Updated: 2025/Nov/16

Please read Privacy Policy. It's for your privacy.

QSORT(3)                   Library Functions Manual                   QSORT(3)

NAME
     qsort, heapsort, mergesort, qsort_r, heapsort_r, mergesort_r - 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 *));

     void
     qsort_r(void *base, size_t nmemb, size_t size,
         int (*compar)(const void *, const void *, void *), void *cookie);

     int
     heapsort(void *base, size_t nmemb, size_t size,
         int (*compar)(const void *, const void *));

     int
     heapsort_r(void *base, size_t nmemb, size_t size,
         int (*compar)(const void *, const void *, void *), void *cookie);

     int
     mergesort(void *base, size_t nmemb, size_t size,
         int (*compar)(const void *, const void *));

     int
     mergesort_r(void *base, size_t nmemb, size_t size,
         int (*compar)(const void *, const void *, void *), void *cookie);

DESCRIPTION
     The 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_r(), heapsort_r(), and mergesort_r() functions pass an
     additional cookie argument through to the comparison function.

     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.

RETURN VALUES
     The qsort() and qsort_r() functions return no value.

     Upon successful completion, heapsort(), mergesort() heapsort_r(), and
     mergesort_r() return 0.  Otherwise, they return -1 and the global
     variable errno is set to indicate the error.

COMPATIBILITY
     Previous versions of qsort() did not permit the comparison routine itself
     to call qsort().  This is no longer true.

ERRORS
     The heapsort(), mergesort() heapsort_r(), and heapsort_r() functions
     succeed unless:

     [EINVAL]           The size argument is zero, or, the size argument to
                        mergesort() or mergesort_r() is less than "sizeof(void
                        *) / 2".

     [ENOMEM]           heapsort(), heapsort_r(), mergesort(), or
                        mergesort_r() were unable to allocate memory.

SEE ALSO
     sort(1), radixsort(3)

     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.

STANDARDS
     The qsort() function conforms to ANSI X3.159-1989 ("ANSI C89").

HISTORY
     The qsort_r(), heapsort_r(), and mergesort_r() functions were added in
     NetBSD 11.0.

     The qsort_r() function conforms to IEEE Std 1003.1-2024 ("POSIX.1").

NetBSD 11.99                     July 20, 2025                    NetBSD 11.99