152 lines
13 KiB
HTML
152 lines
13 KiB
HTML
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<title>std::fseek</title>
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<body class="mediawiki ltr sitedir-ltr ns-0 ns-subject page-cpp_io_c_fseek skin-cppreference2 action-view cpp-navbar">
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<h1 id="firstHeading" class="firstHeading"><span style="font-size:0.7em; line-height:130%">std::</span>fseek</h1>
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<div id="contentSub"><span class="subpages">< <a href="cpp.html">cpp</a>‎ | <a href="cpp-io.html">io</a>‎ | <a href="cpp-io-c.html">c</a></span></div>
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<div>定义于头文件 <code><a href="cpp-header-cstdio.html"><cstdio></a></code></div>
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<div><span class="mw-geshi cpp source-cpp"><span class="kw4">int</span> fseek<span class="br0">(</span> <a href="cpp-io-c.html"><span class="kw1876">std::<span class="me2">FILE</span></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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<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="cpp-io-c.html">SEEK_SET</a></span> ）或当前文件位置后（若 <code>origin</code> 为 <span class="t-lc"><a href="cpp-io-c.html">SEEK_CUR</a></span> ），或文件结尾后（若 <code>origin</code> 为 <span class="t-lc"><a href="cpp-io-c.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="cpp-io-c.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="cpp-io-c-ftell.html">std::ftell</a></span> 的调用的返回值（仅可用于 <span class="t-lc"><a href="cpp-io-c.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="cpp-io-c-ftell.html">std::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="cpp-io-c-ungetc.html">std::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="cpp-io-c-ferror.html">std::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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<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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<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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<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="cpp-io-c.html">SEEK_SET</a></span> 、 <span class="t-lc"><a href="cpp-io-c.html">SEEK_CUR</a></span> 、 <span class="t-lc"><a href="cpp-io-c.html">SEEK_END</a></span></td>
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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 cpp source-cpp">​<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.E6.84.8F">注意</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 cpp source-cpp">​<span class="nu0">0</span>​</span></span> （可应用于任意 <code>origin</code> ）和先前 <span class="t-lc">ftell</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">fflush</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）。标准 C++ 文件流保证冲入并反迁移： <span class="t-lc"><a href="cpp-io-basic_streambuf-pubseekoff.html">std::basic_filebuf::seekoff</a></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 dir="ltr" class="mw-geshi" style="text-align: left;">
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<div class="cpp source-cpp">
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<pre class="de1"><span class="co2">#include <cstdio></span>
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<span class="co2">#include <cstdint></span>
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<span class="co2">#include <vector></span>
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<span class="co2">#include <fstream></span>
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<span class="co2">#include <cassert></span>
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<span class="kw4">int</span> main<span class="br0">(</span><span class="br0">)</span>
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<span class="br0">{</span>
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<a href="cpp-io-basic_ofstream.html"><span class="kw1733">std::<span class="me2">ofstream</span></span></a><span class="br0">(</span><span class="st0">"dummy.nfo"</span><span class="br0">)</span> <span class="sy1"><<</span> <span class="st0">"sample data<span class="es1">\n</span>"</span><span class="sy4">;</span>
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<a href="cpp-io-c.html"><span class="kw1876">std::<span class="me2">FILE</span></span></a><span class="sy2">*</span> fp <span class="sy1">=</span> <a href="cpp-io-c-fopen.html"><span class="kw1813">std::<span class="me2">fopen</span></span></a><span class="br0">(</span><span class="st0">"dummy.nfo"</span>, <span class="st0">"rb"</span><span class="br0">)</span><span class="sy4">;</span>
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<a href="cpp-error-assert.html"><span class="kw774">assert</span></a><span class="br0">(</span>fp<span class="br0">)</span><span class="sy4">;</span>
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std<span class="sy4">::</span><span class="me2">fseek</span><span class="br0">(</span>fp, <span class="nu0">0</span>, <a href="cpp-io-c.html"><span class="kw1890">SEEK_END</span></a><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// 寻位到结尾</span>
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<a href="cpp-types-size_t.html"><span class="kw100">std::<span class="me2">size_t</span></span></a> filesize <span class="sy1">=</span> <a href="cpp-io-c-ftell.html"><span class="kw1865">std::<span class="me2">ftell</span></span></a><span class="br0">(</span>fp<span class="br0">)</span><span class="sy4">;</span>
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std<span class="sy4">::</span><span class="me2">fseek</span><span class="br0">(</span>fp, <span class="nu0">0</span>, <a href="cpp-io-c.html"><span class="kw1888">SEEK_SET</span></a><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// 寻位到起始</span>
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<a href="cpp-container-vector.html"><span class="kw1269">std::<span class="me2">vector</span></span></a><span class="sy1"><</span>uint8_t<span class="sy1">></span> buffer<span class="br0">(</span>filesize<span class="br0">)</span><span class="sy4">;</span>
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<a href="cpp-io-c-fread.html"><span class="kw1819">std::<span class="me2">fread</span></span></a><span class="br0">(</span>buffer.<span class="me1">data</span><span class="br0">(</span><span class="br0">)</span>, sizeof<span class="br0">(</span>uint8_t<span class="br0">)</span>, buffer.<span class="me1">size</span><span class="br0">(</span><span class="br0">)</span>, fp<span class="br0">)</span><span class="sy4">;</span>
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<a href="cpp-io-c-fclose.html"><span class="kw1816">std::<span class="me2">fclose</span></span></a><span class="br0">(</span>fp<span class="br0">)</span><span class="sy4">;</span>
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<a href="cpp-io-c-fprintf.html"><span class="kw1845">std::<span class="me2">printf</span></span></a><span class="br0">(</span><span class="st0">"i've read %zi bytes<span class="es1">\n</span>"</span>, filesize<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">i've read 12 bytes</pre></div>
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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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<div class="t-dsc-member-div">
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<div><a href="cpp-io-c-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>移动文件位置指示器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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<div class="t-dsc-member-div">
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<div><a href="cpp-io-c-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 class="t-dsc">
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<div class="t-dsc-member-div">
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<div><a href="cpp-io-c-ftell.html"><span class="t-lines"><span>ftell</span></span></a></div>
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<td>返回当前文件位置指示器<br />
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<span class="t-mark">(函数)</span></td>
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<div class="t-dsc-member-div">
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<div><a href="cpp-io-c-rewind.html"><span class="t-lines"><span>rewind</span></span></a></div>
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</td>
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<td>移动文件位置指示器到文件起始<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 class="visualClear"></div>
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