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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) 2001-2004 IEEE and The Open Group, All Rights Reserved --><title>log</title></head><body bgcolor="white"><script type="text/javascript" language="JavaScript" src="../jscript/codes.js"></script><basefont size="3"> <a name="log"></a> <a name="tag_03_343"></a><!-- log --> <!--header start--><center><font size="2">The Open Group Base Specifications Issue 6<br>IEEE Std 1003.1, 2004 Edition<br>Copyright © 2001-2004 The IEEE and The Open Group, All Rights reserved.</font></center><!--header end--><hr size="2" noshade><h4><a name="tag_03_343_01"></a>NAME</h4><blockquote>log, logf, logl - natural logarithm function</blockquote><h4><a name="tag_03_343_02"></a>SYNOPSIS</h4><blockquote class="synopsis"><p><code><tt>#include <<a href="../basedefs/math.h.html">math.h</a>><br><br> double log(double</tt> <i>x</i><tt>);<br> float logf(float</tt> <i>x</i><tt>);<br> long double logl(long double</tt> <i>x</i><tt>);<br></tt></code></p></blockquote><h4><a name="tag_03_343_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 compute the natural logarithm of their argument <i>x</i>,log<sub><small><i>e</i></small></sub>(<i>x</i>).</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_343_04"></a>RETURN VALUE</h4><blockquote><p>Upon successful completion, these functions shall return the natural logarithm of <i>x</i>.</p><p>If <i>x</i> is ±0, a pole error shall occur and <i>log</i>(), <i>logf</i>(), and <i>logl</i>() shall return -HUGE_VAL,-HUGE_VALF, and -HUGE_VALL, respectively.</p><p>For finite values of <i>x</i> that are less than 0, <sup>[<a href="javascript:open_code('MX')">MX</a>]</sup> <img src="../images/opt-start.gif" alt="[Option Start]" border="0"> or if <i>x</i> is -Inf, <img src="../images/opt-end.gif" alt="[Option End]" border="0"> a domain error shall occur, and <sup>[<a href="javascript:open_code('MX')">MX</a>]</sup> <img src="../images/opt-start.gif" alt="[Option Start]" border="0"> either a NaN (if supported), or <img src="../images/opt-end.gif"alt="[Option End]" border="0"> an implementation-defined value shall be returned.</p><p><sup>[<a href="javascript:open_code('MX')">MX</a>]</sup> <img src="../images/opt-start.gif" alt="[Option Start]" border="0"> If<i>x</i> is NaN, a NaN shall be returned.</p><p>If <i>x</i> is 1, +0 shall be returned.</p><p>If <i>x</i> is +Inf, <i>x</i> shall be returned. <img src="../images/opt-end.gif" alt="[Option End]" border="0"></p></blockquote><h4><a name="tag_03_343_05"></a>ERRORS</h4><blockquote><p>These functions shall fail if:</p><dl compact><dt>Domain Error</dt><dd>The finite value of <i>x</i> is negative, <sup>[<a href="javascript:open_code('MX')">MX</a>]</sup> <img src="../images/opt-start.gif" alt="[Option Start]" border="0"> or <i>x</i> is -Inf. <img src="../images/opt-end.gif" alt="[Option End]" border="0"> <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 beraised.</p></dd><dt>Pole Error</dt><dd>The value of <i>x</i> is zero. <p>If the integer expression (math_errhandling & MATH_ERRNO) is non-zero, then <i>errno</i> shall be set to [ERANGE]. If theinteger expression (math_errhandling & MATH_ERREXCEPT) is non-zero, then the divide-by-zero floating-point exception shall beraised.</p></dd></dl></blockquote><hr><div class="box"><em>The following sections are informative.</em></div><h4><a name="tag_03_343_06"></a>EXAMPLES</h4><blockquote><p>None.</p></blockquote><h4><a name="tag_03_343_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_343_08"></a>RATIONALE</h4><blockquote><p>None.</p></blockquote><h4><a name="tag_03_343_09"></a>FUTURE DIRECTIONS</h4><blockquote><p>None.</p></blockquote><h4><a name="tag_03_343_10"></a>SEE ALSO</h4><blockquote><p><a href="exp.html"><i>exp</i>()</a>, <a href="feclearexcept.html"><i>feclearexcept</i>()</a>, <a href="fetestexcept.html"><i>fetestexcept</i>()</a>, <a href="isnan.html"><i>isnan</i>()</a>, <a href="log10.html"><i>log10</i>()</a>,<a href="log1p.html"><i>log1p</i>()</a>, the Base Definitions volume of IEEE Std 1003.1-2001, <a href="../basedefs/xbd_chap04.html#tag_04_18">Section 4.18, Treatment of Error Conditions for Mathematical Functions</a>, <a href="../basedefs/math.h.html"><i><math.h></i></a></p></blockquote><h4><a name="tag_03_343_11"></a>CHANGE HISTORY</h4><blockquote><p>First released in Issue 1. Derived from Issue 1 of the SVID.</p></blockquote><h4><a name="tag_03_343_12"></a>Issue 5</h4><blockquote><p>The DESCRIPTION is updated to indicate how an application should check for an error. This text was previously published in theAPPLICATION USAGE section.</p></blockquote><h4><a name="tag_03_343_13"></a>Issue 6</h4><blockquote><p>The DESCRIPTION is updated to avoid use of the term "must" for application requirements.</p><p>The <i>logf</i>() and <i>logl</i>() functions are added for alignment with the ISO/IEC 9899:1999 standard.</p><p>The DESCRIPTION, RETURN VALUE, ERRORS, and APPLICATION USAGE sections are revised to align with the ISO/IEC 9899:1999standard.</p><p>IEC 60559:1989 standard floating-point extensions over the ISO/IEC 9899:1999 standard are marked.</p></blockquote><div class="box"><em>End of informative text.</em></div><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>?? 快捷鍵說明
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