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| <h3 class="section">1.37 <code>logb</code>, <code>logbf</code>–get exponent of floating-point number</h3> |
| |
| <p><a name="index-logb-106"></a><a name="index-logbf-107"></a><strong>Synopsis</strong> |
| <pre class="example"> #include <math.h> |
| double logb(double <var>x</var>); |
| float logbf(float <var>x</var>); |
| |
| </pre> |
| <p><strong>Description</strong><br> |
| The <code>logb</code> functions extract the exponent of <var>x</var>, as a signed integer value |
| in floating-point format. If <var>x</var> is subnormal it is treated as though it were |
| normalized; thus, for positive finite <var>x</var>, |
| 1 <= (<var>x</var> * FLT_RADIX to the power (-logb(<var>x</var>))) < FLT_RADIX. |
| A domain error may occur if the argument is zero. |
| In this floating-point implementation, FLT_RADIX is 2. Which also means |
| that for finite <var>x</var>, <code>logb</code>(<var>x</var>) = <code>floor</code>(<code>log2</code>(<code>fabs</code>(<var>x</var>))). |
| |
| <p>All nonzero, normal numbers can be described as |
| <var>m</var> * 2**<var>p</var>, where 1.0 <= <var>m</var> < 2.0. |
| The <code>logb</code> functions examine the argument <var>x</var>, and return <var>p</var>. |
| The <code>frexp</code> functions are similar to the <code>logb</code> functions, but |
| returning <var>m</var> adjusted to the interval [.5, 1) or 0, and <var>p</var>+1. |
| |
| <p><br> |
| <strong>Returns</strong><br> |
| <!-- Formatting note: "$@" forces a new line --> |
| When <var>x</var> is:<br> |
| +inf or -inf, +inf is returned;<br> |
| NaN, NaN is returned;<br> |
| 0, -inf is returned, and the divide-by-zero exception is raised;<br> |
| otherwise, the <code>logb</code> functions return the signed exponent of <var>x</var>. |
| |
| <p><br> |
| <strong>Portability</strong><br> |
| ANSI C, POSIX |
| |
| <p><br> |
| <strong>See Also</strong><br> |
| frexp, ilogb |
| |
| <p><br> |
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