qsort, heapsort, mergesort  sort functions
Standard C Library (libc, lc)
#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. Its
only advantage over qsort() is that it uses almost no additional 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 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
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() 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.
Previous versions of qsort() did not permit the comparison routine itself
to call qsort(3). This is no longer true.
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.
The qsort() function conforms to ANSI X3.1591989 (``ANSI C'').
BSD June 4, 1993 BSD
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