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| 22 | <title>Exception Flags (GNU C Language Manual)</title>
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| 31 | <link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">
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| 32 | <link href="Floating-Point-in-Depth.html" rel="up" title="Floating Point in Depth">
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| 57 | <body lang="en">
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| 58 | <span id="Exception-Flags"></span><div class="header">
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| 59 | <p>
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| 60 | Next: <a href="Exact-Floating_002dPoint.html" accesskey="n" rel="next">Exact Floating-Point</a>, Previous: <a href="Invalid-Optimizations.html" accesskey="p" rel="prev">Invalid Optimizations</a>, Up: <a href="Floating-Point-in-Depth.html" accesskey="u" rel="up">Floating Point in Depth</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Symbol-Index.html" title="Index" rel="index">Index</a>]</p>
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| 61 | </div>
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| 62 | <hr>
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| 63 | <span id="Floating-Arithmetic-Exception-Flags"></span><h3 class="section">28.5 Floating Arithmetic Exception Flags</h3>
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| 64 | <span id="index-floating-arithmetic-exception-flags"></span>
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| 65 | <span id="index-exception-flags-_0028floating-point_0029"></span>
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| 66 | <span id="index-sticky-exception-flags-_0028floating-point_0029"></span>
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| 67 | <span id="index-floating-overflow"></span>
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| 68 | <span id="index-overflow_002c-floating"></span>
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| 69 | <span id="index-floating-underflow-1"></span>
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| 70 | <span id="index-underflow_002c-floating-1"></span>
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| 71 |
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| 72 | <p><em>Sticky exception flags</em> record the occurrence of particular
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| 73 | conditions: once set, they remain set until the program explicitly
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| 74 | clears them.
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| 75 | </p>
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| 76 | <p>The conditions include <em>invalid operand</em>,
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| 77 | <em>division-by_zero</em>, <em>inexact result</em> (i.e., one that
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| 78 | required rounding), <em>underflow</em>, and <em>overflow</em>. Some
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| 79 | extended floating-point designs offer several additional exception
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| 80 | flags. The functions <code>feclearexcept</code>, <code>feraiseexcept</code>,
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| 81 | <code>fetestexcept</code>, <code>fegetexceptflags</code>, and
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| 82 | <code>fesetexceptflags</code> provide a standardized interface to those
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| 83 | flags. See <a href="https://www.gnu.org/software/libc/manual/html_node/Status-bit-operations.html#Status-bit-operations">Status bit operations</a> in <cite>The GNU C Library
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| 84 | Reference Manual</cite>.
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| 85 | </p>
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| 86 | <p>One important use of those <span id="fetestexcept"></span>flags is to do a
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| 87 | computation that is normally expected to be exact in floating-point
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| 88 | arithmetic, but occasionally might not be, in which case, corrective
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| 89 | action is needed. You can clear the <em>inexact result</em> flag with a
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| 90 | call to <code>feclearexcept (FE_INEXACT)</code>, do the computation, and
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| 91 | then test the flag with <code>fetestexcept (FE_INEXACT)</code>; the result
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| 92 | of that call is 0 if the flag is not set (there was no rounding), and
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| 93 | 1 when there was rounding (which, we presume, implies the program has
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| 94 | to correct for that).
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| 95 | </p>
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| 96 |
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| 97 |
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| 98 |
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| 99 |
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| 100 |
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| 101 | </body>
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