202 lines
13 KiB
HTML
202 lines
13 KiB
HTML
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<title>memset, memset_s</title>
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<body class="mediawiki ltr sitedir-ltr ns-0 ns-subject page-c_string_byte_memset skin-cppreference2 action-view cpp-navbar">
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<h1 id="firstHeading" class="firstHeading">memset, memset_s</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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<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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<td>
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<div><span class="mw-geshi c source-c"><span class="kw4">void</span> <span class="sy2">*</span>memset<span class="br0">(</span> <span class="kw4">void</span> <span class="sy2">*</span>dest, <span class="kw4">int</span> ch, <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>(1)</td>
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<td class="t-dcl-nopad"></td>
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</tr>
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<tr class="t-dcl t-since-c11">
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<td>
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<div><span class="mw-geshi c source-c">errno_t memset_s<span class="br0">(</span> <span class="kw4">void</span> <span class="sy2">*</span>dest, rsize_t destsz, <span class="kw4">int</span> ch, rsize_t count <span class="br0">)</span><span class="sy4">;</span></span></div>
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</td>
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<td>(2)</td>
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<td><span class="t-mark-rev t-since-c11">(C11 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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</tbody>
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</table>
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<div class="t-li1"><span class="t-li">1)</span> 复制值 <code>ch</code> （如同以 <span class="t-c"><span class="mw-geshi c source-c"><span class="br0">(</span><span class="kw4">unsigned</span> <span class="kw4">char</span><span class="br0">)</span>ch</span></span> 转换到 <span class="t-c"><span class="mw-geshi c source-c"><span class="kw4">unsigned</span> <span class="kw4">char</span></span></span> 后）到 <code>dest</code> 所指向对象的首 <code>count</code> 个字节。</div>
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<div class="t-li1"><span class="t-li"></span> 若出现 dest 数组结尾后的访问则行为未定义。若 <code>dest</code> 为空指针则行为未定义。</div>
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<div class="t-li1"><span class="t-li">2)</span> 同 <span class="t-v">(1)</span> ，除了若 <code>dest</code> 与 <code>destsz</code> 自身有效，则存储 <code>ch</code> 于目标范围 <span class="t-c"><span class="mw-geshi c source-c"><span class="br0">[</span>dest, dest<span class="sy2">+</span>destsz<span class="br0">)</span></span></span> 的每个位置后，在运行时检测下列错误，并调用当前安装的<a href="c-error-set_constraint_handler_s.html">制约处理</a>函数：
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<dl>
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<dd>
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<ul>
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<li><code>dest</code> 是空指针</li>
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<li><code>destsz</code> 或 <code>count</code> 大于 <span class="t-lc">RSIZE_MAX</span></li>
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<li><code>count</code> 大于 <code>destsz</code> （会发生缓冲区溢出）</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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<div class="t-li1"><span class="t-li"></span> 若 <code>dest</code> 所指向的字符数组大小 < <code>count</code> <= <code>destsz</code>; 则行为未定义，换言之，错误的 <code>destsz</code> 值不暴露行将发生的缓冲区溢出。
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<dl>
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<dd></dd>
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<dd>同所有边界检查函数， <code>memset_s</code> 仅若实现定义了 <span class="t-c"><span class="mw-geshi c source-c">__STDC_LIB_EXT1__</span></span> ，且用户在包含 <code>string.h</code> 前定义 <span class="t-c"><span class="mw-geshi c source-c">__STDC_WANT_LIB_EXT1__</span></span> 为整数常量 <span class="t-c"><span class="mw-geshi c source-c"><span class="nu0">1</span></span></span> 才保证可用。</dd>
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</dl>
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</div>
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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>dest</td>
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<td>-</td>
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<td>指向要填充的对象的指针</td>
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</tr>
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<tr class="t-par">
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<td>ch</td>
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<td>-</td>
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<td>填充字节</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>要填充的字节数</td>
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</tr>
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<tr class="t-par">
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<td>destsz</td>
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<td>-</td>
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<td>目标数组的大小</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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<div class="t-li1"><span class="t-li">1)</span> <code>dest</code> 的副本，本质为更底层操作的临时内存地址，在实际操作中不建议直接使用此地址，操作完成以后，真正有意义的地址是dest本身。</div>
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<div class="t-li1"><span class="t-li">2)</span> 成功时为零，失败时为非零。在失败时，若 <code>dest</code> 不是空指针且 <code>destsz</code> 合法，则亦写入 <code>destsz</code> 个填充字节 <code>ch</code> 到目标数组。</div>
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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>若 <code>memset</code> 所修改的对象在其生存期的剩余部分不再被访问，则此函数可以被优化掉（在<a href="https://zh.cppreference.com/mwiki/index.php?title=c/language/as_if&action=edit&redlink=1" class="new" title="c/language/as if（页面不存在）">如同</a>规则下）（例如 <a rel="nofollow" class="external text" href="https://gcc.gnu.org/bugzilla/show_bug.cgi?id=8537">gcc 漏洞 8537</a> ）。为此，此函数不能用于擦洗内存（例如以令填充存储密码的数组）。对 <code>memset_s</code> 禁止此优化：保证进行内存写。该问题的第三方解决方案包含 FreeBSD <a rel="nofollow" class="external text" href="https://www.freebsd.org/cgi/man.cgi?query=explicit_bzero">explicit_bzero</a> 或 Microsoft <a rel="nofollow" class="external text" href="https://msdn.microsoft.com/en-us/library/windows/desktop/aa366877.aspx">SecureZeroMemory</a> 。</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">#define __STDC_WANT_LIB_EXT1__ 1</span>
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<span class="co2">#include <stdio.h></span>
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<span class="co2">#include <string.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">char</span> str<span class="br0">[</span><span class="br0">]</span> <span class="sy1">=</span> <span class="st0">"ghghghghghghghghghghgh"</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>str<span class="br0">)</span><span class="sy4">;</span>
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memset<span class="br0">(</span>str,<span class="st0">'a'</span>,<span class="nu0">5</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>str<span class="br0">)</span><span class="sy4">;</span>
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<span class="co2">#ifdef __STDC_LIB_EXT1__</span>
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set_constraint_handler_s<span class="br0">(</span>ignore_handler_s<span class="br0">)</span><span class="sy4">;</span>
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<span class="kw4">int</span> r <span class="sy1">=</span> memset_s<span class="br0">(</span>str, <span class="kw4">sizeof</span> str, <span class="st0">'b'</span>, <span class="nu0">5</span><span class="br0">)</span><span class="sy4">;</span>
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<a href="c-io-fprintf.html"><span class="kw850">printf</span></a><span class="br0">(</span><span class="st0">"str = <span class="es1">\"</span>%s<span class="es1">\"</span>, r = %d<span class="es1">\n</span>"</span>, str, r<span class="br0">)</span><span class="sy4">;</span>
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r <span class="sy1">=</span> memset_s<span class="br0">(</span>str, <span class="nu0">5</span>, <span class="st0">'c'</span>, <span class="nu0">10</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// count 大于 destsz </span>
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<a href="c-io-fprintf.html"><span class="kw850">printf</span></a><span class="br0">(</span><span class="st0">"str = <span class="es1">\"</span>%s<span class="es1">\"</span>, r = %d<span class="es1">\n</span>"</span>, str, r<span class="br0">)</span><span class="sy4">;</span>
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<span class="co2">#endif</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">ghghghghghghghghghghgh
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aaaaahghghghghghghghgh
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str = "bbbbbhghghghghghghghgh", r = 0
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str = "ccccchghghghghghghghgh", r = 22</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>7.24.6.1 The memset function (p: 371)</li>
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</ul>
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</dd>
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</dl>
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<dl>
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<dd>
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<ul>
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<li>K.3.7.4.1 The memset_s function (p: 621-622)</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>7.21.6.1 The memset function (p: 333)</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>4.11.6.1 The memset 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-memcpy.html"><span class="t-lines"><span>memcpy</span><span>memcpy_s</span></span></a></div>
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<div><span class="t-lines"><span></span><span><span class="t-mark-rev t-since-c11">(C11)</span></span></span></div>
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</div>
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</td>
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<td>将一个缓冲区复制到另一个<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-wide-wmemset.html"><span class="t-lines"><span>wmemset</span></span></a></div>
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<div><span class="t-lines"><span><span class="t-mark-rev t-since-c95">(C95)</span></span></span></div>
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</div>
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</td>
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<td>将给定的宽字符复制到宽字符数组的所有位置<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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