169 lines
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169 lines
14 KiB
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<title>FE_DFL_ENV</title>
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<body class="mediawiki ltr sitedir-ltr ns-0 ns-subject page-c_numeric_fenv_FE_DFL_ENV skin-cppreference2 action-view cpp-navbar">
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<h1 id="firstHeading" class="firstHeading">FE_DFL_ENV</h1>
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<div id="contentSub"><span class="subpages">< <a href="c.html">c</a>‎ | <a href="c-numeric.html">numeric</a>‎ | <a href="c-numeric-fenv.html">fenv</a></span></div>
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<div>定义于头文件 <code><fenv.h></code></div>
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<td></td>
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<div><span class="mw-geshi c source-c"><span class="co2">#define FE_DFL_ENV /*implementation defined*/</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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<p>宏常量 <span class="t-lc"><strong class="selflink">FE_DFL_ENV</strong></span> 展开成 <span class="t-c"><span class="mw-geshi c source-c"><span class="kw4">const</span> fenv_t<span class="sy2">*</span></span></span> 类型表达式，指向默认浮点环境，即在程序启动时加载的浮点环境的完整副本。</p>
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<p>实现可以支持以 <code>FE_</code> 后随大写字母开始，且拥有 <span class="t-c"><span class="mw-geshi c source-c"><span class="kw4">const</span> fenv_t<span class="sy2">*</span></span></span> 类型的附加宏。</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 <fenv.h></span>
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<span class="co2">#pragma STDC FENV_ACCESS ON</span>
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<span class="kw4">void</span> show_fe_exceptions<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-fprintf.html"><span class="kw850">printf</span></a><span class="br0">(</span><span class="st0">"current exceptions raised: "</span><span class="br0">)</span><span class="sy4">;</span>
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<span class="kw1">if</span><span class="br0">(</span><a href="c-numeric-fenv-fetestexcept.html"><span class="kw719">fetestexcept</span></a><span class="br0">(</span><a href="c-numeric-fenv-FE_exceptions.html"><span class="kw730">FE_DIVBYZERO</span></a><span class="br0">)</span><span class="br0">)</span> <a href="c-io-fprintf.html"><span class="kw850">printf</span></a><span class="br0">(</span><span class="st0">" FE_DIVBYZERO"</span><span class="br0">)</span><span class="sy4">;</span>
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<span class="kw1">if</span><span class="br0">(</span><a href="c-numeric-fenv-fetestexcept.html"><span class="kw719">fetestexcept</span></a><span class="br0">(</span><a href="c-numeric-fenv-FE_exceptions.html"><span class="kw731">FE_INEXACT</span></a><span class="br0">)</span><span class="br0">)</span> <a href="c-io-fprintf.html"><span class="kw850">printf</span></a><span class="br0">(</span><span class="st0">" FE_INEXACT"</span><span class="br0">)</span><span class="sy4">;</span>
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<span class="kw1">if</span><span class="br0">(</span><a href="c-numeric-fenv-fetestexcept.html"><span class="kw719">fetestexcept</span></a><span class="br0">(</span><a href="c-numeric-fenv-FE_exceptions.html"><span class="kw732">FE_INVALID</span></a><span class="br0">)</span><span class="br0">)</span> <a href="c-io-fprintf.html"><span class="kw850">printf</span></a><span class="br0">(</span><span class="st0">" FE_INVALID"</span><span class="br0">)</span><span class="sy4">;</span>
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<span class="kw1">if</span><span class="br0">(</span><a href="c-numeric-fenv-fetestexcept.html"><span class="kw719">fetestexcept</span></a><span class="br0">(</span><a href="c-numeric-fenv-FE_exceptions.html"><span class="kw733">FE_OVERFLOW</span></a><span class="br0">)</span><span class="br0">)</span> <a href="c-io-fprintf.html"><span class="kw850">printf</span></a><span class="br0">(</span><span class="st0">" FE_OVERFLOW"</span><span class="br0">)</span><span class="sy4">;</span>
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<span class="kw1">if</span><span class="br0">(</span><a href="c-numeric-fenv-fetestexcept.html"><span class="kw719">fetestexcept</span></a><span class="br0">(</span><a href="c-numeric-fenv-FE_exceptions.html"><span class="kw734">FE_UNDERFLOW</span></a><span class="br0">)</span><span class="br0">)</span> <a href="c-io-fprintf.html"><span class="kw850">printf</span></a><span class="br0">(</span><span class="st0">" FE_UNDERFLOW"</span><span class="br0">)</span><span class="sy4">;</span>
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<span class="kw1">if</span><span class="br0">(</span><a href="c-numeric-fenv-fetestexcept.html"><span class="kw719">fetestexcept</span></a><span class="br0">(</span><a href="c-numeric-fenv-FE_exceptions.html"><span class="kw729">FE_ALL_EXCEPT</span></a><span class="br0">)</span><span class="sy1">==</span><span class="nu0">0</span><span class="br0">)</span> <a href="c-io-fprintf.html"><span class="kw850">printf</span></a><span class="br0">(</span><span class="st0">" none"</span><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">"<span class="es1">\n</span>"</span><span class="br0">)</span><span class="sy4">;</span>
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<span class="br0">}</span>
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<span class="kw4">void</span> show_fe_rounding_method<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-fprintf.html"><span class="kw850">printf</span></a><span class="br0">(</span><span class="st0">"current rounding method: "</span><span class="br0">)</span><span class="sy4">;</span>
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<span class="kw1">switch</span> <span class="br0">(</span><a href="c-numeric-fenv-feround.html"><span class="kw723">fegetround</span></a><span class="br0">(</span><span class="br0">)</span><span class="br0">)</span> <span class="br0">{</span>
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<span class="kw1">case</span> <a href="c-numeric-fenv-FE_round.html"><span class="kw736">FE_TONEAREST</span></a><span class="sy4">:</span> <a href="c-io-fprintf.html"><span class="kw850">printf</span></a> <span class="br0">(</span><span class="st0">"FE_TONEAREST"</span><span class="br0">)</span><span class="sy4">;</span> <span class="kw1">break</span><span class="sy4">;</span>
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<span class="kw1">case</span> <a href="c-numeric-fenv-FE_round.html"><span class="kw735">FE_DOWNWARD</span></a><span class="sy4">:</span> <a href="c-io-fprintf.html"><span class="kw850">printf</span></a> <span class="br0">(</span><span class="st0">"FE_DOWNWARD"</span><span class="br0">)</span><span class="sy4">;</span> <span class="kw1">break</span><span class="sy4">;</span>
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<span class="kw1">case</span> <a href="c-numeric-fenv-FE_round.html"><span class="kw738">FE_UPWARD</span></a><span class="sy4">:</span> <a href="c-io-fprintf.html"><span class="kw850">printf</span></a> <span class="br0">(</span><span class="st0">"FE_UPWARD"</span><span class="br0">)</span><span class="sy4">;</span> <span class="kw1">break</span><span class="sy4">;</span>
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<span class="kw1">case</span> <a href="c-numeric-fenv-FE_round.html"><span class="kw737">FE_TOWARDZERO</span></a><span class="sy4">:</span> <a href="c-io-fprintf.html"><span class="kw850">printf</span></a> <span class="br0">(</span><span class="st0">"FE_TOWARDZERO"</span><span class="br0">)</span><span class="sy4">;</span> <span class="kw1">break</span><span class="sy4">;</span>
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<span class="kw1">default</span><span class="sy4">:</span> <a href="c-io-fprintf.html"><span class="kw850">printf</span></a> <span class="br0">(</span><span class="st0">"unknown"</span><span class="br0">)</span><span class="sy4">;</span>
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<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">"<span class="es1">\n</span>"</span><span class="br0">)</span><span class="sy4">;</span>
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<span class="br0">}</span>
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<span class="kw4">void</span> show_fe_environment<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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show_fe_exceptions<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span>
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show_fe_rounding_method<span class="br0">(</span><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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<a href="c-io-fprintf.html"><span class="kw850">printf</span></a><span class="br0">(</span><span class="st0">"On startup:<span class="es1">\n</span>"</span><span class="br0">)</span><span class="sy4">;</span>
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show_fe_environment<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span>
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<span class="co1">// 更改环境</span>
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<a href="c-numeric-fenv-feround.html"><span class="kw724">fesetround</span></a><span class="br0">(</span><a href="c-numeric-fenv-FE_round.html"><span class="kw735">FE_DOWNWARD</span></a><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// 更改舍入模式</span>
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<a href="c-numeric-fenv-feraiseexcept.html"><span class="kw720">feraiseexcept</span></a><span class="br0">(</span><a href="c-numeric-fenv-FE_exceptions.html"><span class="kw732">FE_INVALID</span></a><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// 引发异常</span>
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<a href="c-io-fprintf.html"><span class="kw850">printf</span></a><span class="br0">(</span><span class="st0">"<span class="es1">\n</span>Before restoration:<span class="es1">\n</span>"</span><span class="br0">)</span><span class="sy4">;</span>
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show_fe_environment<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span>
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<a href="c-numeric-fenv-feenv.html"><span class="kw726">fesetenv</span></a><span class="br0">(</span>FE_DFL_ENV<span class="br0">)</span><span class="sy4">;</span> <span class="co1">// 恢复</span>
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<a href="c-io-fprintf.html"><span class="kw850">printf</span></a><span class="br0">(</span><span class="st0">"<span class="es1">\n</span>After restoring default environment:<span class="es1">\n</span>"</span><span class="br0">)</span><span class="sy4">;</span>
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show_fe_environment<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>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">On startup:
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current exceptions raised: none
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current rounding method: FE_TONEAREST
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Before restoration:
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current exceptions raised: FE_INVALID
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current rounding method: FE_DOWNWARD
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After restoring default environment:
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current exceptions raised: none
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current rounding method: FE_TONEAREST</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>7.6/9 Floating-point environment <fenv.h> (p: 208)</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>7.6/8 Floating-point environment <fenv.h> (p: 188-189)</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-numeric-fenv-feenv.html"><span class="t-lines"><span>fegetenv</span><span>fesetenv</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-c99">(C99)</span></span></span></div>
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</div>
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</td>
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<td>保存或恢复当前浮点环境，包括异常的标志和数字的舍弃模式<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-numeric-fenv-feupdateenv.html"><span class="t-lines"><span>feupdateenv</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>恢复之前保存的浮点环境，并引发之前已经引发过的异常，使其存在于当前内存环境中<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 class="visualClear"></div>
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