110 lines
9.6 KiB
HTML
110 lines
9.6 KiB
HTML
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<title>aligned_alloc</title>
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<body class="mediawiki ltr sitedir-ltr ns-0 ns-subject page-c_memory_aligned_alloc skin-cppreference2 action-view cpp-navbar">
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<h1 id="firstHeading" class="firstHeading">aligned_alloc</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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<table class="t-dcl-begin">
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<td>
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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>aligned_alloc<span class="br0">(</span> <a href="c-types-size_t.html"><span class="kw100">size_t</span></a> alignment, <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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<td><span class="t-mark-rev t-since-c11">(C11 起)</span></td>
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<p>分配 <code>size</code> 字节未初始化的存储空间，按照 <code>alignment</code> 指定对齐。 <code>size</code> 参数必须是 <code>alignment</code> 的整数倍。</p>
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<p><code>aligned_alloc</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> 归还一块内存区域的先前调用，<i>同步于</i>令 <span class="t-lc">aligned_alloc</span> 分配相同或部分相同的内存区域的调用。此同步出现于任何解分配函数所做的内存访问之后，和任何通过 <code>aligned_alloc</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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<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>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的指针。为避免内存泄漏，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> 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> ，或传递实现不支持的 <code>alignment</code> ，会令函数失败并返回空指针（出版的 C11 指定此为未定义行为，这已经为 DR 460 所更正）。 <a rel="nofollow" class="external text" href="http://www.open-std.org/jtc1/sc22/wg14/www/docs/n2072.htm">n2072</a> 提议移除大小限制，使之能在限制性的对齐边界分配小对象（类似 <a href="c-language-_Alignas.html"><tt>_Alignas</tt></a> ）。</p>
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<p>作为“实现支持”的要求一例， 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="c-memory-malloc.html">malloc</a></span> 为任何对象类型对齐内存（实际上，这表示对齐到 <span class="t-c"><span class="mw-geshi c source-c">_Alignof<span class="br0">(</span><a href="c-types-max_align_t.html"><span class="kw104">max_align_t</span></a><span class="br0">)</span></span></span> ）。此函数对于过度对齐的内存分配有用，譬如 SSE 、缓存线路，或是虚拟机页边界。</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="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> <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>p1<span class="br0">)</span><span class="sy4">;</span>
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<a href="c-io-fprintf.html"><span class="kw851">printf</span></a><span class="br0">(</span><span class="st0">"default-aligned addr: %p<span class="es1">\n</span>"</span>, <span class="br0">(</span><span class="kw4">void</span><span class="sy2">*</span><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="kw406">free</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> aligned_alloc<span class="br0">(</span><span class="nu0">1024</span>, <span class="nu0">1024</span><span class="sy2">*</span><span class="kw4">sizeof</span> <span class="sy2">*</span>p2<span class="br0">)</span><span class="sy4">;</span>
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<a href="c-io-fprintf.html"><span class="kw851">printf</span></a><span class="br0">(</span><span class="st0">"1024-byte aligned addr: %p<span class="es1">\n</span>"</span>, <span class="br0">(</span><span class="kw4">void</span><span class="sy2">*</span><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="kw406">free</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 addr: 0x1e40c20
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1024-byte aligned addr: 0x1e41000</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>7.22.3.1 The aligned_alloc function (p: 347-348)</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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<div class="visualClear"></div>
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