qsort, heapsort, mergesort  sort functions
#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 *));
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 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 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.
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 preexisting 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 preexisting order in
the data can
make this untrue.
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.
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() or mergesort() were unable to allocate memory.
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, pp.
467464,
January 1993.
Bentley, J.L. and McIlroy, M.D., "Engineering a Sort Function", Software
 Practice and Experience, Vol. 23(11), pp. 12491265,
November 1993.
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 X3.1591989 (``ANSI
C'').
OpenBSD 3.6 June 4, 1993
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