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<title>GNU Scientific Library – Reference Manual: Special Function Usage</title>
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<a name="Special-Function-Usage"></a>
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<p>
Next: <a href="The-gsl_005fsf_005fresult-struct.html#The-gsl_005fsf_005fresult-struct" accesskey="n" rel="next">The gsl_sf_result struct</a>, Up: <a href="Special-Functions.html#Special-Functions" accesskey="u" rel="up">Special Functions</a> [<a href="Function-Index.html#Function-Index" title="Index" rel="index">Index</a>]</p>
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<a name="Usage"></a>
<h3 class="section">7.1 Usage</h3>
<p>The special functions are available in two calling conventions, a
<em>natural form</em> which returns the numerical value of the function and
an <em>error-handling form</em> which returns an error code. The two types
of function provide alternative ways of accessing the same underlying
code.
</p>
<p>The <em>natural form</em> returns only the value of the function and can be
used directly in mathematical expressions. For example, the following
function call will compute the value of the Bessel function
<em>J_0(x)</em>,
</p>
<div class="example">
<pre class="example">double y = gsl_sf_bessel_J0 (x);
</pre></div>
<p>There is no way to access an error code or to estimate the error using
this method. To allow access to this information the alternative
error-handling form stores the value and error in a modifiable argument,
</p>
<div class="example">
<pre class="example">gsl_sf_result result;
int status = gsl_sf_bessel_J0_e (x, &result);
</pre></div>
<p>The error-handling functions have the suffix <code>_e</code>. The returned
status value indicates error conditions such as overflow, underflow or
loss of precision. If there are no errors the error-handling functions
return <code>GSL_SUCCESS</code>.
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