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| Previous: <a rel="previous" accesskey="p" href="Parsing-of-Integers.html#Parsing-of-Integers">Parsing of Integers</a>, |
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| <h4 class="subsection">20.11.2 Parsing of Floats</h4> |
| |
| <p><a name="index-stdlib_002eh-2569"></a>The ‘<samp><span class="samp">str</span></samp>’ functions are declared in <samp><span class="file">stdlib.h</span></samp> and those |
| beginning with ‘<samp><span class="samp">wcs</span></samp>’ are declared in <samp><span class="file">wchar.h</span></samp>. One might |
| wonder about the use of <code>restrict</code> in the prototypes of the |
| functions in this section. It is seemingly useless but the ISO C<!-- /@w --> |
| standard uses it (for the functions defined there) so we have to do it |
| as well. |
| |
| <!-- stdlib.h --> |
| <!-- ISO --> |
| <div class="defun"> |
| — Function: double <b>strtod</b> (<var>const char *restrict string, char **restrict tailptr</var>)<var><a name="index-strtod-2570"></a></var><br> |
| <blockquote><p>The <code>strtod</code> (“string-to-double”) function converts the initial |
| part of <var>string</var> to a floating-point number, which is returned as a |
| value of type <code>double</code>. |
| |
| <p>This function attempts to decompose <var>string</var> as follows: |
| |
| <ul> |
| <li>A (possibly empty) sequence of whitespace characters. Which characters |
| are whitespace is determined by the <code>isspace</code> function |
| (see <a href="Classification-of-Characters.html#Classification-of-Characters">Classification of Characters</a>). These are discarded. |
| |
| <li>An optional plus or minus sign (‘<samp><span class="samp">+</span></samp>’ or ‘<samp><span class="samp">-</span></samp>’). |
| |
| <li>A floating point number in decimal or hexadecimal format. The |
| decimal format is: |
| <ul> |
| <li>A nonempty sequence of digits optionally containing a decimal-point |
| character—normally ‘<samp><span class="samp">.</span></samp>’, but it depends on the locale |
| (see <a href="General-Numeric.html#General-Numeric">General Numeric</a>). |
| |
| <li>An optional exponent part, consisting of a character ‘<samp><span class="samp">e</span></samp>’ or |
| ‘<samp><span class="samp">E</span></samp>’, an optional sign, and a sequence of digits. |
| |
| </ul> |
| |
| <p>The hexadecimal format is as follows: |
| <ul> |
| <li>A 0x or 0X followed by a nonempty sequence of hexadecimal digits |
| optionally containing a decimal-point character—normally ‘<samp><span class="samp">.</span></samp>’, but |
| it depends on the locale (see <a href="General-Numeric.html#General-Numeric">General Numeric</a>). |
| |
| <li>An optional binary-exponent part, consisting of a character ‘<samp><span class="samp">p</span></samp>’ or |
| ‘<samp><span class="samp">P</span></samp>’, an optional sign, and a sequence of digits. |
| |
| </ul> |
| |
| <li>Any remaining characters in the string. If <var>tailptr</var> is not a null |
| pointer, a pointer to this tail of the string is stored in |
| <code>*</code><var>tailptr</var>. |
| </ul> |
| |
| <p>If the string is empty, contains only whitespace, or does not contain an |
| initial substring that has the expected syntax for a floating-point |
| number, no conversion is performed. In this case, <code>strtod</code> returns |
| a value of zero and the value returned in <code>*</code><var>tailptr</var> is the |
| value of <var>string</var>. |
| |
| <p>In a locale other than the standard <code>"C"</code> or <code>"POSIX"</code> locales, |
| this function may recognize additional locale-dependent syntax. |
| |
| <p>If the string has valid syntax for a floating-point number but the value |
| is outside the range of a <code>double</code>, <code>strtod</code> will signal |
| overflow or underflow as described in <a href="Math-Error-Reporting.html#Math-Error-Reporting">Math Error Reporting</a>. |
| |
| <p><code>strtod</code> recognizes four special input strings. The strings |
| <code>"inf"</code> and <code>"infinity"</code> are converted to &infin;, |
| or to the largest representable value if the floating-point format |
| doesn't support infinities. You can prepend a <code>"+"</code> or <code>"-"</code> |
| to specify the sign. Case is ignored when scanning these strings. |
| |
| <p>The strings <code>"nan"</code> and <code>"nan(</code><var>chars<small class="dots">...</small></var><code>)"</code> are converted |
| to NaN. Again, case is ignored. If <var>chars<small class="dots">...</small></var> are provided, they |
| are used in some unspecified fashion to select a particular |
| representation of NaN (there can be several). |
| |
| <p>Since zero is a valid result as well as the value returned on error, you |
| should check for errors in the same way as for <code>strtol</code>, by |
| examining <var>errno</var> and <var>tailptr</var>. |
| </p></blockquote></div> |
| |
| <!-- stdlib.h --> |
| <!-- ISO --> |
| <div class="defun"> |
| — Function: float <b>strtof</b> (<var>const char *string, char **tailptr</var>)<var><a name="index-strtof-2571"></a></var><br> |
| <blockquote><!-- stdlib.h --> |
| <!-- ISO --> |
| — Function: long double <b>strtold</b> (<var>const char *string, char **tailptr</var>)<var><a name="index-strtold-2572"></a></var><br> |
| <blockquote><p>These functions are analogous to <code>strtod</code>, but return <code>float</code> |
| and <code>long double</code> values respectively. They report errors in the |
| same way as <code>strtod</code>. <code>strtof</code> can be substantially faster |
| than <code>strtod</code>, but has less precision; conversely, <code>strtold</code> |
| can be much slower but has more precision (on systems where <code>long |
| double</code> is a separate type). |
| |
| <p>These functions have been GNU extensions and are new to ISO C99<!-- /@w -->. |
| </p></blockquote></div> |
| |
| <!-- wchar.h --> |
| <!-- ISO --> |
| <div class="defun"> |
| — Function: double <b>wcstod</b> (<var>const wchar_t *restrict string, wchar_t **restrict tailptr</var>)<var><a name="index-wcstod-2573"></a></var><br> |
| <blockquote><!-- stdlib.h --> |
| <!-- ISO --> |
| — Function: float <b>wcstof</b> (<var>const wchar_t *string, wchar_t **tailptr</var>)<var><a name="index-wcstof-2574"></a></var><br> |
| <blockquote><!-- stdlib.h --> |
| <!-- ISO --> |
| — Function: long double <b>wcstold</b> (<var>const wchar_t *string, wchar_t **tailptr</var>)<var><a name="index-wcstold-2575"></a></var><br> |
| <blockquote><p>The <code>wcstod</code>, <code>wcstof</code>, and <code>wcstol</code> functions are |
| equivalent in nearly all aspect to the <code>strtod</code>, <code>strtof</code>, and |
| <code>strtold</code> functions but it handles wide character string. |
| |
| <p>The <code>wcstod</code> function was introduced in Amendment 1<!-- /@w --> of ISO C90<!-- /@w -->. The <code>wcstof</code> and <code>wcstold</code> functions were introduced in |
| ISO C99<!-- /@w -->. |
| </p></blockquote></div> |
| |
| <!-- stdlib.h --> |
| <!-- ISO --> |
| <div class="defun"> |
| — Function: double <b>atof</b> (<var>const char *string</var>)<var><a name="index-atof-2576"></a></var><br> |
| <blockquote><p>This function is similar to the <code>strtod</code> function, except that it |
| need not detect overflow and underflow errors. The <code>atof</code> function |
| is provided mostly for compatibility with existing code; using |
| <code>strtod</code> is more robust. |
| </p></blockquote></div> |
| |
| <p>The GNU C library also provides ‘<samp><span class="samp">_l</span></samp>’ versions of these functions, |
| which take an additional argument, the locale to use in conversion. |
| See <a href="Parsing-of-Integers.html#Parsing-of-Integers">Parsing of Integers</a>. |
| |
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