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<a name="atof_002c-atoff_002d_002d_002dstring-to-double-or-float"></a>
<h3 class="section">2.7 <code>atof</code>, <code>atoff</code>—string to double or float</h3>
<a name="index-atof"></a>
<a name="index-atoff"></a>
<p><strong>Synopsis</strong>
</p><div class="example">
<pre class="example">#include <stdlib.h>
double atof(const char *<var>s</var>);
float atoff(const char *<var>s</var>);
</pre></div>
<p><strong>Description</strong><br>
<code>atof</code> converts the initial portion of a string to a <code>double</code>.
<code>atoff</code> converts the initial portion of a string to a <code>float</code>.
</p>
<p>The functions parse the character string <var>s</var>,
locating a substring which can be converted to a floating-point
value. The substring must match the format:
</p><div class="smallexample">
<pre class="smallexample"> [+|-]<var>digits</var>[.][<var>digits</var>][(e|E)[+|-]<var>digits</var>]
</pre></div>
<p>The substring converted is the longest initial
fragment of <var>s</var> that has the expected format, beginning with
the first non-whitespace character. The substring
is empty if <code>str</code> is empty, consists entirely
of whitespace, or if the first non-whitespace character is
something other than <code>+</code>, <code>-</code>, <code>.</code>, or a digit.
</p>
<p><code>atof(<var>s</var>)</code> is implemented as <code>strtod(<var>s</var>, NULL)</code>.
<code>atoff(<var>s</var>)</code> is implemented as <code>strtof(<var>s</var>, NULL)</code>.
</p>
<br>
<p><strong>Returns</strong><br>
<code>atof</code> returns the converted substring value, if any, as a
<code>double</code>; or <code>0.0</code>, if no conversion could be performed.
If the correct value is out of the range of representable values, plus
or minus <code>HUGE_VAL</code> is returned, and <code>ERANGE</code> is stored in
<code>errno</code>.
If the correct value would cause underflow, <code>0.0</code> is returned
and <code>ERANGE</code> is stored in <code>errno</code>.
</p>
<p><code>atoff</code> obeys the same rules as <code>atof</code>, except that it
returns a <code>float</code>.
</p>
<br>
<p><strong>Portability</strong><br>
<code>atof</code> is ANSI C. <code>atof</code>, <code>atoi</code>, and <code>atol</code> are subsumed by <code>strod</code>
and <code>strol</code>, but are used extensively in existing code. These functions are
less reliable, but may be faster if the argument is verified to be in a valid
range.
</p>
<p>Supporting OS subroutines required: <code>close</code>, <code>fstat</code>, <code>isatty</code>,
<code>lseek</code>, <code>read</code>, <code>sbrk</code>, <code>write</code>.
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