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<a name="logb_002c-logbf_002d_002d_002dget-exponent-of-floating_002dpoint-number"></a>
<h3 class="section">1.38 <code>logb</code>, <code>logbf</code>&mdash;get exponent of floating-point number</h3>
<a name="index-logb"></a>
<a name="index-logbf"></a>
<p><strong>Synopsis</strong>
</p><div class="example">
<pre class="example">#include &lt;math.h&gt;
double logb(double <var>x</var>);
float logbf(float <var>x</var>);

</pre></div>
<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 &lt;= (<var>x</var> * FLT_RADIX to the power (-logb(<var>x</var>))) &lt; 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>
<p>All nonzero, normal numbers can be described as
<var>m</var> * 2**<var>p</var>, where 1.0 &lt;= <var>m</var> &lt; 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>
<p><strong>Returns</strong><br>
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>
<p><strong>Portability</strong><br>
ANSI C, POSIX
</p>
<br>
<p><strong>See Also</strong><br>
frexp, ilogb
</p>
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