275 lines
18 KiB
HTML
275 lines
18 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>freopen, freopen_s</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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</head>
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<body class="mediawiki ltr sitedir-ltr ns-0 ns-subject page-c_io_freopen 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">freopen, freopen_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-io.html">io</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><stdio.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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</tbody>
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<tbody class="t-dcl-rev t-dcl-rev-num">
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<tr class="t-dcl-rev-aux">
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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>
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<tr class="t-dcl t-until-c99">
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<td>
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<div><span class="mw-geshi c source-c"><a href="c-io.html"><span class="kw883">FILE</span></a> <span class="sy2">*</span>freopen<span class="br0">(</span> <span class="kw4">const</span> <span class="kw4">char</span> <span class="sy2">*</span>filename, <span class="kw4">const</span> <span class="kw4">char</span> <span class="sy2">*</span>mode,<br />
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<a href="c-io.html"><span class="kw883">FILE</span></a> <span class="sy2">*</span>stream <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>
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<tr class="t-dcl t-since-c99">
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<td>
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<div><span class="mw-geshi c source-c"><a href="c-io.html"><span class="kw883">FILE</span></a> <span class="sy2">*</span>freopen<span class="br0">(</span> <span class="kw4">const</span> <span class="kw4">char</span> <span class="sy2">*</span><span class="kw4">restrict</span> filename, <span class="kw4">const</span> <span class="kw4">char</span> <span class="sy2">*</span><span class="kw4">restrict</span> mode,<br />
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<a href="c-io.html"><span class="kw883">FILE</span></a> <span class="sy2">*</span><span class="kw4">restrict</span> stream <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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</tr>
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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 freopen_s<span class="br0">(</span><a href="c-io.html"><span class="kw883">FILE</span></a> <span class="sy2">*</span><span class="kw4">restrict</span> <span class="sy2">*</span><span class="kw4">restrict</span> newstreamptr,<br /></span>
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<p> <span class="kw4">const</span> <span class="kw4">char</span> <span class="sy2">*</span><span class="kw4">restrict</span> filename, <span class="kw4">const</span> <span class="kw4">char</span> <span class="sy2">*</span><span class="kw4">restrict</span> mode,<br /></p>
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<a href="c-io.html"><span class="kw883">FILE</span></a> <span class="sy2">*</span><span class="kw4">restrict</span> stream<span class="br0">)</span><span class="sy4">;</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>
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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> 首先，试图关闭与 <code>stream</code> 关联的文件，忽略任何错误。然后，若 <code>filename</code> 非空，则试图用 <code>mode</code> 打开 <code>filename</code> 所指定的文件，如同用 <span class="t-lc"><a href="c-io-fopen.html">fopen</a></span> ，然后将该文件与 <code>stream</code> 所指向的文件流关联。若 <code>filename</code> 为空指针，则函数试图重打开已与 <code>stream</code> 关联的文件（此情况下是否允许模式改变是实现定义的）。</div>
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<div class="t-li1"><span class="t-li">2)</span> 同 <span class="t-v">(1)</span> ，除了以 <span class="t-lc">fopen_s</span> 中的方式处理 <code>mode</code> ，并将指向文件流的指针写入 <code>newstreamptr</code> ，还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>newstreamptr</code> 是空指针</li>
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<li><code>stream</code> 是空指针</li>
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<li><code>mode</code> 是空指针</li>
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</ul>
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</dd>
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<dd></dd>
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<dd>同所有边界检查函数， <code>freopen_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><stdio.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>filename</td>
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<td>-</td>
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<td>要关联到文件流的文件名</td>
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</tr>
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<tr class="t-par">
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<td>mode</td>
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<td>-</td>
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<td>确定文件访问模式的空终止字符串
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<table class="wikitable">
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<tr>
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<th>文件访问<br />
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模式字符串</th>
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<th>含义</th>
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<th>解释</th>
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<th>若文件已存在的动作</th>
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<th>若文件不存在的动作</th>
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</tr>
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<tr>
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<td><code>"r"</code></td>
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<td>读</td>
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<td>打开文件以读取</td>
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<td>从头读</td>
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<td>打开失败</td>
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</tr>
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<tr>
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<td><code>"w"</code></td>
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<td>写</td>
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<td>创建文件以写入</td>
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<td>销毁内容</td>
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<td>创建新文件</td>
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</tr>
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<tr>
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<td><code>"a"</code></td>
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<td>后附</td>
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<td>后附到文件</td>
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<td>写到结尾</td>
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<td>创建新文件</td>
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</tr>
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<tr>
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<td><code>"r+"</code></td>
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<td>读扩展</td>
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<td>打开文件以读/写</td>
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<td>从头读</td>
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<td>错误</td>
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</tr>
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<tr>
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<td><code>"w+"</code></td>
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<td>写扩展</td>
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<td>创建文件以读/写</td>
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<td>销毁内容</td>
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<td>创建新文件</td>
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</tr>
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<tr>
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<td><code>"a+"</code></td>
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<td>后附扩展</td>
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<td>打开文件以读/写</td>
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<td>写到结尾</td>
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<td>创建新文件</td>
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</tr>
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<tr>
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<td>文件访问模式标签 <code>"b"</code> 可以可选地指定以二进制模式打开文件。此标签仅在 Windows 系统上生效。<br />
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在附加文件访问模式下，数据被写入到文件尾，而不考虑文件位置指示器的当前位置。</td>
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</tr>
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<tr>
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<td>当文件以更新模式打开时（ '+' 作为上述模式参数值列表的第二或第三个字符），访问流可以进行输入和输出。然而输出不应直接紧随输入，而中间无对 <span class="t-lc"><a href="c-io-fflush.html">fflush</a></span> 函数或对文件定位函数（ <span class="t-lc"><a href="c-io-fseek.html">fseek</a></span> 、 <span class="t-lc"><a href="c-io-fsetpos.html">fsetpos</a></span> 或 <span class="t-lc"><a href="c-io-rewind.html">rewind</a></span> ）的调用，且输入不应直接紧随输出，而中间无对文件定位函数的调用，除非输入操作遇到文件尾。在一些实现中，以更新模式打开（或创建）文本文件可能取代打开（或创建）二进制流。</td>
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</tr>
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<tr>
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<td>文件访问模式标签 <code>"x"</code> 可以可选地后附于 "w" 或 "w+" 指定符。若文件存在，则此标签强制函数失败，而不重写它。<span class="t-mark-rev t-since-c11">(C11)</span></td>
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</tr>
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<tr>
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<td>使用 <code>fopen_s</code> 或 <code>freopen_s</code> 时，任何以 "w" 或 "a" 创建的文件的文件访问许可禁止其他用户访问它。文件访问模式标签 <code>"u"</code> 可以可选地前附于任何以为 "w" 或 "a" 始的指定符，以启用默认的 <code>fopen</code> 许可。<span class="t-mark-rev t-since-c11">(C11)</span></td>
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</tr>
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</table>
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</td>
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</tr>
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<tr class="t-par">
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<td>stream</td>
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<td>-</td>
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<td>要修改的文件流</td>
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</tr>
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<tr class="t-par">
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<td>newstreamptr</td>
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<td>-</td>
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<td>指向函数存储结果所用指针的指针（输出参数）</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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<div class="t-li1"><span class="t-li">1)</span> 成功时为 <code>stream</code> 值的副本，失败时为空指针。</div>
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<div class="t-li1"><span class="t-li">2)</span> 成功时为零（并将 <code>stream</code> 值的副本写入 <code>*newstreamptr</code> ），失败时为非零（并写入空指针到 <code>*newstreamptr</code> ，除非 <code>newstreamptr</code> 自身为空指针）。</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><code>freopen</code> 是一旦由 I/O 操作或 <span class="t-lc">fwide</span> 建立面向后，改变文件流窄/宽面向的唯一方式。</p>
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<p><code>freopen</code> 的 Microsoft CRT 版本在 <code>filename</code> 为空指针时不支持任何模式更改并将它当作错误（见<a rel="nofollow" class="external text" href="https://docs.microsoft.com/en-us/cpp/c-runtime-library/reference/freopen-wfreopen">文档</a>）。可行的替代方案是非标准函数 <a rel="nofollow" class="external text" href="https://docs.microsoft.com/en-us/cpp/c-runtime-library/reference/setmode"><code>_setmode()</code></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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<p>下列代码重定向 <code>stdout</code> 到文件。</p>
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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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<a href="c-io-puts.html"><span class="kw835">puts</span></a><span class="br0">(</span><span class="st0">"stdout is printed to console"</span><span class="br0">)</span><span class="sy4">;</span>
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<span class="kw1">if</span> <span class="br0">(</span>freopen<span class="br0">(</span><span class="st0">"redir.txt"</span>, <span class="st0">"w"</span>, <a href="c-io.html"><span class="kw886">stdout</span></a><span class="br0">)</span> <span class="sy1">==</span> <a href="c-types-NULL.html"><span class="kw103">NULL</span></a><span class="br0">)</span>
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<span class="br0">{</span>
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<a href="c-io-perror.html"><span class="kw878">perror</span></a><span class="br0">(</span><span class="st0">"freopen() failed"</span><span class="br0">)</span><span class="sy4">;</span>
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<span class="kw1">return</span> <a href="c-program-EXIT_status.html"><span class="kw494">EXIT_FAILURE</span></a><span class="sy4">;</span>
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<span class="br0">}</span>
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<a href="c-io-puts.html"><span class="kw835">puts</span></a><span class="br0">(</span><span class="st0">"stdout is redirected to a file"</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// 写入 redir.txt</span>
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<a href="c-io-fclose.html"><span class="kw821">fclose</span></a><span class="br0">(</span><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="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">stdout is printed to console</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>7.21.5.4 The freopen function (p: 307)</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.3.5.2.2 The freopen_s function (p: 590)</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>7.19.5.4 The freopen function (p: 272-273)</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>4.9.5.4 The freopen 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-io-fopen.html"><span class="t-lines"><span>fopen</span><span>fopen_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>打开文件<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-io-fclose.html"><span class="t-lines"><span>fclose</span></span></a></div>
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</div>
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</td>
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<td>关闭文件<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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