
QSORT(3) BSD Library Functions Manual QSORT(3)
NAME
qsort, heapsort, mergesort  sort functions
LIBRARY
Standard C Library (libc, lc)
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 partitionexchange 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 preexisting 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 func
tion pointed to by compar, which requires two arguments pointing to the objects being com
pared.
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 vari
ant of partitionexchange 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
worstcase 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 worstcase time. Its only advantage over qsort() is that it uses almost no addi
tional memory; while qsort() does not allocate memory, it is implemented using recursion.
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 preexist
ing order; its worst case time is O N lg N; its best case is O N.
Normally, qsort() is faster than mergesort() is faster than heapsort(). Memory availability
and preexisting 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() function succeeds unless:
[EINVAL] The size argument is zero, or, the size argument to mergesort() is less
than ``sizeof(void *) / 2''.
[ENOMEM] heapsort() or mergesort() were unable to allocate memory.
COMPATIBILITY
Previous versions of qsort() did not permit the comparison routine itself to call qsort().
This is no longer true.
SEE ALSO
sort(1), radixsort(3)
Hoare, C.A.R., "Quicksort", The Computer Journal, 5:1, pp. 1015, 1962.
Williams, J.W.J, "Heapsort", Communications of the ACM, 7:1, pp. 347348, 1964.
Knuth, D.E., "Sorting and Searching", The Art of Computer Programming, Vol. 3, pp. 114123,
145149, 1968.
McIlroy, P.M., "Optimistic Sorting and Information Theoretic Complexity", Proceedings of the
Fourth Annual ACMSIAM Symposium on Discrete Algorithms, pp. 467474, 1993.
Bentley, J.L. and McIlroy, M.D., "Engineering a Sort Function", SoftwarePractice and
Experience, Vol. 23, pp. 12491265, 1993.
STANDARDS
The qsort() function conforms to ANSI X3.1591989 (``ANSI C89'').
BSD June 4, 1993 BSD 
