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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>strtod</title></head><body bgcolor="white"><script type="text/javascript" language="JavaScript" src="../jscript/codes.js"></script><basefont size="3"> <a name="strtod"></a> <a name="tag_03_740"></a><!-- strtod --> <!--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_740_01"></a>NAME</h4><blockquote>strtod, strtof, strtold - convert a string to a double-precision number</blockquote><h4><a name="tag_03_740_02"></a>SYNOPSIS</h4><blockquote class="synopsis"><p><code><tt>#include <<a href="../basedefs/stdlib.h.html">stdlib.h</a>><br><br> double strtod(const char *restrict</tt> <i>nptr</i><tt>, char **restrict</tt> <i>endptr</i><tt>);<br> float strtof(const char *restrict</tt> <i>nptr</i><tt>, char **restrict</tt> <i>endptr</i><tt>);<br> long double strtold(const char *restrict</tt> <i>nptr</i><tt>, char **restrict</tt> <i>endptr</i><tt>);<br></tt></code></p></blockquote><h4><a name="tag_03_740_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 convert the initial portion of the string pointed to by <i>nptr</i> to <b>double</b>, <b>float</b>, and<b>long double</b> representation, respectively. First, they decompose the input string into three parts:</p><ol><li><p>An initial, possibly empty, sequence of white-space characters (as specified by <a href="../functions/isspace.html"><i>isspace</i>()</a>)</p></li><li><p>A subject sequence interpreted as a floating-point constant or representing infinity or NaN</p></li><li><p>A final string of one or more unrecognized characters, including the terminating null byte of the input string</p></li></ol><p>Then they shall attempt to convert the subject sequence to a floating-point number, and return the result.</p><p>The expected form of the subject sequence is an optional plus or minus sign, then one of the following:</p><ul><li><p>A non-empty sequence of decimal digits optionally containing a radix character, then an optional exponent part</p></li><li><p>A 0x or 0X, then a non-empty sequence of hexadecimal digits optionally containing a radix character, then an optional binaryexponent part</p></li><li><p>One of INF or INFINITY, ignoring case</p></li><li><p>One of NAN or NAN(<i>n-char-sequence<small><sub>opt</sub></small></i>), ignoring case in the NAN part, where:</p><pre><tt>n-char-sequence: digit nondigit n-char-sequence digit n-char-sequence nondigit</tt></pre></li></ul><p>The subject sequence is defined as the longest initial subsequence of the input string, starting with the first non-white-spacecharacter, that is of the expected form. The subject sequence contains no characters if the input string is not of the expectedform.</p><p>If the subject sequence has the expected form for a floating-point number, the sequence of characters starting with the firstdigit or the decimal-point character (whichever occurs first) shall be interpreted as a floating constant of the C language, exceptthat the radix character shall be used in place of a period, and that if neither an exponent part nor a radix character appears ina decimal floating-point number, or if a binary exponent part does not appear in a hexadecimal floating-point number, an exponentpart of the appropriate type with value zero is assumed to follow the last digit in the string. If the subject sequence begins witha minus sign, the sequence shall be interpreted as negated. A character sequence INF or INFINITY shall be interpreted as aninfinity, if representable in the return type, else as if it were a floating constant that is too large for the range of the returntype. A character sequence NAN or NAN(<i>n-char-sequence<small><sub>opt</sub></small></i>) shall be interpreted as a quiet NaN, ifsupported in the return type, else as if it were a subject sequence part that does not have the expected form; the meaning of the<i>n</i>-char sequences is implementation-defined. A pointer to the final string is stored in the object pointed to by<i>endptr</i>, provided that <i>endptr</i> is not a null pointer.</p><p>If the subject sequence has the hexadecimal form and FLT_RADIX is a power of 2, the value resulting from the conversion iscorrectly rounded.</p><p><sup>[<a href="javascript:open_code('CX')">CX</a>]</sup> <img src="../images/opt-start.gif" alt="[Option Start]" border="0"> Theradix character is defined in the program's locale (category <i>LC_NUMERIC ).</i> In the POSIX locale, or in a locale where theradix character is not defined, the radix character shall default to a period ( <tt>'.'</tt> ). <img src="../images/opt-end.gif"alt="[Option End]" border="0"></p><p>In other than the C <sup>[<a href="javascript:open_code('CX')">CX</a>]</sup> <img src="../images/opt-start.gif" alt="[Option Start]" border="0"> or POSIX <img src="../images/opt-end.gif" alt="[Option End]" border="0"> locales, otherimplementation-defined subject sequences may be accepted.</p><p>If the subject sequence is empty or does not have the expected form, no conversion shall be performed; the value of <i>str</i>is stored in the object pointed to by <i>endptr</i>, provided that <i>endptr</i> is not a null pointer.</p><p><sup>[<a href="javascript:open_code('CX')">CX</a>]</sup> <img src="../images/opt-start.gif" alt="[Option Start]" border="0"> The<i>strtod</i>() function shall not change the setting of <i>errno</i> if successful.</p><p>Since 0 is returned on error and is also a valid return on success, an application wishing to check for error situations shouldset <i>errno</i> to 0, then call <i>strtod</i>(), <i>strtof</i>(), or <i>strtold</i>(), then check <i>errno</i>. <img src="../images/opt-end.gif" alt="[Option End]" border="0"></p></blockquote><h4><a name="tag_03_740_04"></a>RETURN VALUE</h4><blockquote><p>Upon successful completion, these functions shall return the converted value. If no conversion could be performed, 0 shall bereturned, and <i>errno</i> may be set to [EINVAL].</p><p>If the correct value is outside the range of representable values, ±HUGE_VAL, ±HUGE_VALF, or ±HUGE_VALLshall be returned (according to the sign of the value), and <i>errno</i> shall be set to [ERANGE].</p><p>If the correct value would cause an underflow, a value whose magnitude is no greater than the smallest normalized positivenumber in the return type shall be returned and <i>errno</i> set to [ERANGE].</p></blockquote><h4><a name="tag_03_740_05"></a>ERRORS</h4><blockquote><p>These functions shall fail if:</p><dl compact><dt>[ERANGE]</dt><dd>The value to be returned would cause overflow <sup>[<a href="javascript:open_code('CX')">CX</a>]</sup> <img src="../images/opt-start.gif" alt="[Option Start]" border="0"> or underflow. <img src="../images/opt-end.gif" alt="[Option End]"border="0"></dd></dl><p>These functions may fail if:</p><dl compact><dt>[EINVAL]</dt><dd><sup>[<a href="javascript:open_code('CX')">CX</a>]</sup> <img src="../images/opt-start.gif" alt="[Option Start]" border="0"> Noconversion could be performed. <img src="../images/opt-end.gif" alt="[Option End]" border="0"></dd></dl></blockquote><hr><div class="box"><em>The following sections are informative.</em></div><h4><a name="tag_03_740_06"></a>EXAMPLES</h4><blockquote><p>None.</p></blockquote><h4><a name="tag_03_740_07"></a>APPLICATION USAGE</h4><blockquote><p>If the subject sequence has the hexadecimal form and FLT_RADIX is not a power of 2, and the result is not exactly representable,the result should be one of the two numbers in the appropriate internal format that are adjacent to the hexadecimal floating sourcevalue, with the extra stipulation that the error should have a correct sign for the current rounding direction.</p><p>If the subject sequence has the decimal form and at most DECIMAL_DIG (defined in <a href="../basedefs/float.h.html"><i><float.h></i></a>) significant digits, the result should be correctly rounded. If the subjectsequence <i>D</i> has the decimal form and more than DECIMAL_DIG significant digits, consider the two bounding, adjacent decimalstrings <i>L</i> and <i>U</i>, both having DECIMAL_DIG significant digits, such that the values of <i>L</i>, <i>D</i>, and <i>U</i>satisfy <i>L</i> <= <i>D</i> <= <i>U</i>. The result should be one of the (equal or adjacent) values that would be obtainedby correctly rounding <i>L</i> and <i>U</i> according to the current rounding direction, with the extra stipulation that the errorwith respect to <i>D</i> should have a correct sign for the current rounding direction.</p><p>The changes to <i>strtod</i>() introduced by the ISO/IEC 9899:1999 standard can alter the behavior of well-formedapplications complying with the ISO/IEC 9899:1990 standard and thus earlier versions of the base documents. One such examplewould be:</p><pre><tt>intwhat_kind_of_number (char *s){ char *endp; double d; long l;<br> d = strtod(s, &endp); if (s != endp && *endp == `\0') printf("It's a float with value %g\n", d); else { l = strtol(s, &endp, 0); if (s != endp && *endp == `\0') printf("It's an integer with value %ld\n", 1); else return 1; } return 0;}</tt></pre><p>If the function is called with:</p><pre><tt>what_kind_of_number ("0x10")</tt></pre><p>an ISO/IEC 9899:1990 standard-compliant library will result in the function printing:</p><pre><tt>It's an integer with value 16</tt></pre><p>With the ISO/IEC 9899:1999 standard, the result is:</p><pre><tt>It's a float with value 16</tt></pre><p>The change in behavior is due to the inclusion of floating-point numbers in hexadecimal notation without requiring that either adecimal point or the binary exponent be present.</p></blockquote><h4><a name="tag_03_740_08"></a>RATIONALE</h4><blockquote><p>None.</p></blockquote><h4><a name="tag_03_740_09"></a>FUTURE DIRECTIONS</h4><blockquote><p>None.</p></blockquote><h4><a name="tag_03_740_10"></a>SEE ALSO</h4><blockquote><p><a href="isspace.html"><i>isspace</i>()</a>, <a href="localeconv.html"><i>localeconv</i>()</a>, <a href="scanf.html"><i>scanf</i>()</a>, <a href="setlocale.html"><i>setlocale</i>()</a>, <a href="strtol.html"><i>strtol</i>()</a>, theBase Definitions volume of IEEE Std 1003.1-2001, <a href="../basedefs/xbd_chap07.html">Chapter 7, Locale</a>, <a href="../basedefs/float.h.html"><i><float.h></i></a>, <a href="../basedefs/stdlib.h.html"><i><stdlib.h></i></a></p></blockquote><h4><a name="tag_03_740_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_740_12"></a>Issue 5</h4><blockquote><p>The DESCRIPTION is updated to indicate that <i>errno</i> is not changed if the function is successful.</p></blockquote><h4><a name="tag_03_740_13"></a>Issue 6</h4><blockquote><p>Extensions beyond the ISO C standard are marked.</p><p>The following new requirements on POSIX implementations derive from alignment with the Single UNIX Specification:</p><ul><li><p>In the RETURN VALUE and ERRORS sections, the [EINVAL] optional error condition is added if no conversion could be performed.</p></li></ul><p>The following changes are made for alignment with the ISO/IEC 9899:1999 standard:</p><ul><li><p>The <i>strtod</i>() function is updated.</p></li><li><p>The <i>strtof</i>() and <i>strtold</i>() functions are added.</p></li><li><p>The DESCRIPTION is extensively revised.</p></li></ul><p>ISO/IEC 9899:1999 standard, Technical Corrigendum 1 is incorporated.</p><p>IEEE Std 1003.1-2001/Cor 1-2002, item XSH/TC1/D6/61 is applied, correcting the second paragraph in the RETURN VALUEsection. This change clarifies the sign of the return value.</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>⌨️ 快捷键说明
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