Quick Sort Algorithm in C

Quicksort, or partition-exchange sort, is a sorting algorithm  that, on average, makes O(n log n) comparisons to sort n  items.  It was developed by Tony Hoare. Quicksort is  faster in practice than other O(n log n) algorithms such  as Bubble sort or Insertion Sort. Quicksort can be  implemented with an in-place partitioning algorithm, so  the entire sort can be done with only O(log n) additional space.
Quicksort is a comparison sort and is not a stable sort.
Its complexity is as follows: Best Case - O(n log n) Worst Case - O(n^2)  Average Case - O(n log n)
Quicksort is a divide and conquer algorithm. Quicksort first  divides a large list into two smaller sub-lists: the low  elements and the high elements. Quicksort can then  recursively sort the sub-lists.
The steps are: 1. Pick an element, called a pivot, from the list.
2. Reorder the list so that all elements with values less  than the pivot come before the pivot, while all elements  with values greater than the pivot come after it (equal  values can go either way). After this partitioning, the  pivot is in its final position. This is called the partition  operation.
3. Recursively sort the sub-list of lesser elements and  the sub-list of greater elements. The base case of the  recursion are lists of size zero or one, which never  need to be sorted.

Algorithm/Pseudo-code:

function quicksort('array')
  if length('array') ≤ 1
     return 'array' // an array of zero or 

                   //one elements is already sorted
  select and remove a pivot value 'pivot' from 'array'
  create empty lists 'less' and 'greater'
  for each 'x' in 'array'
     if 'x' ≤ 'pivot' then append 'x' to 'less'
     else append 'x' to 'greater'
  return concatenate(quicksort('less'), 'pivot', 

                quicksort('greater')) // two recursive calls

Here is an Image depicting Quick Sort:

In-place partition in action on a small list. 
The boxed element is the pivot element, 
blue elements are less or equal, and red 
elements are larger.