184 lines
7.9 KiB
HTML
184 lines
7.9 KiB
HTML
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<!-- Copyright (c) 2001 The Open Group, All Rights Reserved -->
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<title>ilogb</title>
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<basefont size="3"> <a name="ilogb"></a> <a name="tag_03_274"></a><!-- ilogb -->
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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_274_01"></a>NAME</h4>
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<blockquote>ilogb, ilogbf, ilogbl - return an unbiased exponent</blockquote>
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<h4><a name="tag_03_274_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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int ilogb(double</tt> <i>x</i><tt>);<br>
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int ilogbf(float</tt> <i>x</i><tt>);<br>
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int ilogbl(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_274_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 return the exponent part of their argument <i>x</i>. Formally, the return value is the integral part of
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log<small><sub>r</sub></small>|x| as a signed integral 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 <a href="../basedefs/float.h.html"><i><float.h></i></a>.</p>
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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_274_04"></a>RETURN VALUE</h4>
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<blockquote>
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<p>Upon successful completion, these functions shall return the exponent part of <i>x</i> as a signed integer value. They are
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equivalent to calling the corresponding <a href="../functions/logb.html"><i>logb</i>()</a> function and casting the returned value
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to type <b>int</b>.</p>
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<p>If <i>x</i> is 0, <sup>[<a href="javascript:open_code('XSI')">XSI</a>]</sup> <img src="../images/opt-start.gif" alt=
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"[Option Start]" border="0"> a domain error shall occur, and <img src="../images/opt-end.gif" alt="[Option End]" border="0">
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the value FP_ILOGB0 shall be returned.</p>
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<p>If <i>x</i> is ±Inf, <sup>[<a href="javascript:open_code('XSI')">XSI</a>]</sup> <img src="../images/opt-start.gif" alt=
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"[Option Start]" border="0"> a domain error shall occur, and <img src="../images/opt-end.gif" alt="[Option End]" border="0">
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the value {INT_MAX} shall be returned.</p>
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<p>If <i>x</i> is a NaN, <sup>[<a href="javascript:open_code('XSI')">XSI</a>]</sup> <img src="../images/opt-start.gif" alt=
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"[Option Start]" border="0"> a domain error shall occur, and <img src="../images/opt-end.gif" alt="[Option End]" border="0">
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the value FP_ILOGBNAN shall be returned.</p>
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<p><sup>[<a href="javascript:open_code('XSI')">XSI</a>]</sup> <img src="../images/opt-start.gif" alt="[Option Start]" border="0">
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If the correct value is greater than {INT_MAX}, {INT_MAX} shall be returned and a domain error shall occur.</p>
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<p>If the correct value is less than {INT_MIN}, {INT_MIN} shall be returned and a domain error shall occur. <img src=
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"../images/opt-end.gif" alt="[Option End]" border="0"></p>
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</blockquote>
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<h4><a name="tag_03_274_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>Domain Error</dt>
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<dd><sup>[<a href="javascript:open_code('XSI')">XSI</a>]</sup> <img src="../images/opt-start.gif" alt="[Option Start]" border="0">
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The <i>x</i> argument is zero, NaN, or ±Inf, or the correct value is not representable as an integer.
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<p>If the integer expression (math_errhandling & MATH_ERRNO) is non-zero, then <i>errno</i> shall be set to [EDOM]. If the
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integer expression (math_errhandling & MATH_ERREXCEPT) is non-zero, then the invalid floating-point exception shall be raised.
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<img src="../images/opt-end.gif" alt="[Option End]" border="0"></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_274_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_274_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_274_08"></a>RATIONALE</h4>
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<blockquote>
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<p>The errors come from taking the expected floating-point value and converting it to <b>int</b>, which is an invalid operation in
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IEEE Std 754-1985 (since overflow, infinity, and NaN are not representable in a type <b>int</b>), so should be a domain
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error.</p>
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<p>There are no known implementations that overflow. For overflow to happen, {INT_MAX} must be less than
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LDBL_MAX_EXP*<i>log2</i>(FLT_RADIX) or {INT_MIN} must be greater than LDBL_MIN_EXP*<i>log2</i>(FLT_RADIX) if subnormals are not
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supported, or {INT_MIN} must be greater than (LDBL_MIN_EXP-LDBL_MANT_DIG)*<i>log2</i>(FLT_RADIX) if subnormals are supported.</p>
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</blockquote>
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<h4><a name="tag_03_274_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_274_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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"logb.html"><i>logb</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_274_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_274_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_274_13"></a>Issue 6</h4>
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<blockquote>
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<p>The <i>ilogb</i>() function is no longer marked as an extension.</p>
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<p>The <i>ilogbf</i>() and <i>ilogbl</i>() functions are added for alignment with the ISO/IEC 9899:1999 standard.</p>
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<p>The RETURN VALUE section is revised for alignment with the ISO/IEC 9899:1999 standard.</p>
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<p>XSI extensions are marked.<br>
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</p>
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</blockquote>
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<div class="box"><em>End of informative text.</em></div>
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<hr>
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<hr size="2" noshade>
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<center><font size="2"><!--footer start-->
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UNIX ® is a registered Trademark of The Open Group.<br>
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POSIX ® is a registered Trademark of The IEEE.<br>
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[ <a href="../mindex.html">Main Index</a> | <a href="../basedefs/contents.html">XBD</a> | <a href=
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"../utilities/contents.html">XCU</a> | <a href="../functions/contents.html">XSH</a> | <a href="../xrat/contents.html">XRAT</a>
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