191 lines
11 KiB
HTML
191 lines
11 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>变长参数</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_language_variadic 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">变长参数</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-language.html">language</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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<p>变参数函数是能以不同数目参数调用的函数。</p>
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<p>只有新式（原型）<a href="c-language-function_declaration.html">函数声明</a>可以为变长参数的。它必须通过出现在参数列表最后的 <span class="t-c"><span class="mw-geshi c source-c">...</span></span> 形式的参数指定，且至少跟随一个具名参数。</p>
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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="co1">//新式声明</span>
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<span class="kw4">int</span> printx<span class="br0">(</span><span class="kw4">const</span> <span class="kw4">char</span><span class="sy2">*</span> fmt, ...<span class="br0">)</span><span class="sy4">;</span> <span class="co1">// 此方法声明的函数</span>
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printx<span class="br0">(</span><span class="st0">"hello world"</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// 可能会以一个</span>
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printx<span class="br0">(</span><span class="st0">"a=%d b=%d"</span>, a, b<span class="br0">)</span><span class="sy4">;</span> <span class="co1">// 或更多参数调用</span>
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<span class="co1">// int printy(..., const char* fmt); // 错误： ... 必须在最后</span>
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<span class="co1">// int printz(...); // 错误： ... 必须跟随至少一个具名参数</span></pre></div>
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</div>
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<p>在<a href="c-language-operator_other.html#.E5.87.BD.E6.95.B0.E8.B0.83.E7.94.A8">函数调用</a>中，每个属于变长参数列表一部分的参数会经历名为<a href="c-language-conversion.html#.E9.BB.98.E8.AE.A4.E5.8F.82.E6.95.B0.E6.8F.90.E5.8D.87">默认参数提升</a>的隐式转换。</p>
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<p>在函数体内使用变长参数时，这些参数的值必须用 <a href="c-variadic.html"><stdarg.h> 库工具</a>访问：</p>
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<table class="t-dsc-begin">
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<tr class="t-dsc-header">
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<td colspan="2">
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<div>定义于头文件 <code><stdarg.h></code></div>
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</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-variadic-va_start.html"><span class="t-lines"><span>va_start</span></span></a></div>
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</div>
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</td>
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<td>令函数得以访问可变参数<br />
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<span class="t-mark">(宏函数)</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-variadic-va_arg.html"><span class="t-lines"><span>va_arg</span></span></a></div>
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</div>
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</td>
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<td>访问下一个函数可变参数<br />
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<span class="t-mark">(宏函数)</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-variadic-va_copy.html"><span class="t-lines"><span>va_copy</span></span></a></div>
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<div><span class="t-lines"><span><span class="t-mark-rev t-since-c99">(C99)</span></span></span></div>
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</div>
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</td>
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<td>创造函数可变参数的副本<br />
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<span class="t-mark">(宏函数)</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-variadic-va_end.html"><span class="t-lines"><span>va_end</span></span></a></div>
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</div>
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</td>
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<td>结束函数可变参数的行程<br />
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<span class="t-mark">(宏函数)</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-variadic-va_list.html"><span class="t-lines"><span>va_list</span></span></a></div>
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</div>
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</td>
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<td>保有 va_start 、 va_arg 、 va_end 及 va_copy 所需信息<br />
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<span class="t-mark">(typedef)</span></td>
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</tr>
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</table>
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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>虽然旧式（无原型）<a href="c-language-function_declaration.html">函数声明</a>允许后继的函数调用使用任意参数，它们也不允许是变长参数（ C89 起）。这种函数的定义必须指定固定数目的参数，并且不能使用 <code>stdarg.h</code> 中的宏。</p>
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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="co1">//旧式声明</span>
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<span class="kw4">int</span> printx<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// 此方式定义的函数</span>
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printx<span class="br0">(</span><span class="st0">"hello world"</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// 可以以一个</span>
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printx<span class="br0">(</span><span class="st0">"a=%d b=%d"</span>, a, b<span class="br0">)</span><span class="sy4">;</span> <span class="co1">// 或更多参数调用</span>
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<span class="co1">// 上述调用行为至少有一个是未定义的，取决于函数定义所接收的参数数</span></pre></div>
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</div>
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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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<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 <time.h></span>
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<span class="co2">#include <stdarg.h></span>
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<span class="kw4">void</span> tlog<span class="br0">(</span><span class="kw4">const</span> <span class="kw4">char</span><span class="sy2">*</span> fmt,...<span class="br0">)</span>
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<span class="br0">{</span>
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<span class="kw4">char</span> msg<span class="br0">[</span><span class="nu0">50</span><span class="br0">]</span><span class="sy4">;</span>
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<a href="c-chrono-strftime.html"><span class="kw517">strftime</span></a><span class="br0">(</span>msg, <span class="kw4">sizeof</span> msg, <span class="st0">"%T"</span>, <a href="c-chrono-localtime.html"><span class="kw520">localtime</span></a><span class="br0">(</span><span class="sy3">&</span><span class="br0">(</span><a href="c-chrono-time_t.html"><span class="kw524">time_t</span></a><span class="br0">)</span><span class="br0">{</span><a href="c-chrono-time.html"><span class="kw513">time</span></a><span class="br0">(</span><a href="c-types-NULL.html"><span class="kw103">NULL</span></a><span class="br0">)</span><span class="br0">}</span><span class="br0">)</span><span class="br0">)</span><span class="sy4">;</span>
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<a href="c-io-fprintf.html"><span class="kw851">printf</span></a><span class="br0">(</span><span class="st0">"[%s] "</span>, msg<span class="br0">)</span><span class="sy4">;</span>
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va_list args<span class="sy4">;</span>
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va_start<span class="br0">(</span>args, fmt<span class="br0">)</span><span class="sy4">;</span>
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<a href="c-io-vfprintf.html"><span class="kw855">vprintf</span></a><span class="br0">(</span>fmt, args<span class="br0">)</span><span class="sy4">;</span>
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va_end<span class="br0">(</span>args<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> 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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tlog<span class="br0">(</span><span class="st0">"logging %d %d %d...<span class="es1">\n</span>"</span>, <span class="nu0">1</span>, <span class="nu0">2</span>, <span class="nu0">3</span><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">[10:21:38] logging 1 2 3...</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>6.7.6.3/9 Function declarators (including prototypes) (p: 133)</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>7.16 Variable arguments <stdarg.h> (p: 269-272)</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>6.7.5.3/9 Function declarators (including prototypes) (p: 119)</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>7.15 Variable arguments <stdarg.h> (p: 249-252)</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>3.5.4.3/5 Function declarators (including prototypes)</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>4.8 VARIABLE ARGUMENTS <stdarg.h></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"></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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