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| 22 | <title>Handling Infinity (GNU C Language Manual)</title>
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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="Handling-Infinity"></span><div class="header">
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| 59 | <p>
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| 60 | Next: <a href="Handling-NaN.html" accesskey="n" rel="next">Handling NaN</a>, Previous: <a href="Exact-Floating-Constants.html" accesskey="p" rel="prev">Exact Floating Constants</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="Handling-Infinity-1"></span><h3 class="section">28.13 Handling Infinity</h3>
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| 64 | <span id="index-infinity-in-floating_002dpoint-arithmetic"></span>
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| 65 | <span id="index-floating_002dpoint-infinity"></span>
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| 66 |
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| 67 | <p>As we noted earlier, the IEEE 754 model of computing is not to stop
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| 68 | the program when exceptional conditions occur. It takes note of
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| 69 | exceptional values or conditions by setting sticky <em>exception
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| 70 | flags</em>, or by producing results with the special values Infinity and
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| 71 | QNaN. In this section, we discuss Infinity; see <a href="Handling-NaN.html">Handling NaN</a> for
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| 72 | the other.
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| 73 | </p>
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| 74 | <p>In GNU C, you can create a value of negative Infinity in software like
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| 75 | this:
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| 76 | </p>
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| 77 | <pre class="verbatim">double x;
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| 78 |
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| 79 | x = -1.0 / 0.0;
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| 80 | </pre>
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| 81 | <p>GNU C supplies the <code>__builtin_inf</code>, <code>__builtin_inff</code>, and
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| 82 | <code>__builtin_infl</code> macros, and the GNU C Library provides the
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| 83 | <code>INFINITY</code> macro, all of which are compile-time constants for
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| 84 | positive infinity.
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| 85 | </p>
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| 86 | <p>GNU C also provides a standard function to test for an Infinity:
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| 87 | <code>isinf (x)</code> returns <code>1</code> if the argument is a signed
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| 88 | infinity, and <code>0</code> if not.
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| 89 | </p>
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| 90 | <p>Infinities can be compared, and all Infinities of the same sign are
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| 91 | equal: there is no notion in IEEE 754 arithmetic of different kinds of
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| 92 | Infinities, as there are in some areas of mathematics. Positive
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| 93 | Infinity is larger than any finite value, and negative Infinity is
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| 94 | smaller than finite value.
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| 95 | </p>
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| 96 | <p>Infinities propagate in addition, subtraction, multiplication,
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| 97 | and square root, but in division, they disappear, because of the
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| 98 | rule that <code>finite / Infinity</code> is <code>0.0</code>. Thus, an
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| 99 | overflow in an intermediate computation that produces an Infinity
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| 100 | is likely to be noticed later in the final results. The programmer can
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| 101 | then decide whether the overflow is expected, and acceptable, or whether
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| 102 | the code possibly has a bug, or needs to be run in higher
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| 103 | precision, or redesigned to avoid the production of the Infinity.
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| 104 | </p>
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| 105 |
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| 106 | <hr>
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| 107 | <div class="header">
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| 108 | <p>
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| 109 | Next: <a href="Handling-NaN.html" accesskey="n" rel="next">Handling NaN</a>, Previous: <a href="Exact-Floating-Constants.html" accesskey="p" rel="prev">Exact Floating Constants</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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