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| <h3 class="section">1.36 <code>log2</code>, <code>log2f</code>–base 2 logarithm</h3> |
| |
| <p><a name="index-log2-104"></a><a name="index-log2f-105"></a><strong>Synopsis</strong> |
| <pre class="example"> #include <math.h> |
| double log2(double <var>x</var>); |
| float log2f(float <var>x</var>); |
| |
| </pre> |
| <p><strong>Description</strong><br> |
| The <code>log2</code> functions compute the base-2 logarithm of <var>x</var>. A domain error |
| occurs if the argument is less than zero. A range error occurs if the |
| argument is zero. |
| |
| <p>The Newlib implementations are not full, intrinisic calculations, but |
| rather are derivatives based on <code>log</code>. (Accuracy might be slightly off from |
| a direct calculation.) In addition to functions, they are also implemented as |
| macros defined in math.h: |
| <pre class="smallexample"> #define log2(x) (log (x) / _M_LN2) |
| #define log2f(x) (logf (x) / (float) _M_LN2) |
| </pre> |
| <p>To use the functions instead, just undefine the macros first. |
| |
| <p>You can use the (non-ANSI) function <code>matherr</code> to specify error |
| handling for these functions, indirectly through the respective <code>log</code> |
| function. |
| |
| <p><br> |
| <strong>Returns</strong><br> |
| The <code>log2</code> functions return |
| <code>log base-2(</code><var>x</var><code>)</code> |
| on success. |
| When <var>x</var> is zero, the |
| returned value is <code>-HUGE_VAL</code> and <code>errno</code> is set to <code>ERANGE</code>. |
| When <var>x</var> is negative, the returned value is NaN (not a number) and |
| <code>errno</code> is set to <code>EDOM</code>. You can control the error behavior via |
| <code>matherr</code>. |
| |
| <p><br> |
| <strong>Portability</strong><br> |
| C99, POSIX, System V Interface Definition (Issue 6). |
| |
| <p><br> |
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