The qsort() function sorts an array with nmemb elements of size
size. The base argument points to the start of the array.
The contents of the array are sorted in ascending order according to a
comparison function pointed to by compar, which is called with two
arguments that point to the objects being compared.
The comparison function must return an integer less than, equal to, or greater
than zero if the first argument is considered to be respectively less than,
equal to, or greater than the second. If two members compare as equal, their
order in the sorted array is undefined.