140 lines
12 KiB
HTML
140 lines
12 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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<head><meta http-equiv="x-ua-compatible" content="ie=edge">
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<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
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<title>std::malloc</title>
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<link rel="stylesheet" href="ext.css" />
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<link rel="stylesheet" href="site_modules.css" />
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<body class="mediawiki ltr sitedir-ltr ns-0 ns-subject page-cpp_memory_c_malloc skin-cppreference2 action-view cpp-navbar">
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<div id="cpp-content-base">
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<div id="content"><a id="top"></a>
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<h1 id="firstHeading" class="firstHeading"><span style="font-size:0.7em; line-height:130%">std::</span>malloc</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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<tbody>
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<tr class="t-dsc-header">
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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>
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<td></td>
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<td></td>
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</tr>
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<tr class="t-dcl">
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<td class="t-dcl-nopad">
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<div><span class="mw-geshi cpp source-cpp"><span class="kw4">void</span><span class="sy2">*</span> malloc<span class="br0">(</span> <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 class="t-dcl-nopad"></td>
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</tr>
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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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</tr>
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</tbody>
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</table>
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<p>分配 <code>size</code> 字节的未初始化存储。</p>
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<p>若分配成功，则返回指向分配的适合对任何标量类型对齐的内存块中，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，或某个不可用于访问存储，但必须传递给 <span class="t-lc"><a href="cpp-memory-c-free.html">std::free</a></span> 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"><a href="cpp-memory-c-calloc.html">std::calloc</a></span> 、 <span class="t-lc"><strong class="selflink">std::malloc</strong></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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</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>size</td>
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<td>-</td>
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<td>要分配的字节数</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起始的指针。返回的指针必须用 <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.E8.A7.A3">注解</span></h3>
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<p>任何情况下，此函数不调用构造函数或初始化内存。无能保证调用匹配的解分配函数的预备使用的智能指针。 C++ 中偏好的内存分配方法是用 RAII 预备函数 <span class="t-lc"><a href="cpp-memory-unique_ptr-make_unique.html">std::make_unique</a></span> 、 <span class="t-lc"><a href="cpp-memory-shared_ptr-make_shared.html">std::make_shared</a></span> 、容器构造函数等，而在低层代码中为 <a href="cpp-language-new.html">new 表达式</a>。</p>
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<p>对于加载大文件，经由如 POSIX 上的 <a rel="nofollow" class="external text" href="https://pubs.opengroup.org/onlinepubs/9699919799/functions/mmap.html"><code>mmap</code></a> 或 Windows 上的 <code>CreateFileMapping</code>(<a rel="nofollow" class="external text" href="https://docs.microsoft.com/en-us/windows/win32/api/winbase/nf-winbase-createfilemappinga"><code>A</code></a>/<a rel="nofollow" class="external text" href="https://docs.microsoft.com/en-us/windows/win32/api/memoryapi/nf-memoryapi-createfilemappingw"><code>W</code></a>) 伴随 <a rel="nofollow" class="external text" href="https://docs.microsoft.com/en-us/windows/win32/api/memoryapi/nf-memoryapi-mapviewoffile"><code>MapViewOfFile</code></a> 的操作系统特定函数进行文件映射，比为文件读取分配缓冲区更适合。</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 <iostream> </span>
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<span class="co2">#include <cstdlib> </span>
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<span class="co2">#include <string></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="co1">// 为 4 个 string 的数组分配足够空间</span>
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<span class="kw1">if</span><span class="br0">(</span><span class="kw4">auto</span> p <span class="sy1">=</span> <span class="br0">(</span><a href="cpp-string-basic_string.html"><span class="kw1229">std::<span class="me2">string</span></span></a><span class="sy2">*</span><span class="br0">)</span>std<span class="sy4">::</span><span class="me2">malloc</span><span class="br0">(</span><span class="nu0">4</span> <span class="sy2">*</span> sizeof<span class="br0">(</span><a href="cpp-string-basic_string.html"><span class="kw1229">std::<span class="me2">string</span></span></a><span class="br0">)</span><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> i <span class="sy1">=</span> <span class="nu0">0</span><span class="sy4">;</span>
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<span class="kw1">try</span>
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<span class="br0">{</span>
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<span class="kw1">for</span><span class="br0">(</span><span class="sy4">;</span> i <span class="sy3">!</span><span class="sy1">=</span> <span class="nu0">4</span><span class="sy4">;</span> <span class="sy2">++</span>i<span class="br0">)</span> <span class="co1">// 填充数组</span>
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new<span class="br0">(</span>p <span class="sy2">+</span> i<span class="br0">)</span> <a href="cpp-string-basic_string.html"><span class="kw1229">std::<span class="me2">string</span></span></a><span class="br0">(</span><span class="nu0">5</span>, <span class="st0">'a'</span> <span class="sy2">+</span> i<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> j <span class="sy1">=</span> <span class="nu0">0</span><span class="sy4">;</span> j <span class="sy3">!</span><span class="sy1">=</span> <span class="nu0">4</span><span class="sy4">;</span> <span class="sy2">++</span>j<span class="br0">)</span> <span class="co1">// 打印出来</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">"p["</span> <span class="sy1"><<</span> j <span class="sy1"><<</span> <span class="st0">"] == "</span> <span class="sy1"><<</span> p<span class="br0">[</span>j<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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<span class="br0">}</span>
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<span class="kw1">catch</span><span class="br0">(</span>...<span class="br0">)</span> <span class="br0">{</span><span class="br0">}</span>
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<span class="kw1">for</span><span class="br0">(</span><span class="sy4">;</span> i <span class="sy3">!</span><span class="sy1">=</span> <span class="nu0">0</span><span class="sy4">;</span> <span class="sy2">--</span>i<span class="br0">)</span> <span class="co1">// 清理</span>
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p<span class="br0">[</span>i <span class="sy2">-</span> <span class="nu0">1</span><span class="br0">]</span>.~basic_string<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>p<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">p[0] == aaaaa
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p[1] == bbbbb
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p[2] == ccccc
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p[3] == ddddd</pre></div>
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</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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<tr class="t-dsc">
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<td>
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<div class="t-dsc-member-div">
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<div><a href="cpp-memory-new-operator_new.html"><span class="t-lines"><span>operator new</span><span>operator new[]</span></span></a></div>
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</div>
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</td>
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<td>分配函数<br />
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<span class="t-mark">(函数)</span></td>
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</tr>
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<tr class="t-dsc">
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<td>
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<div class="t-dsc-member-div">
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<div><a href="cpp-memory-get_temporary_buffer.html"><span class="t-lines"><span>get_temporary_buffer</span></span></a></div>
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<div><span class="t-lines"><span><span class="t-mark-rev t-deprecated-cxx17">(C++17 中弃用)</span></span><span><span class="t-mark-rev t-until-cxx20">(C++20 中移除)</span></span></span></div>
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</div>
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</td>
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<td>获得未初始化存储<br />
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<span class="t-mark">(函数模板)</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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</div>
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</div>
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</div>
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</body>
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</html>
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