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<h3 class="section">9.3 Array Sort Function</h3>

<p><a name="index-sort-function-_0028for-arrays_0029-828"></a><a name="index-quick-sort-function-_0028for-arrays_0029-829"></a><a name="index-array-sort-function-830"></a>
To sort an array using an arbitrary comparison function, use the
<code>qsort</code> function.  The prototype for this function is in
<samp><span class="file">stdlib.h</span></samp>. 
<a name="index-stdlib_002eh-831"></a>
<!-- stdlib.h -->
<!-- ISO -->

<div class="defun">
&mdash; Function: void <b>qsort</b> (<var>void *array, size_t count, size_t size, comparison_fn_t compare</var>)<var><a name="index-qsort-832"></a></var><br>
<blockquote><p>The <var>qsort</var> function sorts the array <var>array</var>.  The array contains
<var>count</var> elements, each of which is of size <var>size</var>.

        <p>The <var>compare</var> function is used to perform the comparison on the
array elements.  This function is called with two pointer arguments and
should return an integer less than, equal to, or greater than zero
corresponding to whether its first argument is considered less than,
equal to, or greater than its second argument.

        <p><a name="index-stable-sorting-833"></a><strong>Warning:</strong> If two objects compare as equal, their order after
sorting is unpredictable.  That is to say, the sorting is not stable. 
This can make a difference when the comparison considers only part of
the elements.  Two elements with the same sort key may differ in other
respects.

        <p>If you want the effect of a stable sort, you can get this result by
writing the comparison function so that, lacking other reason
distinguish between two elements, it compares them by their addresses. 
Note that doing this may make the sorting algorithm less efficient, so
do it only if necessary.

        <p>Here is a simple example of sorting an array of doubles in numerical
order, using the comparison function defined above (see <a href="Comparison-Functions.html#Comparison-Functions">Comparison Functions</a>):

     <pre class="smallexample">          {
            double *array;
            int size;
            ...
            qsort (array, size, sizeof (double), compare_doubles);
          }
</pre>
        <p>The <code>qsort</code> function derives its name from the fact that it was
originally implemented using the &ldquo;quick sort&rdquo; algorithm.

        <p>The implementation of <code>qsort</code> in this library might not be an
in-place sort and might thereby use an extra amount of memory to store
the array. 
</p></blockquote></div>

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