qsort(3)

NAME

qsort, qsort_r, heapsort, mergesort - 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, void *thunk,
        int (*compar)(void *, 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 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. The mergesort() function behaves sim
ilarly, 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 qsort_r() function behaves identically to qsort(), ex
cept that it takes an additional argument, thunk, which is passed un
changed as the first argument to function pointed to compar. This allows
the comparison function to access additional data without using global
variables, and
thus qsort_r() is suitable for use in functions which must
be reentrant.
The algorithms implemented by qsort(), qsort_r(), and
heapsort() are not
stable, that is, if two members compare as equal, their or
der in the
sorted array is undefined. The mergesort() algorithm is
stable.
The qsort() and qsort_r() functions are an implementation of
C.A.R. Hoare's ``quicksort'' algorithm, a variant of partition-ex
change sorting;
in particular, see D.E. Knuth's Algorithm Q. Quicksort
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 imple
mented using recursion.
The function mergesort() requires additional memory of size
nmemb * size bytes; it should be used only when space is not at a premi
um. The
mergesort() function 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.

The heapsort() function returns the value 0 if successful;
otherwise the value -1 is returned 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(3). This is no longer true.

ERRORS

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] The heapsort() or mergesort() functions
were unable to: allocate memory.

STANDARDS

The qsort() function conforms to ISO/IEC 9899:1990 (``ISO
C89'').
BSD September 30, 2003

docs.sk

comprehensive documentation repository

See also