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DESCRIPTION

The qsort() and heapsort() functions sort an array of nmemb objects, the initial member of which is pointed to by base. The size of each object is specified by size. mergesort() behaves similarly, but requires that size be greater than “sizeof(void *) / 2”.

The contents of the array base are sorted in ascending order according to a comparison function pointed to by compar, which requires two arguments pointing to the objects being compared.

The comparison function must return an int 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.

The functions qsort() and heapsort() are not stable, that is, if two members compare as equal, their order in the sorted array is undefined. The function mergesort() is stable.

The qsort() function is an implementation of C.A.R. Hoare's “quicksort” algorithm, a variant of partition-exchange sorting; in particular, see D.E. Knuth's Algorithm Q. qsort() takes O N lg N average time. This implementation uses median selection to avoid its O N**2 worst-case behavior and will fall back to heapsort() if the recursion depth exceeds 2 lg N.

The heapsort() function is an implementation of J.W.J. William's “heapsort” algorithm, a variant of selection sorting; in particular, see D.E. Knuth's Algorithm H. heapsort() takes O N lg N worst-case time. This implementation of heapsort() is implemented without recursive function calls.

The function mergesort() requires additional memory of size nmemb * size bytes; it should be used only when space is not at a premium. mergesort() is optimized for data with pre-existing order; its worst case time is O N lg N; its best case is O N.

Normally, qsort() is faster than mergesort(), which is faster than heapsort(). Memory availability and pre-existing order in the data can make this untrue.

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

char *array[] = { "XX", "YYY", "Z" };
#define N (sizeof(array) / sizeof(array[0]))

int
cmp(const void *a, const void *b)
{
	/*
	 * a and b point to elements of the array.
	 * Cast and dereference to obtain the actual elements,
	 * which are also pointers in this case.
	 */
	size_t lena = strlen(*(const char **)a);
	size_t lenb = strlen(*(const char **)b);
	/*
	 * Do not subtract the lengths. The difference between values
	 * cannot be represented by an int.
	 */
	return lena < lenb ? -1 : lena > lenb;
}

int
main()
{
	size_t i;

	qsort(array, N, sizeof(array[0]), cmp);
	for (i =  0; i < N; i++)
		printf("%s\n", array[i]);
}