166 lines
10 KiB
HTML
166 lines
10 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>memcmp</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-c_string_byte_memcmp 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">memcmp</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-string.html">string</a>‎ | <a href="c-string-byte.html">byte</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><string.h></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 c source-c"><span class="kw4">int</span> memcmp<span class="br0">(</span> <span class="kw4">const</span> <span class="kw4">void</span><span class="sy2">*</span> lhs, <span class="kw4">const</span> <span class="kw4">void</span><span class="sy2">*</span> rhs, <a href="c-types-size_t.html"><span class="kw100">size_t</span></a> count <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>比较 <code>lhs</code> 和 <code>rhs</code> 所指向对象的首 <code>count</code> 个字节。比较按字典序进行。</p>
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<p>结果的符号是在被比较对象中相异的首对字节的值（都转译成 <span class="t-c"><span class="mw-geshi c source-c"><span class="kw4">unsigned</span> <span class="kw4">char</span></span></span> ）的差。</p>
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<p>若在 <code>lhs</code> 和 <code>rhs</code> 所指向的任一对象结尾后出现访问，则行为未定义。若 <code>lhs</code> 或 <code>rhs</code> 为空指针则行为未定义。</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>lhs, rhs</td>
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<td>-</td>
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<td>指向要比较的对象的指针</td>
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</tr>
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<tr class="t-par">
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<td>count</td>
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<td>-</td>
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<td>要检验的字节数</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>若 <code>lhs</code> 以字典序出现前于 <code>rhs</code> 则为负值。</p>
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<p>若 <code>lhs</code> 与 <code>rhs</code> 比较相等，或 count 为零则为零。</p>
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<p>若 <code>lhs</code> 以字典序出现后于 <code>rhs</code> 则为正值。</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>此函数读取<a href="c-language-object.html">对象表示</a>，而非对象值，而且典型地只对字节数组有意义：结构体可以含有填充字节而其值不确定，存储于联合体最近存储成员后的任何字节的值是不确定的，且一个类型可以对相同值拥有二种或多种表示（对于 +0 和 -0 或 +0.0 和 –0.0 的相异编码、类型中不确定填充位）。</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 <string.h></span>
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<span class="kw4">void</span> demo<span class="br0">(</span><span class="kw4">const</span> <span class="kw4">char</span><span class="sy2">*</span> lhs, <span class="kw4">const</span> <span class="kw4">char</span><span class="sy2">*</span> rhs, <a href="c-types-size_t.html"><span class="kw100">size_t</span></a> sz<span class="br0">)</span>
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<span class="br0">{</span>
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<span class="kw1">for</span><span class="br0">(</span><a href="c-types-size_t.html"><span class="kw100">size_t</span></a> n <span class="sy1">=</span> <span class="nu0">0</span><span class="sy4">;</span> n <span class="sy1"><</span> sz<span class="sy4">;</span> <span class="sy2">++</span>n<span class="br0">)</span>
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<a href="c-io-putchar.html"><span class="kw834">putchar</span></a><span class="br0">(</span>lhs<span class="br0">[</span>n<span class="br0">]</span><span class="br0">)</span><span class="sy4">;</span>
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<span class="kw4">int</span> rc <span class="sy1">=</span> memcmp<span class="br0">(</span>lhs, rhs, sz<span class="br0">)</span><span class="sy4">;</span>
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<span class="kw4">const</span> <span class="kw4">char</span> <span class="sy2">*</span>rel <span class="sy1">=</span> rc <span class="sy1"><</span> <span class="nu0">0</span> <span class="sy4">?</span> <span class="st0">" precedes "</span> <span class="sy4">:</span> rc <span class="sy1">></span> <span class="nu0">0</span> <span class="sy4">?</span> <span class="st0">" follows "</span> <span class="sy4">:</span> <span class="st0">" compares equal "</span><span class="sy4">;</span>
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<a href="c-io-fputs.html"><span class="kw831">fputs</span></a><span class="br0">(</span>rel, <a href="c-io.html"><span class="kw886">stdout</span></a><span class="br0">)</span><span class="sy4">;</span>
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<span class="kw1">for</span><span class="br0">(</span><a href="c-types-size_t.html"><span class="kw100">size_t</span></a> n <span class="sy1">=</span> <span class="nu0">0</span><span class="sy4">;</span> n <span class="sy1"><</span> sz<span class="sy4">;</span> <span class="sy2">++</span>n<span class="br0">)</span>
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<a href="c-io-putchar.html"><span class="kw834">putchar</span></a><span class="br0">(</span>rhs<span class="br0">[</span>n<span class="br0">]</span><span class="br0">)</span><span class="sy4">;</span>
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<a href="c-io-puts.html"><span class="kw835">puts</span></a><span class="br0">(</span><span class="st0">" in lexicographical order"</span><span class="br0">)</span><span class="sy4">;</span>
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<span class="br0">}</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">char</span> a1<span class="br0">[</span><span class="br0">]</span> <span class="sy1">=</span> <span class="br0">{</span><span class="st0">'a'</span>,<span class="st0">'b'</span>,<span class="st0">'c'</span><span class="br0">}</span><span class="sy4">;</span>
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<span class="kw4">char</span> a2<span class="br0">[</span><span class="kw4">sizeof</span> a1<span class="br0">]</span> <span class="sy1">=</span> <span class="br0">{</span><span class="st0">'a'</span>,<span class="st0">'b'</span>,<span class="st0">'d'</span><span class="br0">}</span><span class="sy4">;</span>
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demo<span class="br0">(</span>a1, a2, <span class="kw4">sizeof</span> a1<span class="br0">)</span><span class="sy4">;</span>
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demo<span class="br0">(</span>a2, a1, <span class="kw4">sizeof</span> a1<span class="br0">)</span><span class="sy4">;</span>
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demo<span class="br0">(</span>a1, a1, <span class="kw4">sizeof</span> a1<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">abc precedes abd in lexicographical order
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abd follows abc in lexicographical order
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abc compares equal to abc in lexicographical order</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>7.24.4.1 The memcmp function (p: 365)</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>7.21.4.1 The memcmp function (p: 328)</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>4.11.4.1 The memcmp 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">
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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="c-string-byte-strcmp.html"><span class="t-lines"><span>strcmp</span></span></a></div>
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</div>
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</td>
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<td>比较两个字符串<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="c-string-byte-strncmp.html"><span class="t-lines"><span>strncmp</span></span></a></div>
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</div>
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</td>
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<td>比较两个字符串的一定数量字符<br />
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<span class="t-mark">(函数)</span></td>
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</tr>
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</table>
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</div>
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</div>
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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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