197 lines
15 KiB
HTML
197 lines
15 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>fseek</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_fseek 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">fseek</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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<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> fseek<span class="br0">(</span> <a href="c-io.html"><span class="kw883">FILE</span></a> <span class="sy2">*</span>stream, <span class="kw4">long</span> offset, <span class="kw4">int</span> origin <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>设置文件流 <code>stream</code> 的文件位置指示器为 <code>offset</code> 所指向的值。</p>
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<p>若 <code>stream</code> 以二进制模式打开，则新位置准确地是文件起始后（若 <code>origin</code> 为 <span class="t-lc"><a href="c-io.html">SEEK_SET</a></span> ）或当前文件位置后（若 <code>origin</code> 为 <span class="t-lc"><a href="c-io.html">SEEK_CUR</a></span> ），或文件结尾后（若 <code>origin</code> 为 <span class="t-lc"><a href="c-io.html">SEEK_END</a></span> ）的 <code>offset</code> 字节。不要求二进制流支持 <span class="t-lc"><a href="c-io.html">SEEK_END</a></span> ，尤其是是否输出附加的空字节。</p>
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<p>若 <code>stream</code> 以文本模式打开，则仅有的受支持 <code>offset</code> 值为零（可用于任何 <code>origin</code> ）和先前在关联到同一个文件的流上对 <span class="t-lc"><a href="c-io-ftell.html">ftell</a></span> 的调用的返回值（仅可用于 <span class="t-lc"><a href="c-io.html">SEEK_SET</a></span> 的 <code>origin</code> ）。</p>
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<p>若 <code>stream</code> 为宽面向，则一同应用对文本和二进制流的限制（允许 <span class="t-lc"><a href="c-io-ftell.html">ftell</a></span> 的结果与 SEEK_SET 一同使用，并允许零 offset 以 SEEK_SET 和 SEEK_CUR 但非 SEEK_END 为基准）。</p>
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<p>除了更改文件位置指示器， <code>fseek</code> 还撤销 <span class="t-lc"><a href="c-io-ungetc.html">ungetc</a></span> 的效果并清除文件尾状态，若可应用。</p>
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<p>若发生读或写错误，则设置流的错误指示器（ <span class="t-lc"><a href="c-io-ferror.html">ferror</a></span> ）而不影响文件位置。</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>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>offset</td>
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<td>-</td>
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<td>相对 origin 迁移的字符数</td>
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</tr>
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<tr class="t-par">
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<td>origin</td>
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<td>-</td>
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<td><code>offset</code> 所加上的位置。它能拥有下列值之一： <span class="t-lc"><a href="c-io.html">SEEK_SET</a></span> 、 <span class="t-lc"><a href="c-io.html">SEEK_CUR</a></span> 、 <span class="t-lc"><a href="c-io.html">SEEK_END</a></span></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>成功时为 <span class="t-c"><span class="mw-geshi c source-c">​<span class="nu0">0</span>​</span></span> ，否则为非零。</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>在巡位到宽流的非结尾位置后，下个对任意输出函数的调用可能令剩下的文件内容未定义，例如通过输出一个长度不同的多字节序列。</p>
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<p>对于文本流， <code>offset</code> 仅有的合法值是 <span class="t-c"><span class="mw-geshi c source-c">​<span class="nu0">0</span>​</span></span> （可应用于任意 <code>origin</code> ）和先前 <span class="t-lc"><a href="c-io-ftell.html">ftell</a></span> 调用的返回值（仅可应用于 <code>SEEK_SET</code> ）。</p>
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<p>POSIX 允许在文件尾之后巡位。若在此巡位后进行输出，则任何间隙中的读取将返回零字节。在文件系统支持的场合，这会创建一个<i>稀疏文件</i>。</p>
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<p>POSIX 亦要求 fseek 先进行 <span class="t-lc"><a href="c-io-fflush.html">fflush</a></span> ，若有任何未写入数据（但是否恢复迁移状态是实现定义的）。</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>fseek 带错误检查</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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<span class="co1">// 准备浮点值的数组。</span>
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<span class="co2">#define SIZE 5</span>
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<span class="kw4">double</span> A<span class="br0">[</span>SIZE<span class="br0">]</span> <span class="sy1">=</span> <span class="br0">{</span><span class="nu0">1</span>.,<span class="nu0">2</span>.,<span class="nu0">3</span>.,<span class="nu0">4</span>.,<span class="nu0">5</span>.<span class="br0">}</span><span class="sy4">;</span>
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<span class="co1">// 写入数组到文件。</span>
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<a href="c-io.html"><span class="kw883">FILE</span></a> <span class="sy2">*</span> fp <span class="sy1">=</span> <a href="c-io-fopen.html"><span class="kw819">fopen</span></a><span class="br0">(</span><span class="st0">"test.bin"</span>, <span class="st0">"wb"</span><span class="br0">)</span><span class="sy4">;</span>
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<a href="c-io-fwrite.html"><span class="kw826">fwrite</span></a><span class="br0">(</span>A,<span class="kw4">sizeof</span><span class="br0">(</span><span class="kw4">double</span><span class="br0">)</span>,SIZE,fp<span class="br0">)</span><span class="sy4">;</span>
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<a href="c-io-fclose.html"><span class="kw822">fclose</span></a> <span class="br0">(</span>fp<span class="br0">)</span><span class="sy4">;</span>
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<span class="co1">// 读浮点值入数组 B。</span>
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<span class="kw4">double</span> B<span class="br0">[</span>SIZE<span class="br0">]</span><span class="sy4">;</span>
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fp <span class="sy1">=</span> <a href="c-io-fopen.html"><span class="kw819">fopen</span></a><span class="br0">(</span><span class="st0">"test.bin"</span>,<span class="st0">"rb"</span><span class="br0">)</span><span class="sy4">;</span>
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<span class="co1">// 设置文件位置指示器到第三个浮点值之前。</span>
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<span class="kw1">if</span> <span class="br0">(</span>fseek<span class="br0">(</span>fp,<span class="kw4">sizeof</span><span class="br0">(</span><span class="kw4">double</span><span class="br0">)</span><span class="sy2">*</span><span class="nu0">2L</span>,<a href="c-io.html"><span class="kw895">SEEK_SET</span></a><span class="br0">)</span> <span class="sy3">!</span><span class="sy1">=</span> <span class="nu0">0</span><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">"fseek()"</span><span class="br0">)</span><span class="sy4">;</span>
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<a href="c-io-fprintf.html"><span class="kw852">fprintf</span></a><span class="br0">(</span><a href="c-io.html"><span class="kw887">stderr</span></a>,<span class="st0">"fseek() failed in file %s at line # %d<span class="es1">\n</span>"</span>, __FILE__,__LINE__<span class="sy2">-</span><span class="nu0">5</span><span class="br0">)</span><span class="sy4">;</span>
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<a href="c-program-exit.html"><span class="kw488">exit</span></a><span class="br0">(</span><a href="c-program-EXIT_status.html"><span class="kw494">EXIT_FAILURE</span></a><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> ret_code <span class="sy1">=</span> <a href="c-io-fread.html"><span class="kw825">fread</span></a><span class="br0">(</span>B,<span class="kw4">sizeof</span><span class="br0">(</span><span class="kw4">double</span><span class="br0">)</span>,<span class="nu0">1</span>,fp<span class="br0">)</span><span class="sy4">;</span> <span class="co1">// 读取一个浮点值</span>
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<a href="c-io-fprintf.html"><span class="kw851">printf</span></a><span class="br0">(</span><span class="st0">"%.1f<span class="es1">\n</span>"</span>, B<span class="br0">[</span><span class="nu0">0</span><span class="br0">]</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// 打印一个浮点值</span>
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<a href="c-io-fclose.html"><span class="kw822">fclose</span></a><span class="br0">(</span>fp<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="kw493">EXIT_SUCCESS</span></a><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">3.0</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>7.21.9.2 The fseek function (p: 336-337)</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>7.19.9.2 The fseek function (p: 302-303)</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>4.9.9.2 The fseek 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-fsetpos.html"><span class="t-lines"><span>fsetpos</span></span></a></div>
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</div>
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</td>
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<td>将文件位置指示器移动到文件中的指定位置<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-fgetpos.html"><span class="t-lines"><span>fgetpos</span></span></a></div>
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</div>
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</td>
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<td>获取文件位置指示器<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-ftell.html"><span class="t-lines"><span>ftell</span></span></a></div>
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</div>
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</td>
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<td>返回当前的文件位置指示值<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-rewind.html"><span class="t-lines"><span>rewind</span></span></a></div>
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</div>
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</td>
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<td>将文件位置指示器移动到文件首<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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