120 lines
11 KiB
HTML
120 lines
11 KiB
HTML
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<title>std::aligned_alloc</title>
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<body class="mediawiki ltr sitedir-ltr ns-0 ns-subject page-cpp_memory_c_aligned_alloc 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>aligned_alloc</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> aligned_alloc<span class="br0">(</span> <a href="cpp-types-size_t.html"><span class="kw100">std::<span class="me2">size_t</span></span></a> alignment, <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>
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<td class="t-dcl-nopad"></td>
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<td><span class="t-mark-rev t-since-cxx17">(C++17 起)</span></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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<td></td>
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<p>分配 <code>size</code> 字节的未初始化存储，由 <code>alignment</code> 指定其对齐。 <code>size</code> 参数必须是 <code>alignment</code> 的整数倍。</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"><a href="cpp-memory-c-calloc.html">std::calloc</a></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"><strong class="selflink">std::aligned_alloc</strong></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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<td>alignment</td>
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<td>-</td>
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<td>指定对齐。必须是实现支持的合法对齐。</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>分配的字节数。 <code>alignment</code> 的整数倍。</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。为避免内存泄漏，返回的指针必须由 <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>传递不是 <code>alignment</code> 整数倍的 <code>size</code> 或不受实现支持或非法的 <code>alignment</code> 导致函数失败并返回空指针（ C11 刚发布时，曾指定此情况为未定义行为，这为 DR 460 所纠正）。</p>
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<p>作为“实现支持”要求的例子， POSIX 函数 <a rel="nofollow" class="external text" href="http://pubs.opengroup.org/onlinepubs/9699919799/functions/posix_memalign.html">posix_memalign</a> 接受任何是二的幂且为 <code>sizeof(void*)</code> 倍数的 <code>alignment</code> ，而基于 POSIX 的 <code>aligned_alloc</code> 实现继承此要求。</p>
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<p>常规的 <span class="t-lc"><a href="cpp-memory-c-malloc.html">std::malloc</a></span> 分配适用于任何对象的大小的内存（实际上，意味着内存对齐到 <span class="t-c"><span class="mw-geshi cpp source-cpp">alignof<span class="br0">(</span><a href="cpp-types-max_align_t.html"><span class="kw104">std::<span class="me2">max_align_t</span></span></a><span class="br0">)</span></span></span> ）。此函数适用于过对齐分配，例如对 SSE 、缓存线或 VM 页边界。</p>
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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 <cstdio></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="kw1">static_cast</span><span class="sy1"><</span><span class="kw4">int</span><span class="sy2">*</span><span class="sy1">></span><span class="br0">(</span><a href="cpp-memory-c-malloc.html"><span class="kw759">std::<span class="me2">malloc</span></span></a><span class="br0">(</span><span class="nu0">10</span><span class="sy2">*</span>sizeof <span class="sy2">*</span>p1<span class="br0">)</span><span class="br0">)</span><span class="sy4">;</span>
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<a href="cpp-io-c-fprintf.html"><span class="kw1844">std::<span class="me2">printf</span></span></a><span class="br0">(</span><span class="st0">"default-aligned address: %p<span class="es1">\n</span>"</span>, <span class="kw1">static_cast</span><span class="sy1"><</span><span class="kw4">void</span><span class="sy2">*</span><span class="sy1">></span><span class="br0">(</span>p1<span class="br0">)</span><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>p1<span class="br0">)</span><span class="sy4">;</span>
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<span class="kw4">int</span><span class="sy2">*</span> p2 <span class="sy1">=</span> <span class="kw1">static_cast</span><span class="sy1"><</span><span class="kw4">int</span><span class="sy2">*</span><span class="sy1">></span><span class="br0">(</span>std<span class="sy4">::</span><span class="me2">aligned_alloc</span><span class="br0">(</span><span class="nu0">1024</span>, <span class="nu0">10</span><span class="sy2">*</span>sizeof <span class="sy2">*</span>p2<span class="br0">)</span><span class="br0">)</span><span class="sy4">;</span>
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<a href="cpp-io-c-fprintf.html"><span class="kw1844">std::<span class="me2">printf</span></span></a><span class="br0">(</span><span class="st0">"1024-byte aligned address: %p<span class="es1">\n</span>"</span>, <span class="kw1">static_cast</span><span class="sy1"><</span><span class="kw4">void</span><span class="sy2">*</span><span class="sy1">></span><span class="br0">(</span>p2<span class="br0">)</span><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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<span class="br0">}</span></pre></div>
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</div>
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<p>可能的输出：</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">default-aligned address: 0x2221c20
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1024-byte aligned address: 0x2222400</pre></div>
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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">
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<td>
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<div class="t-dsc-member-div">
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<div><a href="cpp-types-aligned_storage.html"><span class="t-lines"><span>aligned_storage</span></span></a></div>
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<div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx11">(C++11)</span></span></span></div>
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</div>
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</td>
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<td>定义适于用作给定大小的类型的未初始化存储的类型<br />
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<span class="t-mark">(类模板)</span></td>
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</tr>
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</table>
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</div>
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<div class="visualClear"></div>
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</body>
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