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(libc.info)Array Sort Function

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Array Sort Function
===================

   To sort an array using an arbitrary comparison function, use the
`qsort' function.  The prototype for this function is in `stdlib.h'.

 - Function: void qsort (void *ARRAY, size_t COUNT, size_t SIZE,
          comparison_fn_t COMPARE)
     The QSORT function sorts the array ARRAY.  The array contains
     COUNT elements, each of which is of size SIZE.

     The COMPARE 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.

     *Warning:* 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.

     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.

     Here is a simple example of sorting an array of doubles in
     numerical order, using the comparison function defined above
     (Note: Comparison Functions):

          {
            double *array;
            int size;
            ...
            qsort (array, size, sizeof (double), compare_doubles);
          }

     The `qsort' function derives its name from the fact that it was
     originally implemented using the "quick sort" algorithm.

     The implementation of `qsort' in this library might not be an
     in-place sort and might thereby use an extra amount of memory to
     store the array.