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<title>GNU Scientific Library &ndash; Reference Manual: Using GSL error reporting in your own functions</title>

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<body lang="en" bgcolor="#FFFFFF" text="#000000" link="#0000FF" vlink="#800080" alink="#FF0000">
<a name="Using-GSL-error-reporting-in-your-own-functions"></a>
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<p>
Next: <a href="Error-Reporting-Examples.html#Error-Reporting-Examples" accesskey="n" rel="next">Error Reporting Examples</a>, Previous: <a href="Error-Handlers.html#Error-Handlers" accesskey="p" rel="previous">Error Handlers</a>, Up: <a href="Error-Handling.html#Error-Handling" accesskey="u" rel="up">Error Handling</a> &nbsp; [<a href="Function-Index.html#Function-Index" title="Index" rel="index">Index</a>]</p>
</div>
<hr>
<a name="Using-GSL-error-reporting-in-your-own-functions-1"></a>
<h3 class="section">3.4 Using GSL error reporting in your own functions</h3>
<a name="index-error-handling-macros"></a>
<p>If you are writing numerical functions in a program which also uses GSL
code you may find it convenient to adopt the same error reporting
conventions as in the library.
</p>
<p>To report an error you need to call the function <code>gsl_error</code> with a
string describing the error and then return an appropriate error code
from <code>gsl_errno.h</code>, or a special value, such as <code>NaN</code>.  For
convenience the file <samp>gsl_errno.h</samp> defines two macros which carry
out these steps:
</p>
<dl>
<dt><a name="index-GSL_005fERROR"></a>Macro: <strong>GSL_ERROR</strong> <em>(<var>reason</var>, <var>gsl_errno</var>)</em></dt>
<dd>
<p>This macro reports an error using the GSL conventions and returns a
status value of <code>gsl_errno</code>.  It expands to the following code fragment,
</p>
<div class="example">
<pre class="example">gsl_error (reason, __FILE__, __LINE__, gsl_errno);
return gsl_errno;
</pre></div>

<p>The macro definition in <samp>gsl_errno.h</samp> actually wraps the code
in a <code>do { ... } while (0)</code> block to prevent possible
parsing problems.
</p></dd></dl>

<p>Here is an example of how the macro could be used to report that a
routine did not achieve a requested tolerance.  To report the error the
routine needs to return the error code <code>GSL_ETOL</code>.
</p>
<div class="example">
<pre class="example">if (residual &gt; tolerance) 
  {
    GSL_ERROR(&quot;residual exceeds tolerance&quot;, GSL_ETOL);
  }
</pre></div>

<dl>
<dt><a name="index-GSL_005fERROR_005fVAL"></a>Macro: <strong>GSL_ERROR_VAL</strong> <em>(<var>reason</var>, <var>gsl_errno</var>, <var>value</var>)</em></dt>
<dd>
<p>This macro is the same as <code>GSL_ERROR</code> but returns a user-defined
value of <var>value</var> instead of an error code.  It can be used for
mathematical functions that return a floating point value.
</p></dd></dl>

<p>The following example shows how to return a <code>NaN</code> at a mathematical
singularity using the <code>GSL_ERROR_VAL</code> macro,
</p>
<div class="example">
<pre class="example">if (x == 0) 
  {
    GSL_ERROR_VAL(&quot;argument lies on singularity&quot;, 
                  GSL_ERANGE, GSL_NAN);
  }
</pre></div>


<hr>
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<p>
Next: <a href="Error-Reporting-Examples.html#Error-Reporting-Examples" accesskey="n" rel="next">Error Reporting Examples</a>, Previous: <a href="Error-Handlers.html#Error-Handlers" accesskey="p" rel="previous">Error Handlers</a>, Up: <a href="Error-Handling.html#Error-Handling" accesskey="u" rel="up">Error Handling</a> &nbsp; [<a href="Function-Index.html#Function-Index" title="Index" rel="index">Index</a>]</p>
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