| QSORT(3) | Library Functions Manual | QSORT(3) | 
qsort, heapsort,
  mergesort —
#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 *));
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. 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
    pre-existing 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 pre-existing
    order in the data can make this untrue.
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.
qsort() did not permit the
  comparison routine itself to call qsort(). This is no
  longer true.
heapsort() function succeeds unless:
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, Proceedings of the Fourth Annual ACM-SIAM Symposium on Discrete Algorithms, pp. 467-474, 1993.
Bentley, J.L. and McIlroy, M.D., Engineering a Sort Function, Software-Practice and Experience, Vol. 23, pp. 1249-1265, 1993.
qsort() function conforms to ANSI
  X3.159-1989 (“ANSI C89”).
| June 4, 1993 | NetBSD 9.3 |