a href="cpp-memory-new-operator_new.html"><tt>operator new</tt></a> 与 <a href="cpp-memory-new-operator_delete.html"><tt>operator delete</tt></a> 的用户替换版本</li>
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<li><span class="t-lc"><strong class="selflink">std::calloc</strong></span> 、 <span class="t-lc"><a href="cpp-memory-c-malloc.html">std::malloc</a></span> 、 <span class="t-lc"><a href="cpp-memory-c-realloc.html">std::realloc</a></span> <span class="t-rev-inl t-since-cxx17"><span>、 <span class="t-lc"><a href="cpp-memory-c-aligned_alloc.html">std::aligned_alloc</a></span></span> <span><span class="t-mark-rev t-since-cxx17">(C++17 起)</span></span></span> 、 <span class="t-lc"><a href="cpp-memory-c-free.html">std::free</a></span></li>
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</ul>
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<p>对这些分配或解分配特定存储单元的函数调用以单独全序出现，并且在此顺序中，每个解分配调用<a href="cpp-atomic-memory_order.html">先发生于</a>下个分配（若存在）。</p>
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</td>
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<td><span class="t-mark-rev t-since-cxx11">(C++11 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>对象数量</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起始的指针。返回的指针必须以 <span class="t-lc"><a href="cpp-memory-c-free.html">std::free()</a></span> 或 <span class="t-lc"><a href="cpp-memory-c-realloc.html">std::realloc()</a></span> 解分配。</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>因为对齐要求，分配的字节数不需要等于 <code>num*size</code> 。</p>
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<p>将所有位初始化为零不保证浮点数或指针各被初始化到 0.0 与空指针值（尽管大多数平台上这为 true ）</p>
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<p>最初（ C89 中），添加对零大小的支持是为了接纳下面这种代码</p>
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<div dir="ltr" class="mw-geshi" style="text-align: left;">
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<div class="cpp source-cpp">
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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>, sizeof<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> sizeof<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">// 可能更改 c 或打破循环的代码</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="cpp source-cpp">
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<pre class="de1"><span class="co2">#include <iostream></span>
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<span class="co2">#include <cstdlib></span>
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<span class="kw4">int</span> main<span class="br0">(</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> <span class="br0">(</span><span class="kw4">int</span><span class="sy2">*</span><span class="br0">)</span>std<span class="sy4">::</span><span class="me2">calloc</span><span class="br0">(</span><span class="nu0">4</span>, sizeof<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> <span class="br0">(</span><span class="kw4">int</span><span class="sy2">*</span><span class="br0">)</span>std<span class="sy4">::</span><span class="me2">calloc</span><span class="br0">(</span><span class="nu0">1</span>, sizeof<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">// 同上，直接指名数组类型</span>
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<span class="kw4">int</span><span class="sy2">*</span> p3 <span class="sy1">=</span> <span class="br0">(</span><span class="kw4">int</span><span class="sy2">*</span><span class="br0">)</span>std<span class="sy4">::</span><span class="me2">calloc</span><span class="br0">(</span><span class="nu0">4</span>, sizeof <span class="sy2">*</span>p3<span class="br0">)</span><span class="sy4">;</span> <span class="co1">// 同上，不重复类型名</span>
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<span class="kw1">if</span><span class="br0">(</span>p2<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="cpp-io-cout.html"><span class="kw1757">std::<span class="me2">cout</span></span></a> <span class="sy1"><<</span> <span class="st0">"p2["</span> <span class="sy1"><<</span> n <span class="sy1"><<</span> <span class="st0">"] == "</span> <span class="sy1"><<</span> p2<span class="br0">[</span>n<span class="br0">]</span> <span class="sy1"><<</span> <span class="st0">'<span class="es1">\n</span>'</span><span class="sy4">;</span>
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<a href="cpp-memory-c-free.html"><span class="kw762">std::<span class="me2">free</span></span></a><span class="br0">(</span>p1<span class="br0">)</span><span class="sy4">;</span>
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<a href="cpp-memory-c-free.html"><span class="kw762">std::<span class="me2">free</span></span></a><span class="br0">(</span>p2<span class="br0">)</span><span class="sy4">;</span>
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<a href="cpp-memory-c-free.html"><span class="kw762">std::<span class="me2">free</span></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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<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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<div class="visualClear"></div>
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