NAME
qsort, heapsort,
mergesort —
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 *));
int
mergesort(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 mergesort() function is a modified merge sort
with exponential search intended for sorting data with pre-existing
order.
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 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 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.
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.
RETURN VALUES
The qsort() function returns no value.
Upon successful completion, heapsort() and
mergesort() return 0. Otherwise, they return -1 and
the global variable errno is set to indicate the
error.
ERRORS
The heapsort() and
mergesort() functions succeed unless:
- [
EINVAL] - The size argument is zero, or the
size argument to
mergesort() is less than “sizeof(void *) / 2”. - [
ENOMEM] heapsort() ormergesort() were unable to allocate memory.
SEE ALSO
Hoare, C.A.R., Quicksort, The Computer Journal, 5:1, pp. 10-15, 1962.
Williams, J.W.J, Heapsort, Communications of the ACM, 7:1, pp. 347-348, 1964.
Knuth, D.E., Sorting and Searching, The Art of Computer Programming, Vol. 3, pp. 114-123, 145-149, 1968.
McIlroy, P.M., Optimistic Sorting and Information Theoretic Complexity, Fourth Annual ACM-SIAM Symposium on Discrete Algorithms, pp. 467-464, January 1993.
Bentley, J.L. and McIlroy, M.D., Engineering a Sort Function, Software - Practice and Experience, Vol. 23(11), pp. 1249-1265, November 1993.
STANDARDS
Previous versions of qsort() did not
permit the comparison routine itself to call
qsort(). This is no longer true.
The qsort() function conforms to
ANSI X3.159-1989
(“ANSI C89”).
HISTORY
A qsort() function first appeared in
Version 3 AT&T UNIX.