254 lines
22 KiB
HTML
254 lines
22 KiB
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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<title>strncpy, strncpy_s</title>
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<body class="mediawiki ltr sitedir-ltr ns-0 ns-subject page-c_string_byte_strncpy 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">strncpy, strncpy_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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</tbody>
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<tbody class="t-dcl-rev t-dcl-rev-num">
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<td></td>
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<td rowspan="142">(1)</td>
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<td></td>
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<tr class="t-dcl t-until-c99">
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<div><span class="mw-geshi c source-c"><span class="kw4">char</span> <span class="sy2">*</span>strncpy<span class="br0">(</span> <span class="kw4">char</span> <span class="sy2">*</span>dest, <span class="kw4">const</span> <span class="kw4">char</span> <span class="sy2">*</span>src, <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><span class="t-mark-rev t-until-c99">(C99 前)</span></td>
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<tr class="t-dcl t-since-c99">
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<div><span class="mw-geshi c source-c"><span class="kw4">char</span> <span class="sy2">*</span>strncpy<span class="br0">(</span> <span class="kw4">char</span> <span class="sy2">*</span><span class="kw4">restrict</span> dest, <span class="kw4">const</span> <span class="kw4">char</span> <span class="sy2">*</span><span class="kw4">restrict</span> src, <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><span class="t-mark-rev t-since-c99">(C99 起)</span></td>
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</tbody>
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<tbody>
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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 strncpy_s<span class="br0">(</span><span class="kw4">char</span> <span class="sy2">*</span><span class="kw4">restrict</span> dest, rsize_t destsz,<br />
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<span class="kw4">const</span> <span class="kw4">char</span> <span class="sy2">*</span><span class="kw4">restrict</span> src, 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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<div class="t-li1"><span class="t-li">1)</span> 复制 <code>src</code> 所指向的字符数组的至多 <code>count</code> 个字符（包含空终止字符，但不包含后随空字符的任何字符）到 <code>dest</code> 所指向的字符数组。</div>
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<div class="t-li1"><span class="t-li"></span> 若在完全复制整个 <code>src</code> 数组前抵达 <code>count</code> ，则结果的字符数组不是空终止的。</div>
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<div class="t-li1"><span class="t-li"></span> 若在复制来自 <code>src</code> 的空终止字符后未抵达 <code>count</code> ，则写入额外的空字符到 <code>dest</code> ，直至写入总共 <code>count</code> 个字符。</div>
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<div class="t-li1"><span class="t-li"></span> 若字符数组重叠，若 <code>dest</code> 或 <code>src</code> 不是指向字符数组的指针（包含若 <code>dest</code> 或 <code>src</code> 为空指针），若 <code>dest</code> 所指向的数组大小小于 <code>count</code> ，或若 <code>src</code> 所指向的数组大小小于 <code>count</code> 且它不含空字符，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> ，除了函数不持续写入零到目标数组以填满 <code>count</code> ，它在写入空终止字符后停止（若源中无空字符，则它于 <span class="t-c"><span class="mw-geshi c source-c">dest<span class="br0">[</span>count<span class="br0">]</span></span></span> 写入一个然后停止）。并且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>src</code> 或 <code>dest</code> 是空指针</li>
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<li><code>destsz</code> 零或大于 <span class="t-lc">RSIZE_MAX</span></li>
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<li><code>count</code> 大于 <span class="t-lc">RSIZE_MAX</span></li>
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<li><code>count</code> 大于或等于 <code>destsz</code> ，但 <code>destsz</code> 小于或等于 <span class="t-c"><span class="mw-geshi c source-c">strnlen_s<span class="br0">(</span>src, count<span class="br0">)</span></span></span> ，换言之，会出现截断</li>
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<li>源和目标字符串间会出现重叠</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> 所指的字符数组大小 < <span class="t-c"><span class="mw-geshi c source-c">strnlen_s<span class="br0">(</span>src, destsz<span class="br0">)</span></span></span> <= <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。若 <code>src</code> 所指的字符数组大小 < <span class="t-c"><span class="mw-geshi c source-c">strnlen_s<span class="br0">(</span>src, count<span class="br0">)</span></span></span> < <code>destsz</code> 则行为未定义；换言之，错误的 <code>count</code> 值不暴露行将发生的缓冲区溢出。</div>
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<dl>
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<dd>同所有边界检查函数， <code>strncpy_s</code> 仅若实现定义 <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 1 才保证可用。</dd>
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</dl>
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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>指向要复制到的字符数组的指针</td>
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</tr>
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<tr class="t-par">
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<td>src</td>
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<td>-</td>
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<td>指向复制来源的字符数组的指针</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>要复制的最大字符数</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>目标缓冲区的大小</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> 返回 <code>dest</code> 的副本</div>
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<div class="t-li1"><span class="t-li">2)</span> 成功时返回零，错误时返回非零。而且，在错误时写入零到 <span class="t-c"><span class="mw-geshi c source-c">dest<span class="br0">[</span><span class="nu0">0</span><span class="br0">]</span></span></span> （除非 <code>dest</code> 为空指针，或 <code>destsz</code> 为零或大于 <span class="t-lc">RSIZE_MAX</span> ），而且可能以未指定值破坏目标数组的剩余部分。</div>
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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>按 C11 后的 DR 468 更正， <code>strncpy_s</code> 不同于 <span class="t-lc"><a href="c-string-byte-strcpy.html">strcpy_s</a></span> ，仅若错误发生才被允许破坏目标数组的剩余部分。</p>
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<p>不同于 <code>strncpy</code> ， <code>strncpy_s</code> 不以零填充目标数组。这是转换既存代码到边界检查版本的常见错误源。</p>
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<p>尽管适合目标缓冲区的截断是安全风险，从而是 <code>strncpy_s</code> 的运行时制约违规，还是可通过指定 <code>count</code> 等于目标数组大小减一以获取截断行为：它会复制首 <code>count</code> 个字节，并照常添加空终止符： <span class="t-c"><span class="mw-geshi c source-c">strncpy_s<span class="br0">(</span>dst, <span class="kw4">sizeof</span> dst, src, <span class="br0">(</span><span class="kw4">sizeof</span> dst<span class="br0">)</span><span class="sy2">-</span><span class="nu0">1</span><span class="br0">)</span><span class="sy4">;</span></span></span></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 <string.h></span>
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<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">char</span> src<span class="br0">[</span><span class="br0">]</span> <span class="sy1">=</span> <span class="st0">"hi"</span><span class="sy4">;</span>
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<span class="kw4">char</span> dest<span class="br0">[</span><span class="nu0">6</span><span class="br0">]</span> <span class="sy1">=</span> <span class="st0">"abcdef"</span><span class="sy4">;</span> <span class="co1">// 无空字符</span>
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strncpy<span class="br0">(</span>dest, src, <span class="nu0">5</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// 写入五个字符 'h', 'i', '\0', '\0', '\0' 到 dest</span>
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<a href="c-io-fprintf.html"><span class="kw850">printf</span></a><span class="br0">(</span><span class="st0">"strncpy(dest, src, 5) to a 6-byte dest gives : "</span><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> <span class="kw4">sizeof</span> dest<span class="sy4">;</span> <span class="sy2">++</span>n<span class="br0">)</span> <span class="br0">{</span>
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<span class="kw4">char</span> c <span class="sy1">=</span> dest<span class="br0">[</span>n<span class="br0">]</span><span class="sy4">;</span>
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c <span class="sy4">?</span> <a href="c-io-fprintf.html"><span class="kw850">printf</span></a><span class="br0">(</span><span class="st0">"'%c' "</span>, c<span class="br0">)</span> <span class="sy4">:</span> <a href="c-io-fprintf.html"><span class="kw850">printf</span></a><span class="br0">(</span><span class="st0">"'<span class="es1">\\</span>0' "</span><span class="br0">)</span><span class="sy4">;</span>
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<span class="br0">}</span>
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<a href="c-io-fprintf.html"><span class="kw850">printf</span></a><span class="br0">(</span><span class="st0">"<span class="es1">\n</span>strncpy(dest2, src, 2) to a 2-byte dst gives : "</span><span class="br0">)</span><span class="sy4">;</span>
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<span class="kw4">char</span> dest2<span class="br0">[</span><span class="nu0">2</span><span class="br0">]</span><span class="sy4">;</span>
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strncpy<span class="br0">(</span>dest2, src, <span class="nu0">2</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// 截断：写入二个字符 'h', 'i', 到 dest2</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> <span class="kw4">sizeof</span> dest2<span class="sy4">;</span> <span class="sy2">++</span>n<span class="br0">)</span> <span class="br0">{</span>
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<span class="kw4">char</span> c <span class="sy1">=</span> dest2<span class="br0">[</span>n<span class="br0">]</span><span class="sy4">;</span>
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c <span class="sy4">?</span> <a href="c-io-fprintf.html"><span class="kw850">printf</span></a><span class="br0">(</span><span class="st0">"'%c' "</span>, c<span class="br0">)</span> <span class="sy4">:</span> <a href="c-io-fprintf.html"><span class="kw850">printf</span></a><span class="br0">(</span><span class="st0">"'<span class="es1">\\</span>0' "</span><span class="br0">)</span><span class="sy4">;</span>
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<span class="br0">}</span>
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<a href="c-io-fprintf.html"><span class="kw850">printf</span></a><span class="br0">(</span><span class="st0">"<span class="es1">\n</span>"</span><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">char</span> dst1<span class="br0">[</span><span class="nu0">6</span><span class="br0">]</span>, src1<span class="br0">[</span><span class="nu0">100</span><span class="br0">]</span> <span class="sy1">=</span> <span class="st0">"hello"</span><span class="sy4">;</span>
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<span class="kw4">int</span> r1 <span class="sy1">=</span> strncpy_s<span class="br0">(</span>dst1, <span class="nu0">6</span>, src1, <span class="nu0">100</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// 写入 0 到 r1 ， 6 个字符到 dst1</span>
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<a href="c-io-fprintf.html"><span class="kw850">printf</span></a><span class="br0">(</span><span class="st0">"dst1 = <span class="es1">\"</span>%s<span class="es1">\"</span>, r1 = %d<span class="es1">\n</span>"</span>, dst1,r1<span class="br0">)</span><span class="sy4">;</span> <span class="co1">// 'h','e','l','l','o','\0' 到 dst1</span>
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<span class="kw4">char</span> dst2<span class="br0">[</span><span class="nu0">5</span><span class="br0">]</span>, src2<span class="br0">[</span><span class="nu0">7</span><span class="br0">]</span> <span class="sy1">=</span> <span class="br0">{</span><span class="st0">'g'</span>,<span class="st0">'o'</span>,<span class="st0">'o'</span>,<span class="st0">'d'</span>,<span class="st0">'b'</span>,<span class="st0">'y'</span>,<span class="st0">'e'</span><span class="br0">}</span><span class="sy4">;</span>
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<span class="kw4">int</span> r2 <span class="sy1">=</span> strncpy_s<span class="br0">(</span>dst2, <span class="nu0">5</span>, src2, <span class="nu0">7</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// 复制溢出目标数组</span>
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<a href="c-io-fprintf.html"><span class="kw850">printf</span></a><span class="br0">(</span><span class="st0">"dst2 = <span class="es1">\"</span>%s<span class="es1">\"</span>, r2 = %d<span class="es1">\n</span>"</span>, dst2,r2<span class="br0">)</span><span class="sy4">;</span> <span class="co1">// 写入非零到 r2 ， '\0' 到 dst2[0]</span>
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<span class="kw4">char</span> dst3<span class="br0">[</span><span class="nu0">5</span><span class="br0">]</span><span class="sy4">;</span>
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<span class="kw4">int</span> r3 <span class="sy1">=</span> strncpy_s<span class="br0">(</span>dst3, <span class="nu0">5</span>, src2, <span class="nu0">4</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// 写入 0 到 r3 ， 5 个字符到 dst3</span>
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<a href="c-io-fprintf.html"><span class="kw850">printf</span></a><span class="br0">(</span><span class="st0">"dst3 = <span class="es1">\"</span>%s<span class="es1">\"</span>, r3 = %d<span class="es1">\n</span>"</span>, dst3,r3<span class="br0">)</span><span class="sy4">;</span> <span class="co1">// 'g', 'o', 'o', 'd', '\0' 到 dst3 </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">strncpy(dest, src, 5) to a 6-byte dst gives : 'h' 'i' '\0' '\0' '\0' 'f'
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strncpy(dest2, src, 2) to a 2-byte dst gives : 'h' 'i'
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dst1 = "hello", r1 = 0
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dst2 = "", r2 = 22
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dst3 = "good", r3 = 0</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>7.24.2.4 The strncpy function (p: 363-364)</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.3.7.1.4 The strncpy_s function (p: 616-617)</li>
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|
</ul>
|
|
</dd>
|
|
</dl>
|
|
<div class="t-ref-std-c99">
|
|
<ul>
|
|
<li>C99 标准（ISO/IEC 9899:1999）：</li>
|
|
</ul>
|
|
<dl>
|
|
<dd>
|
|
<ul>
|
|
<li>7.21.2.4 The strncpy function (p: 326-327)</li>
|
|
</ul>
|
|
</dd>
|
|
</dl>
|
|
<div class="t-ref-std-c89">
|
|
<ul>
|
|
<li>C89/C90 标准（ISO/IEC 9899:1990）：</li>
|
|
</ul>
|
|
<dl>
|
|
<dd>
|
|
<ul>
|
|
<li>4.11.2.4 The strncpy function</li>
|
|
</ul>
|
|
</dd>
|
|
</dl>
|
|
</div>
|
|
<h3><span class="mw-headline" id=".E5.8F.82.E9.98.85">参阅</span></h3>
|
|
<table class="t-dsc-begin">
|
|
<tr class="t-dsc">
|
|
<td>
|
|
<div class="t-dsc-member-div">
|
|
<div><a href="c-string-byte-strcpy.html"><span class="t-lines"><span>strcpy</span><span>strcpy_s</span></span></a></div>
|
|
<div><span class="t-lines"><span></span><span><span class="t-mark-rev t-since-c11">(C11)</span></span></span></div>
|
|
</div>
|
|
</td>
|
|
<td>复制一个字符串给另一个<br />
|
|
<span class="t-mark">(函数)</span></td>
|
|
</tr>
|
|
<tr class="t-dsc">
|
|
<td>
|
|
<div class="t-dsc-member-div">
|
|
<div><a href="c-string-byte-memcpy.html"><span class="t-lines"><span>memcpy</span><span>memcpy_s</span></span></a></div>
|
|
<div><span class="t-lines"><span></span><span><span class="t-mark-rev t-since-c11">(C11)</span></span></span></div>
|
|
</div>
|
|
</td>
|
|
<td>将一个缓冲区复制到另一个<br />
|
|
<span class="t-mark">(函数)</span></td>
|
|
</tr>
|
|
<tr class="t-dsc">
|
|
<td>
|
|
<div class="t-dsc-member-div">
|
|
<div><a href="c-experimental-dynamic-strndup.html"><span class="t-lines"><span>strndup</span></span></a></div>
|
|
<div><span class="t-lines"><span><span class="t-mark-rev t-since-dynamic-tr t-mark-ts">(动态内存 TR)</span></span></span></div>
|
|
</div>
|
|
</td>
|
|
<td>分配字符串副本，至多到指定的大小<br />
|
|
<span class="t-mark">(函数)</span></td>
|
|
</tr>
|
|
</table>
|
|
</div>
|
|
</div>
|
|
</div>
|
|
<div class="visualClear"></div>
|
|
</div>
|
|
</div>
|
|
</div>
|
|
</body>
|
|
</html>
|