the code comes from http://programminggeeks.com/c-code-for-quick-sort/

quicksort.c

/*
 * Reference
 * [1] http://programminggeeks.com/c-code-for-quick-sort/ 
 *
 * just add some comments, ALL CODES WAS DONE BY reference[1].
 * If that's not cool, just tell me and I'll delete this.
 */
#include "stdio.h"

int split ( int*, int, int ) ;

void quicksort ( int *, int, int ) ;

int main( )
{
	int arr[10] = { 11, 2, 9, 13, 57, 25, 17, 1, 90, 3 } ;
	int i ;


	quicksort ( arr, 0, 9 ) ;

	printf ( "\nArray after sorting:\n") ;

	for ( i = 0 ; i <= 9 ; i++ )
		printf ( "%d\t", arr[i] ) ;

	return 0;
}

void quicksort ( int a[ ], int lower, int upper )
{
	int i ;
	if ( upper > lower )
	{
		i = split ( a, lower, upper ) ;/* Split into two arrays */
		quicksort ( a, lower, i - 1 ) ;/* Sort both arrays */
		quicksort ( a, i + 1, upper ) ;
	}
}

int split ( int a[ ], int lower, int upper )
{
	int i, p, q, t ;

	p = lower + 1 ;
	q = upper ;
	i = a[lower] ;

	while ( q >= p )
	{
		while ( a[p] < i ) /* iterator, if left side element is less than i(split symbol), check next right one */
			p++ ;

		while ( a[q] > i ) /* another iterator, if right side element is larger than i(split symobl), check left one */
			q-- ;

		if ( q > p ) /* iterator over, at that time, elements before p is less than i (except i), and elements after q is greater than i */
		{
			t = a[p] ;
			a[p] = a[q] ;
			a[q] = t ;
		} /* switch p and q element, so p element is less than i and q element is greater than i */
	}
	
	/* after that while, the array looks like that(x<i,y>i)
	 * [i x x x x x y y y y y]
	 *            ^ ^
	 *			  q p
	 */

	t = a[lower] ;
	a[lower] = a[q] ;
	a[q] = t ;
	
	/* after that swap, the array looks like that(x<i,y>i)
	 * [x x x x x i y y y y y]
	 *            ^ ^
	 *			  q p
	 */
	

	return q ;
}