164 lines
12 KiB
HTML
164 lines
12 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>ungetc</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_ungetc 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">ungetc</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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<tr class="t-dsc-header">
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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> ungetc<span class="br0">(</span> <span class="kw4">int</span> ch, <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 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>若 <code>ch</code> 不等于 <span class="t-lc"><a href="c-io.html">EOF</a></span> ，则推入字符 <code>ch</code> （转译为 <span class="t-c"><span class="mw-geshi c source-c"><span class="kw4">unsigned</span> <span class="kw4">char</span></span></span> ）到与流 <code>stream</code> 关联的输入缓冲区，方式满足从 <code>stream</code> 的后继读取操作将取得该字符。不修改与流关联的外部设备。</p>
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<p>流重寻位操作 <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> 和 <span class="t-lc"><a href="c-io-rewind.html">rewind</a></span> 弃去 <code>ungetc</code> 的效果。</p>
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<p>若调用 <code>ungetc</code> 多于一次，而无中间读取或重寻位，则可能失败（换言之，保证大小为 1 的回放缓冲区，但任何更大的缓冲区是实现定义的）。若成功进行多次 <code>ungetc</code> ，则读取操作以 <code>ungetc</code> 的逆序取得回放的字符。</p>
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<p>若 <code>ch</code> 等于 <span class="t-lc"><a href="c-io.html">EOF</a></span> ，则操作失败而不影响流。</p>
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<p>对 <code>ungetc</code> 的成功调用清除文件尾状态标志 <span class="t-lc"><a href="c-io-feof.html">feof</a></span> 。</p>
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<p>在二进制流上对 <code>ungetc</code> 的成功调用将流位置指示器减少一（若流位置指示器为零，则行为不确定）。</p>
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<p>在文本流上对 <code>ungetc</code> 的成功调用以未指定方式修改流位置指示器，但保证在以读取操作取得所有回放字符后，流位置指示器等于其在 <code>ungetc</code> 之前的值。</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>ch</td>
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<td>-</td>
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<td>要推入输入流缓冲区的字符</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>要回放字符到的文件流</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>成功时返回 <code>ch</code> 。</p>
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<p>失败时返回 <span class="t-lc"><a href="c-io.html">EOF</a></span> ，而给定的流保持不变。</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>实践中，回放缓冲区的大小会在 4k （ Linux 、 MacOS ）和 4 （ Solaris ）或保证的最小值 1 （ HPUX 、 AIX ）间变化。</p>
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<p>若回放的字符等于存在于外部字符序列中该位置的字符，则回放缓冲区的表观大小可以更大（实现可以简单地自减读取的文件位置指示器，并避免维护回放缓冲区）。</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>展示 ungetc 的原目的：实现 scanf</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 <ctype.h></span>
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<span class="co2">#include <stdio.h></span>
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<span class="kw4">void</span> demo_scanf<span class="br0">(</span><span class="kw4">const</span> <span class="kw4">char</span><span class="sy2">*</span> fmt, <a href="c-io.html"><span class="kw883">FILE</span></a><span class="sy2">*</span> s<span class="br0">)</span> <span class="br0">{</span>
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<span class="kw1">if</span><span class="br0">(</span><span class="sy2">*</span>fmt <span class="sy1">==</span> <span class="st0">'%'</span><span class="br0">)</span> <span class="br0">{</span>
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<span class="kw4">int</span> c<span class="sy4">;</span>
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<span class="kw1">switch</span><span class="br0">(</span><span class="sy2">*++</span>fmt<span class="br0">)</span> <span class="br0">{</span>
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<span class="kw1">case</span> <span class="st0">'u'</span><span class="sy4">:</span> <span class="kw1">while</span><span class="br0">(</span><a href="c-string-byte-isspace.html"><span class="kw534">isspace</span></a><span class="br0">(</span>c<span class="sy1">=</span><a href="c-io-fgetc.html"><span class="kw828">getc</span></a><span class="br0">(</span>s<span class="br0">)</span><span class="br0">)</span><span class="br0">)</span> <span class="br0">{</span><span class="br0">}</span> <span class="co1">// 跳过空白符</span>
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<span class="kw4">unsigned</span> <span class="kw4">int</span> num <span class="sy1">=</span> <span class="nu0">0</span><span class="sy4">;</span>
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<span class="kw1">while</span><span class="br0">(</span><a href="c-string-byte-isdigit.html"><span class="kw530">isdigit</span></a><span class="br0">(</span>c<span class="br0">)</span><span class="br0">)</span> <span class="br0">{</span>
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num <span class="sy1">=</span> num<span class="sy2">*</span><span class="nu0">10</span> <span class="sy2">+</span> c<span class="sy2">-</span><span class="st0">'0'</span><span class="sy4">;</span>
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c <span class="sy1">=</span> <a href="c-io-fgetc.html"><span class="kw828">getc</span></a><span class="br0">(</span>s<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">"%%u scanned %u<span class="es1">\n</span>"</span>, num<span class="br0">)</span><span class="sy4">;</span>
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ungetc<span class="br0">(</span>c, s<span class="br0">)</span><span class="sy4">;</span> <span class="co1">// 重处理非数字</span>
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<span class="kw1">case</span> <span class="st0">'c'</span><span class="sy4">:</span> c <span class="sy1">=</span> <a href="c-io-fgetc.html"><span class="kw828">getc</span></a><span class="br0">(</span>s<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">"%%c scanned '%c'<span class="es1">\n</span>"</span>, c<span class="br0">)</span><span class="sy4">;</span>
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<span class="br0">}</span>
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<span class="br0">}</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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<a href="c-io.html"><span class="kw883">FILE</span></a><span class="sy2">*</span> f <span class="sy1">=</span> <a href="c-io-fopen.html"><span class="kw819">fopen</span></a><span class="br0">(</span><span class="st0">"input.txt"</span>, <span class="st0">"w+"</span><span class="br0">)</span><span class="sy4">;</span>
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<a href="c-io-fputs.html"><span class="kw832">fputs</span></a><span class="br0">(</span><span class="st0">"123x"</span>, f<span class="br0">)</span><span class="sy4">;</span>
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<a href="c-io-rewind.html"><span class="kw874">rewind</span></a><span class="br0">(</span>f<span class="br0">)</span><span class="sy4">;</span>
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demo_scanf<span class="br0">(</span><span class="st0">"%u%c"</span>, f<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>f<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">%u scanned 123
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%c scanned 'x'</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>7.21.7.10 The ungetc function (p: 334)</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>7.19.7.11 The ungetc function (p: 300)</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>4.9.7.11 The ungetc 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-fgetc.html"><span class="t-lines"><span>fgetc</span><span>getc</span></span></a></div>
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</div>
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</td>
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<td>从文件流获取一个字符<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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