QSORT(3) BSD Programmer's Manual QSORT(3)
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 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 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 mem
bers 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 partitionexchange sorting; in particular, see
D.E. Knuth's Algorithm Q. Qsort() takes O N lg N average time. This im
plementation uses median selection to avoid its O N**2 worstcase behav
ior.
The heapsort() function is an implementation of J.W.J. William's ``heap
sort'' 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 additional memory;
while qsort() does not allocate memory, it is implemented using recur
sion.
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 preexisting 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 un
true.
RETURN VALUES
The qsort() function returns no value.
Upon successful completion, heapsort() and mergesort() return 0. Other
wise, 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(3). 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",
Fourth Annual ACMSIAM Symposium on Discrete Algorithms, January 1992.
Bentley, J.L., "Engineering a Sort Function", bentley@research.att.com,
January 1992.
STANDARDS
The qsort() function conforms to ANSI C X3.1591989 (``ANSI C '').
4.4BSD June 4, 1993 2
