QSORT(3) BSD Library Functions Manual QSORT(3)NAME
heapsort, heapsort_b, mergesort, mergesort_b, qsort, qsort_b, qsort_r —
sort functions
SYNOPSIS
#include <stdlib.h>
int
heapsort(void *base, size_t nel, size_t width,
int (*compar)(const void *, const void *));
int
heapsort_b(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 *));
int
mergesort_b(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_b(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 or equal to “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 members compare as equal, their order 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-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 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 recur‐
sion.
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.
The heapsort_b(), mergesort_b(), and qsort_b() routines are like the cor‐
responding routines without the _b suffix, expect that the compar call‐
back is a block pointer instead of a function pointer.
RETURN VALUES
The qsort(), qsort_b() and qsort_r() functions return no value.
The heapsort(), heapsort_b(), mergesort(), and mergesort_b() functions
return 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(), heapsort_b(), mergesort(), and mergesort_b() functions
succeed unless:
[EINVAL] The width argument is zero, or, the width argument to
mergesort() or mergesort_b() is less than “sizeof(void
*) / 2”.
[ENOMEM] The heapsort(), heapsort_b(), mergesort(), or
mergesort_b() functions were unable to allocate mem‐
ory.
SEE ALSOsort(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
