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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"><html><head><meta name="generator" content="HTML Tidy, see www.w3.org"><meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1"><link type="text/css" rel="stylesheet" href="style.css"><!-- Generated by The Open Group's rhtm tool v1.2.1 --><!-- Copyright (c) 2001-2003 The Open Group, All Rights Reserved --><title>ilogb</title></head><body bgcolor="white"><script type="text/javascript" language="JavaScript" src="../jscript/codes.js"></script><basefont size="3"> <a name="ilogb"></a> <a name="tag_03_274"></a><!-- ilogb --> <!--header start--><center><font size="2">The Open Group Base Specifications Issue 6<br>IEEE Std 1003.1, 2003 Edition<br>Copyright © 2001-2003 The IEEE and The Open Group, All Rights reserved.</font></center><!--header end--><hr size="2" noshade><h4><a name="tag_03_274_01"></a>NAME</h4><blockquote>ilogb, ilogbf, ilogbl - return an unbiased exponent</blockquote><h4><a name="tag_03_274_02"></a>SYNOPSIS</h4><blockquote class="synopsis"><p><code><tt>#include <<a href="../basedefs/math.h.html">math.h</a>><br><br> int ilogb(double</tt> <i>x</i><tt>);<br> int ilogbf(float</tt> <i>x</i><tt>);<br> int ilogbl(long double</tt> <i>x</i><tt>);<br></tt></code></p></blockquote><h4><a name="tag_03_274_03"></a>DESCRIPTION</h4><blockquote><div class="box"><sup>[<a href="javascript:open_code('CX')">CX</a>]</sup> <img src="../images/opt-start.gif" alt="[Option Start]"border="0"> The functionality described on this reference page is aligned with the ISO C standard. Any conflict between therequirements described here and the ISO C standard is unintentional. This volume of IEEE Std 1003.1-2001 defers tothe ISO C standard. <img src="../images/opt-end.gif" alt="[Option End]" border="0"></div><p>These functions shall return the exponent part of their argument <i>x</i>. Formally, the return value is the integral part oflog<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-pointarithmetic, which is the value of FLT_RADIX defined in <a href="../basedefs/float.h.html"><i><float.h></i></a>.</p><p>An application wishing to check for error situations should set <i>errno</i> to zero and call<i>feclearexcept</i>(FE_ALL_EXCEPT) before calling these functions. On return, if <i>errno</i> is non-zero or<i>fetestexcept</i>(FE_INVALID | FE_DIVBYZERO | FE_OVERFLOW | FE_UNDERFLOW) is non-zero, an error has occurred.</p></blockquote><h4><a name="tag_03_274_04"></a>RETURN VALUE</h4><blockquote><p>Upon successful completion, these functions shall return the exponent part of <i>x</i> as a signed integer value. They areequivalent to calling the corresponding <a href="../functions/logb.html"><i>logb</i>()</a> function and casting the returned valueto type <b>int</b>.</p><p>If <i>x</i> is 0, <sup>[<a href="javascript:open_code('XSI')">XSI</a>]</sup> <img src="../images/opt-start.gif" alt="[Option Start]" border="0"> a domain error shall occur, and <img src="../images/opt-end.gif" alt="[Option End]" border="0">the value FP_ILOGB0 shall be returned.</p><p>If <i>x</i> is ±Inf, <sup>[<a href="javascript:open_code('XSI')">XSI</a>]</sup> <img src="../images/opt-start.gif" alt="[Option Start]" border="0"> a domain error shall occur, and <img src="../images/opt-end.gif" alt="[Option End]" border="0">the value {INT_MAX} shall be returned.</p><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="[Option Start]" border="0"> a domain error shall occur, and <img src="../images/opt-end.gif" alt="[Option End]" border="0">the value FP_ILOGBNAN shall be returned.</p><p><sup>[<a href="javascript:open_code('XSI')">XSI</a>]</sup> <img src="../images/opt-start.gif" alt="[Option Start]" border="0">If the correct value is greater than {INT_MAX}, {INT_MAX} shall be returned and a domain error shall occur.</p><p>If the correct value is less than {INT_MIN}, {INT_MIN} shall be returned and a domain error shall occur. <img src="../images/opt-end.gif" alt="[Option End]" border="0"></p></blockquote><h4><a name="tag_03_274_05"></a>ERRORS</h4><blockquote><p>These functions shall fail if:</p><dl compact><dt>Domain Error</dt><dd><sup>[<a href="javascript:open_code('XSI')">XSI</a>]</sup> <img src="../images/opt-start.gif" alt="[Option Start]" border="0">The <i>x</i> argument is zero, NaN, or ±Inf, or the correct value is not representable as an integer. <p>If the integer expression (math_errhandling & MATH_ERRNO) is non-zero, then <i>errno</i> shall be set to [EDOM]. If theinteger expression (math_errhandling & MATH_ERREXCEPT) is non-zero, then the invalid floating-point exception shall be raised.<img src="../images/opt-end.gif" alt="[Option End]" border="0"></p></dd></dl></blockquote><hr><div class="box"><em>The following sections are informative.</em></div><h4><a name="tag_03_274_06"></a>EXAMPLES</h4><blockquote><p>None.</p></blockquote><h4><a name="tag_03_274_07"></a>APPLICATION USAGE</h4><blockquote><p>On error, the expressions (math_errhandling & MATH_ERRNO) and (math_errhandling & MATH_ERREXCEPT) are independent ofeach other, but at least one of them must be non-zero.</p></blockquote><h4><a name="tag_03_274_08"></a>RATIONALE</h4><blockquote><p>The errors come from taking the expected floating-point value and converting it to <b>int</b>, which is an invalid operation inIEEE Std 754-1985 (since overflow, infinity, and NaN are not representable in a type <b>int</b>), so should be a domainerror.</p><p>There are no known implementations that overflow. For overflow to happen, {INT_MAX} must be less thanLDBL_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 notsupported, or {INT_MIN} must be greater than (LDBL_MIN_EXP-LDBL_MANT_DIG)*<i>log2</i>(FLT_RADIX) if subnormals are supported.</p></blockquote><h4><a name="tag_03_274_09"></a>FUTURE DIRECTIONS</h4><blockquote><p>None.</p></blockquote><h4><a name="tag_03_274_10"></a>SEE ALSO</h4><blockquote><p><a href="feclearexcept.html"><i>feclearexcept</i>()</a> , <a href="fetestexcept.html"><i>fetestexcept</i>()</a> , <a href="logb.html"><i>logb</i>()</a> , <a href="scalb.html"><i>scalb</i>()</a> , the Base Definitions volume ofIEEE Std 1003.1-2001, <a href="../basedefs/xbd_chap04.html#tag_04_18">Section 4.18, Treatment of Error Conditions forMathematical Functions</a>, <a href="../basedefs/float.h.html"><i><float.h></i></a>, <a href="../basedefs/math.h.html"><i><math.h></i></a></p></blockquote><h4><a name="tag_03_274_11"></a>CHANGE HISTORY</h4><blockquote><p>First released in Issue 4, Version 2.</p></blockquote><h4><a name="tag_03_274_12"></a>Issue 5</h4><blockquote><p>Moved from X/OPEN UNIX extension to BASE.</p></blockquote><h4><a name="tag_03_274_13"></a>Issue 6</h4><blockquote><p>The <i>ilogb</i>() function is no longer marked as an extension.</p><p>The <i>ilogbf</i>() and <i>ilogbl</i>() functions are added for alignment with the ISO/IEC 9899:1999 standard.</p><p>The RETURN VALUE section is revised for alignment with the ISO/IEC 9899:1999 standard.</p><p>XSI extensions are marked.<br></p></blockquote><div class="box"><em>End of informative text.</em></div><hr><hr size="2" noshade><center><font size="2"><!--footer start-->UNIX ® is a registered Trademark of The Open Group.<br>POSIX ® is a registered Trademark of The IEEE.<br>[ <a href="../mindex.html">Main Index</a> | <a href="../basedefs/contents.html">XBD</a> | <a href="../utilities/contents.html">XCU</a> | <a href="../functions/contents.html">XSH</a> | <a href="../xrat/contents.html">XRAT</a>]</font></center><!--footer end--><hr size="2" noshade></body></html>⌨️ 快捷键说明
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