168 lines
14 KiB
HTML
168 lines
14 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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<title>std::setvbuf</title>
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<body class="mediawiki ltr sitedir-ltr ns-0 ns-subject page-cpp_io_c_setvbuf 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>setvbuf</h1>
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<div id="bodyContent">
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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 id="mw-content-text" lang="zh" dir="ltr" class="mw-content-ltr" xml:lang="zh">
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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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<td></td>
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<div><span class="mw-geshi cpp source-cpp"><span class="kw4">int</span> setvbuf<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">char</span><span class="sy2">*</span> buffer, <span class="kw4">int</span> mode, <a href="cpp-types-size_t.html"><span class="kw100">std::<span class="me2">size_t</span></span></a> size <span class="br0">)</span><span class="sy4">;</span></span></div>
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<td class="t-dcl-nopad"></td>
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<td></td>
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<td></td>
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<td></td>
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</table>
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<p>以 <code>mode</code> 所指示值更改给定文件流 <code>stream</code> 的缓冲模式。另外，</p>
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<ul>
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<li>若 <code>buffer</code> 为空指针，则重设内部缓冲区大小为 <code>size</code> 。</li>
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<li>若 <code>buffer</code> 不是空指针，则指示流使用始于 <code>buffer</code> 而大小为 <code>size</code> 的用户提供缓冲区。必须在 <code>buffer</code> 所指向的数组的<a href="cpp-language-lifetime.html">生存期</a>结束前（用 <span class="t-lc">fclose</span> ）关闭流。成功调用 <code>setvbuf</code> 后，数组内容不确定，而任何使用它的尝试是未定义行为。</li>
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</ul>
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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>要设置缓冲的文件流</td>
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</tr>
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<tr class="t-par">
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<td>buffer</td>
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<td>-</td>
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<td>指向要使用的流缓冲区的指针，或若仅更改大小和模式则为空指针</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>使用的缓冲模式。它能是下列值之一：
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<table class="wikitable">
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<td><code>_IOFBF</code></td>
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<td>全缓冲：当缓冲区为空时，从流读入数据。或者当缓冲区满时，向流写入数据。</td>
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</tr>
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<tr>
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<td><code>_IOLBF</code></td>
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<td>行缓冲：每次从流中读入一行数据或向流中写入一行数据。</td>
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<td><code>_IONBF</code></td>
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<td>无缓冲：直接从流中读入数据或直接向流中写入数据，缓冲设置无效。</td>
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</table>
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</td>
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<tr class="t-par">
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<td>size</td>
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<td>-</td>
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<td>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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<p>成功时为 <span class="t-c"><span class="mw-geshi cpp source-cpp">​<span class="nu0">0</span>​</span></span> ，失败时为非零。</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>此函数仅可在已将 <code>stream</code> 关联到打开的文件后，但要在任何其他操作（除了对 <span class="t-lc"><a href="cpp-io-c-setbuf.html">std::setbuf</a></span>/<code>std::setvbuf</code> 的失败调用）前使用。</p>
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<p>不是所有 <code>size</code> 字节都需要用于缓冲：实际缓冲区大小通常向下取整到 2 的倍数、页面大小的倍数等。</p>
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<p>多数实现上，行缓冲仅对终端输入流可用。</p>
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<p>一个常见错误是设置 stdin 或 stdout 的缓冲区为生存期在程序终止前结束的数组：</p>
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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="kw4">int</span> main<span class="br0">(</span><span class="br0">)</span> <span class="br0">{</span>
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<span class="kw4">char</span> buf<span class="br0">[</span><a href="cpp-io-c.html"><span class="kw1884">BUFSIZ</span></a><span class="br0">]</span><span class="sy4">;</span>
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<a href="cpp-io-c-setbuf.html"><span class="kw1817">std::<span class="me2">setbuf</span></span></a><span class="br0">(</span><span class="kw2">stdin</span>, buf<span class="br0">)</span><span class="sy4">;</span>
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<span class="br0">}</span> <span class="co1">// buf 的生存期结束，未定义行为</span></pre></div>
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</div>
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<p>期待默认缓冲区大小 <span class="t-lc"><a href="cpp-io-c.html">BUFSIZ</a></span> 为实现上文件 I/O 的最高效缓冲区大小，但 POSIX <a rel="nofollow" class="external text" href="http://pubs.opengroup.org/onlinepubs/9699919799/functions/fstat.html">fstat</a> 经常提供更好的估计。</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>更改缓冲区大小的一个使用情况，是在知道更好的缓冲区大小时</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="cpp source-cpp">
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<pre class="de1"><span class="co2">#include <iostream></span>
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<span class="co2">#include <cstdio></span>
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<span class="co2">#include <stdlib.h></span>
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<span class="co2">#include <sys/stat.h></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-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">"test.txt"</span>, <span class="st0">"r"</span><span class="br0">)</span><span class="sy4">;</span>
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<span class="kw1">if</span><span class="br0">(</span><span class="sy3">!</span>fp<span class="br0">)</span> <span class="br0">{</span>
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<a href="cpp-io-c-perror.html"><span class="kw1872">std::<span class="me2">perror</span></span></a><span class="br0">(</span><span class="st0">"fopen"</span><span class="br0">)</span><span class="sy4">;</span> <span class="kw1">return</span> <span class="nu0">1</span><span class="sy4">;</span>
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<span class="br0">}</span>
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<span class="kw1">struct</span> stat stats<span class="sy4">;</span>
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<span class="kw1">if</span><span class="br0">(</span>fstat<span class="br0">(</span>fileno<span class="br0">(</span>fp<span class="br0">)</span>, <span class="sy3">&</span>stats<span class="br0">)</span> <span class="sy1">==</span> <span class="sy2">-</span><span class="nu0">1</span><span class="br0">)</span> <span class="br0">{</span> <span class="co1">// 仅限 POSIX</span>
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<a href="cpp-io-c-perror.html"><span class="kw1872">std::<span class="me2">perror</span></span></a><span class="br0">(</span><span class="st0">"fstat"</span><span class="br0">)</span><span class="sy4">;</span> <span class="kw1">return</span> <span class="nu0">1</span><span class="sy4">;</span>
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<span class="br0">}</span>
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<a href="cpp-io-cout.html"><span class="kw1758">std::<span class="me2">cout</span></span></a> <span class="sy1"><<</span> <span class="st0">"BUFSIZ is "</span> <span class="sy1"><<</span> <a href="cpp-io-c.html"><span class="kw1884">BUFSIZ</span></a> <span class="sy1"><<</span> <span class="st0">", but optimal block size is "</span>
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<span class="sy1"><<</span> stats.<span class="me1">st_blksize</span> <span class="sy1"><<</span> <span class="st0">'<span class="es1">\n</span>'</span><span class="sy4">;</span>
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<span class="kw1">if</span><span class="br0">(</span>std<span class="sy4">::</span><span class="me2">setvbuf</span><span class="br0">(</span>fp, <a href="cpp-types-NULL.html"><span class="kw103">NULL</span></a>, <a href="cpp-io-c.html"><span class="kw1885">_IOFBF</span></a>, stats.<span class="me1">st_blksize</span><span class="br0">)</span> <span class="sy3">!</span><span class="sy1">=</span> <span class="nu0">0</span><span class="br0">)</span> <span class="br0">{</span>
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perror<span class="br0">(</span><span class="st0">"setvbuf failed"</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// POSIX 版本设置 errno</span>
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<span class="kw1">return</span> <span class="nu0">1</span><span class="sy4">;</span>
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<span class="br0">}</span>
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<span class="kw4">int</span> ch<span class="sy4">;</span>
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<span class="kw1">while</span><span class="br0">(</span><span class="br0">(</span>ch<span class="sy1">=</span><a href="cpp-io-c-fgetc.html"><span class="kw1820">std::<span class="me2">fgetc</span></span></a><span class="br0">(</span>fp<span class="br0">)</span><span class="br0">)</span> <span class="sy3">!</span><span class="sy1">=</span> <a href="cpp-io-c.html"><span class="kw1881">EOF</span></a><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// 读取整个文件：用 truss/strace 观察</span>
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<span class="co1">// 所用的 read(2) syscall</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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<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">BUFSIZ is 8192, but optimal block size is 65536</pre></div>
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</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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<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="cpp-io-c-setbuf.html"><span class="t-lines"><span>setbuf</span></span></a></div>
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</div>
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</td>
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<td>为文件流设置缓冲区<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="cpp-io-basic_filebuf-setbuf.html"><span class="t-lines"><span>setbuf</span></span></a></div>
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<div><span class="t-lines"><span><span class="t-cmark">[虚]</span></span></span></div>
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</div>
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</td>
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<td>提供用户供应的缓冲区，或将此 filebuf 转变为无缓冲<br />
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<span class="t-mark">(<code>std::basic_filebuf<CharT,Traits></code> 的虚受保护成员函数)</span></td>
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</table>
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<div class="visualClear"></div>
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