172 lines
6.3 KiB
HTML
172 lines
6.3 KiB
HTML
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<!-- Copyright (c) 2001 The Open Group, All Rights Reserved -->
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<title>logb</title>
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<body bgcolor="white">
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<script type="text/javascript" language="JavaScript" src="../jscript/codes.js">
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<basefont size="3"> <a name="logb"></a> <a name="tag_03_347"></a><!-- logb -->
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<!--header start-->
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<center><font size="2">The Open Group Base Specifications Issue 6<br>
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IEEE Std 1003.1-2001<br>
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Copyright © 2001 The IEEE and The Open Group, All Rights reserved.</font></center>
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<!--header end-->
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<hr size="2" noshade>
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<h4><a name="tag_03_347_01"></a>NAME</h4>
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<blockquote>logb, logbf, logbl - radix-independent exponent</blockquote>
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<h4><a name="tag_03_347_02"></a>SYNOPSIS</h4>
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<blockquote class="synopsis">
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<p><code><tt>#include <<a href="../basedefs/math.h.html">math.h</a>><br>
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<br>
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double logb(double</tt> <i>x</i><tt>);<br>
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float logbf(float</tt> <i>x</i><tt>);<br>
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long double logbl(long double</tt> <i>x</i><tt>);<br>
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</tt></code></p>
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</blockquote>
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<h4><a name="tag_03_347_03"></a>DESCRIPTION</h4>
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<blockquote>
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<div class="box"><sup>[<a href="javascript:open_code('CX')">CX</a>]</sup> <img src="../images/opt-start.gif" alt="[Option Start]"
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border="0"> The functionality described on this reference page is aligned with the ISO C standard. Any conflict between the
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requirements described here and the ISO C standard is unintentional. This volume of IEEE Std 1003.1-2001 defers to
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the ISO C standard. <img src="../images/opt-end.gif" alt="[Option End]" border="0"></div>
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<p>These functions shall compute the exponent of <i>x</i>, which is the integral part of log<i><sub><small>r</small></sub></i>
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|<i>x</i>|, as a signed floating-point value, for non-zero <i>x</i>, where <i>r</i> is the radix of the machine's floating-point
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arithmetic, which is the value of FLT_RADIX defined in the <a href="../basedefs/float.h.html"><i><float.h></i></a>
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header.</p>
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<p>If <i>x</i> is subnormal it is treated as though it were normalized; thus for finite positive <i>x</i>:</p>
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<pre>
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<tt>1 <=</tt> <i>x</i> <tt>* FLT_RADIX<small><sup>-logb(x)</sup></small> < FLT_RADIX
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</tt>
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</pre>
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<p>An application wishing to check for error situations should set <i>errno</i> to zero and call
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<i>feclearexcept</i>(FE_ALL_EXCEPT) before calling these functions. On return, if <i>errno</i> is non-zero or
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<i>fetestexcept</i>(FE_INVALID | FE_DIVBYZERO | FE_OVERFLOW | FE_UNDERFLOW) is non-zero, an error has occurred.</p>
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</blockquote>
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<h4><a name="tag_03_347_04"></a>RETURN VALUE</h4>
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<blockquote>
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<p>Upon successful completion, these functions shall return the exponent of <i>x</i>.</p>
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<p>If <i>x</i> is ±0, a pole error shall occur and <i>logb</i>(), <i>logbf</i>(), and <i>logbl</i>() shall return -HUGE_VAL,
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-HUGE_VALF, and -HUGE_VALL, respectively.</p>
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<p><sup>[<a href="javascript:open_code('MX')">MX</a>]</sup> <img src="../images/opt-start.gif" alt="[Option Start]" border="0"> If
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<i>x</i> is NaN, a NaN shall be returned.</p>
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<p>If <i>x</i> is ±Inf, +Inf shall be returned. <img src="../images/opt-end.gif" alt="[Option End]" border="0"></p>
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</blockquote>
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<h4><a name="tag_03_347_05"></a>ERRORS</h4>
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<blockquote>
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<p>These functions shall fail if:</p>
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<dl compact>
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<dt>Pole Error</dt>
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<dd>The value of <i>x</i> is ±0.
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<p>If the integer expression (math_errhandling & MATH_ERRNO) is non-zero, then <i>errno</i> shall be set to [ERANGE]. If the
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integer expression (math_errhandling & MATH_ERREXCEPT) is non-zero, then the divide-by-zero floating-point exception shall be
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raised.</p>
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</dd>
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</dl>
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</blockquote>
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<hr>
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<div class="box"><em>The following sections are informative.</em></div>
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<h4><a name="tag_03_347_06"></a>EXAMPLES</h4>
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<blockquote>
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<p>None.</p>
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</blockquote>
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<h4><a name="tag_03_347_07"></a>APPLICATION USAGE</h4>
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<blockquote>
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<p>On error, the expressions (math_errhandling & MATH_ERRNO) and (math_errhandling & MATH_ERREXCEPT) are independent of
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each other, but at least one of them must be non-zero.</p>
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</blockquote>
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<h4><a name="tag_03_347_08"></a>RATIONALE</h4>
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<blockquote>
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<p>None.</p>
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</blockquote>
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<h4><a name="tag_03_347_09"></a>FUTURE DIRECTIONS</h4>
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<blockquote>
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<p>None.</p>
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</blockquote>
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<h4><a name="tag_03_347_10"></a>SEE ALSO</h4>
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<blockquote>
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<p><a href="feclearexcept.html"><i>feclearexcept</i>()</a> , <a href="fetestexcept.html"><i>fetestexcept</i>()</a> , <a href=
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"ilogb.html"><i>ilogb</i>()</a> , <a href="scalb.html"><i>scalb</i>()</a> , the Base Definitions volume of
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IEEE Std 1003.1-2001, <a href="../basedefs/xbd_chap04.html#tag_04_18">Section 4.18, Treatment of Error Conditions for
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Mathematical Functions</a>, <a href="../basedefs/float.h.html"><i><float.h></i></a>, <a href=
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"../basedefs/math.h.html"><i><math.h></i></a></p>
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</blockquote>
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<h4><a name="tag_03_347_11"></a>CHANGE HISTORY</h4>
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<blockquote>
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<p>First released in Issue 4, Version 2.</p>
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</blockquote>
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<h4><a name="tag_03_347_12"></a>Issue 5</h4>
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<blockquote>
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<p>Moved from X/OPEN UNIX extension to BASE.</p>
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</blockquote>
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<h4><a name="tag_03_347_13"></a>Issue 6</h4>
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<blockquote>
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<p>The <i>logb</i>() function is no longer marked as an extension.</p>
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<p>The <i>logbf</i>() and <i>logbl</i>() functions are added for alignment with the ISO/IEC 9899:1999 standard.</p>
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<p>The DESCRIPTION, RETURN VALUE, ERRORS, and APPLICATION USAGE sections are revised to align with the ISO/IEC 9899:1999
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standard.</p>
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<p>IEC 60559:1989 standard floating-point extensions over the ISO/IEC 9899:1999 standard are marked.</p>
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</blockquote>
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<div class="box"><em>End of informative text.</em></div>
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