QSORT(3)

NAME
qsort, heapsort - sort functions

SYNOPSIS
#include <stdlib.h>
void
qsort(void *base, size_t nmemb, size_t size, int (*compar)(const void *, const void *));
int
heapsort(void *base, size_t nmemb, size_t size, int (*compar)(const void *, const void *));

DESCRIPTION
The qsort() function is a modified partition-exchange sort, or quicksort. The heapsort() function is a modified selection sort.

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.

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.

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 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 the traditional O N**2 worst-case behavior.

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. Its only advantage over qsort() is that it uses no additional memory.

RETURN VALUES
The qsort() function returns no value.

Upon successful completion, heapsort() returns 0. Otherwise, it returns -1 and the global variable errno is set to indicate the error.

ERRORS
The heapsort() function succeeds unless:

[EINVAL]
    The size argument is zero.

COMPATIBILITY
Previous versions of qsort() did not permit the comparison routine to itself call qsort(3). This is no longer true.

SEE ALSO
sort(1), radixsort(3) Hoare, C.A.R., 1962, Quicksort, The Computer Journal, 5:1, pp. 10-15. Williams, J.W.J, 1964, Heapsort, Communications of the ACM, 7:1, pp. 347-348. Knuth, D.E., 1968, The Art of Computer Programming, Vol. 3, Sorting and Searching, pp. 114-123, 145-149.

STANDARDS
The qsort() function conforms to ansiC.