QSORT(3) BSD Library Functions Manual QSORT(3)
NAME
heapsort, mergesort, qsort, qsort_r -- sort functions
LIBRARY
Standard C Library (libc, -lc)
SYNOPSIS
#include <stdlib.h>
int
heapsort(void *base, size_t nel, size_t width, int (*compar)(const void *, const void *));
int
mergesort(void *base, size_t nel, size_t width, int (*compar)(const void *, const void *));
void
qsort(void *base, size_t nel, size_t width, int (*compar)(const void *, const void *));
void
qsort_r(void *base, size_t nel, size_t width, void *thunk,
int (*compar)(void *, 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 nel objects, the initial member of which is
pointed to by base. The size of each object is specified by width. The mergesort() function behaves
similarly, but requires that width 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(), except that it takes an additional argument,
thunk, which is passed unchanged 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 mem-bers members
bers compare as equal, their order in the sorted array is undefined. The mergesort() algorithm is sta-ble. stable.
ble.
The qsort() and qsort_r() functions are 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. Quicksort takes O
N lg N average time. This implementation uses median selection to avoid its O N**2 worst-case behav-ior. behavior.
ior.
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 nel * width bytes; it should be used only
when space is not at a premium. 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(), which 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() and mergesort() functions return the value 0 if successful; otherwise the value -1 is
returned and the global variable errno is set to indicate the error.
ERRORS
The heapsort() and mergesort() functions succeed unless:
[EINVAL] The width argument is zero, or, the width argument to mergesort() is less than
``sizeof(void *) / 2''.
[ENOMEM] The heapsort() or mergesort() functions were unable to allocate memory.
COMPATIBILITY
Previous versions of qsort() did not permit the comparison routine itself to call qsort(3). This is no
longer true.
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", Fourth Annual ACM-SIAM
Symposium on Discrete Algorithms, January 1992.
Bentley, J.L. and McIlroy, M.D., "Engineering a Sort Function", Software--Practice and Experience,
Vol. 23(11), pp. 1249-1265, November 1993.
STANDARDS
The qsort() function conforms to ISO/IEC 9899:1990 (``ISO C90'').
BSD September 30, 2003 BSD
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