251 lines
16 KiB
HTML
251 lines
16 KiB
HTML
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<title>strtof, strtod, strtold</title>
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<body class="mediawiki ltr sitedir-ltr ns-0 ns-subject page-c_string_byte_strtof skin-cppreference2 action-view cpp-navbar">
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<h1 id="firstHeading" class="firstHeading">strtof, strtod, strtold</h1>
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<div id="contentSub"><span class="subpages">< <a href="c.html">c</a>‎ | <a href="c-string.html">string</a>‎ | <a href="c-string-byte.html">byte</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><stdlib.h></code></div>
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<td></td>
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<td></td>
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<div><span class="mw-geshi c source-c"><span class="kw4">float</span> strtof<span class="br0">(</span> <span class="kw4">const</span> <span class="kw4">char</span> <span class="sy2">*</span><span class="kw4">restrict</span> str, <span class="kw4">char</span> <span class="sy2">**</span><span class="kw4">restrict</span> str_end <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><span class="t-mark-rev t-since-c99">(C99 起)</span></td>
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<div><span class="mw-geshi c source-c"><span class="kw4">double</span> strtod<span class="br0">(</span> <span class="kw4">const</span> <span class="kw4">char</span> <span class="sy2">*</span>str, <span class="kw4">char</span> <span class="sy2">**</span>str_end <span class="br0">)</span><span class="sy4">;</span></span></div>
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<td><span class="t-mark-rev t-until-c99">(C99 前)</span></td>
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<div><span class="mw-geshi c source-c"><span class="kw4">double</span> strtod<span class="br0">(</span> <span class="kw4">const</span> <span class="kw4">char</span> <span class="sy2">*</span><span class="kw4">restrict</span> str, <span class="kw4">char</span> <span class="sy2">**</span><span class="kw4">restrict</span> str_end <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><span class="t-mark-rev t-since-c99">(C99 起)</span></td>
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<div><span class="mw-geshi c source-c"><span class="kw4">long</span> <span class="kw4">double</span> strtold<span class="br0">(</span> <span class="kw4">const</span> <span class="kw4">char</span> <span class="sy2">*</span><span class="kw4">restrict</span> str, <span class="kw4">char</span> <span class="sy2">**</span><span class="kw4">restrict</span> str_end <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><span class="t-mark-rev t-since-c99">(C99 起)</span></td>
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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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<p>转译 <code>str</code> 所指向的字节字符串中的浮点值。</p>
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<p>函数会舍弃任何空白符（由 <span class="t-c"><span class="mw-geshi c source-c">std<span class="sy4">::</span><a href="c-string-byte-isspace.html"><span class="kw534">isspace</span></a><span class="br0">(</span><span class="br0">)</span></span></span> 确定），直至找到首个非空白符。然后它会取用尽可能多的字符，以构成合法的浮点数表示，并将它们转换成浮点值。合法的浮点值可以为下列之一：</p>
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<ul>
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<li>十进制浮点数表达式。它由下列部分组成：</li>
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<li><span class="t-mark">(可选)</span> 正或负号</li>
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<li>非空的十进制数字序列，可选地包含一个小数点字符（由当前的 C <a href="c-locale-setlocale.html">本地环境</a>确定）（定义有效数字）</li>
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<li><span class="t-mark">(可选)</span> <code><b>e</b></code> 或 <code><b>E</b></code> ，并跟随可选的正或负号，以及非空十进制数字序列（以 10 为底定义指数）</li>
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</ul>
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</dd>
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<ul>
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<li>十六进制浮点数表达式。它由下列部分组成：</li>
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<ul>
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<li><span class="t-mark">(可选)</span> 正或负号</li>
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<li><code><b>0x</b></code> 或 <code><b>0X</b></code></li>
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<li>非空的十六进制数字序列，选地包含一个小数点字符（由当前的 C <a href="c-locale-setlocale.html">本地环境</a>确定）（定义有效数字）</li>
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<li><span class="t-mark">(可选)</span> <code><b>p</b></code> 或 <code><b>P</b></code> ，并跟随可选的正或负号，以及非空十进制数字序列（以 2 为底定义指数）</li>
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</ul>
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</dd>
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</dl>
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<ul>
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<li>无穷大表达式。它由下列部分组成：</li>
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</ul>
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<ul>
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<li><span class="t-mark">(可选)</span> 正或负号</li>
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<li><code><b>INF</b></code> 或 <code><b>INFINITY</b></code> ，忽略大小写</li>
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</ul>
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</dd>
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</dl>
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<ul>
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<li>非数（NaN）表达式。它由下列部分组成：</li>
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</ul>
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<ul>
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<li><span class="t-mark">(可选)</span> 正或负号</li>
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<li><code><b>NAN</b></code> 或 <code><b>NAN(</b></code><i>char_sequence</i><code><b>)</b></code> ，忽略 <code><b>NAN</b></code> 部分的大小写。 <i>char_sequence</i> 只能由数字、拉丁字母和下划线构成。结果是一个静态的 NaN 浮点值。</li>
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</ul>
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</dd>
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</td>
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<td><span class="t-mark-rev t-since-c99">(C99 起)</span></td>
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<ul>
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<li>任何其他可由当前 C <a href="c-locale-setlocale.html">本地环境</a>接受的表达式</li>
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</ul>
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<p>函数设置 <code>str_end</code> 所指向的指针指向最后被转译字符的后一字符，若 <code>str_end</code> 为空指针，则忽略它。</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>str</td>
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<td>-</td>
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<td>指向要转译的空终止字节字符串的指针</td>
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</tr>
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<tr class="t-par">
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<td>str_end</td>
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<td>-</td>
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<td>指向指向字符指针的指针。</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>成功时为对应 <code>str</code> 内容的浮点值。若转换出的值落在对应返回类型的范围外，则发生值域错误并返回 <span class="t-lc"><a href="c-numeric-math-HUGE_VAL.html">HUGE_VAL</a></span> 、 <span class="t-lc"><a href="c-numeric-math-HUGE_VAL.html">HUGE_VALF</a></span> 或 <span class="t-lc"><a href="c-numeric-math-HUGE_VAL.html">HUGE_VALL</a></span> 。若无法进行转换，则返回 <span class="t-c"><span class="mw-geshi c source-c">​<span class="nu0">0</span>​</span></span> 并将 <span class="t-c"><span class="mw-geshi c source-c"><span class="sy2">*</span>str_end</span></span> 设为 <code>str</code> 。</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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<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 <errno.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">// 带错误处理的剖析</span>
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<span class="kw4">const</span> <span class="kw4">char</span> <span class="sy2">*</span>p <span class="sy1">=</span> <span class="st0">"111.11 -2.22 Nan nan(2) inF 0X1.BC70A3D70A3D7P+6 1.18973e+4932zzz"</span><span class="sy4">;</span>
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<a href="c-io-fprintf.html"><span class="kw848">printf</span></a><span class="br0">(</span><span class="st0">"Parsing '%s':<span class="es1">\n</span>"</span>, p<span class="br0">)</span><span class="sy4">;</span>
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<span class="kw4">char</span> <span class="sy2">*</span>end<span class="sy4">;</span>
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<span class="kw1">for</span> <span class="br0">(</span><span class="kw4">double</span> f <span class="sy1">=</span> strtod<span class="br0">(</span>p, <span class="sy3">&</span>end<span class="br0">)</span><span class="sy4">;</span> p <span class="sy3">!</span><span class="sy1">=</span> end<span class="sy4">;</span> f <span class="sy1">=</span> strtod<span class="br0">(</span>p, <span class="sy3">&</span>end<span class="br0">)</span><span class="br0">)</span>
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<span class="br0">{</span>
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<a href="c-io-fprintf.html"><span class="kw848">printf</span></a><span class="br0">(</span><span class="st0">"'%.*s' -> "</span>, <span class="br0">(</span><span class="kw4">int</span><span class="br0">)</span><span class="br0">(</span>end<span class="sy2">-</span>p<span class="br0">)</span>, p<span class="br0">)</span><span class="sy4">;</span>
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p <span class="sy1">=</span> end<span class="sy4">;</span>
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<span class="kw1">if</span> <span class="br0">(</span><span class="kw2">errno</span> <span class="sy1">==</span> <a href="c-error-errno_macros.html"><span class="kw478">ERANGE</span></a><span class="br0">)</span><span class="br0">{</span>
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<a href="c-io-fprintf.html"><span class="kw848">printf</span></a><span class="br0">(</span><span class="st0">"range error, got "</span><span class="br0">)</span><span class="sy4">;</span>
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<span class="kw2">errno</span> <span class="sy1">=</span> <span class="nu0">0</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="kw848">printf</span></a><span class="br0">(</span><span class="st0">"%f<span class="es1">\n</span>"</span>, f<span class="br0">)</span><span class="sy4">;</span>
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<span class="br0">}</span>
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<span class="co1">// 无错误处理的剖析</span>
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<a href="c-io-fprintf.html"><span class="kw848">printf</span></a><span class="br0">(</span><span class="st0">"<span class="es1">\"</span> -0.0000000123junk<span class="es1">\"</span> --> %g<span class="es1">\n</span>"</span>, strtod<span class="br0">(</span><span class="st0">" -0.0000000123junk"</span>, <a href="c-types-NULL.html"><span class="kw103">NULL</span></a><span class="br0">)</span><span class="br0">)</span><span class="sy4">;</span>
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<a href="c-io-fprintf.html"><span class="kw848">printf</span></a><span class="br0">(</span><span class="st0">"<span class="es1">\"</span>junk<span class="es1">\"</span> --> %g<span class="es1">\n</span>"</span>, strtod<span class="br0">(</span><span class="st0">"junk"</span>, <a href="c-types-NULL.html"><span class="kw103">NULL</span></a><span class="br0">)</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>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">Parsing '111.11 -2.22 Nan inF 0X1.BC70A3D70A3D7P+6 1.18973e+4932zzz':
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'111.11' -> 111.110000
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' -2.22' -> -2.220000
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' Nan' -> nan
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' nan(2)' -> nan
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' inF' -> inf
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' 0X1.BC70A3D70A3D7P+6' -> 111.110000
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' 1.18973e+4932' -> range error, got inf
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" -0.0000000123junk" --> -1.23e-08
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"junk" --> 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>7.22.1.3 The strtod, strtof, and strtold functions (p: 342-344)</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>7.20.1.3 The strtod, strtof, and strtold functions (p: 308-310)</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>4.10.1.4 The strtod 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-string-byte-atof.html"><span class="t-lines"><span>atof</span></span></a></div>
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</div>
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</td>
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<td>将字节字符串转换成浮点值<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-string-wide-wcstof.html"><span class="t-lines"><span>wcstof</span><span>wcstod</span><span>wcstold</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><span class="t-mark-rev t-since-c95">(C95)</span></span><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>将宽字符串转换成浮点值<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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