QSORT(3) FreeBSD Library Functions Manual QSORT(3)
NAME
qsort, qsort_b, qsort_r, heapsort, heapsort_b, mergesort, mergesort_b -
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 *));)(const void *, const void *));
void
qsort_b(void *base, size_t nmemb, size_t size,
int (^compar)(const void *, const void *));)(const void *, const void *));
void
qsort_r(void *base, size_t nmemb, size_t size, void *thunk,
int (*compar)(void *, const void *, const void *));)(void *, const void *, const void *));
int
heapsort(void *base, size_t nmemb, size_t size,
int (*compar)(const void *, const void *));)(const void *, const void *));
int
heapsort_b(void *base, size_t nmemb, size_t size,
int (^compar)(const void *, const void *));)(const void *, const void *));
int
mergesort(void *base, size_t nmemb, size_t size,
int (*compar)(const void *, const void *));)(const void *, const void *));
int
mergesort_b(void *base, size_t nmemb, size_t size,
int (^compar)(const void *, const void *));)(const void *, const void *));
#define __STDC_WANT_LIB_EXT1__ 1
errno_t
qsort_s(void *base, rsize_t nmemb, rsize_t size,
int (*compar)(const void *, const void *, void *), void *thunk);)(const void *, const void *, void *), void *thunk);
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 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 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 qsort_b() function behaves identically to qsort(), except that it
takes a block, rather than a function pointer.
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 heapsort_b() function behaves identically
to heapsort(), except that it takes a block, rather than a function
pointer. The mergesort() algorithm is stable. The mergesort_b()
function behaves identically to mergesort(), except that it takes a
block, rather than a function pointer.
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
recursion.
The function mergesort() requires additional memory of size nmemb * size
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() is faster than heapsort().
Memory availability and pre-existing order in the data can make this
untrue.
The qsort_s() function behaves the same as qsort_r(), except that:
- The order of arguments is different
- The order of arguments to compar is different
- if nmemb or size are greater than RSIZE_MAX, or nmemb is not zero and
compar is NULL, then the runtime-constraint handler is called, and
qsort_s() returns an error. Note that the handler is called before
qsort_s() returns the error, and the handler function might not
return.
RETURN VALUES
The qsort() and qsort_r() functions return no value. The qsort_s()
function returns zero on success, non-zero on error.
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.
EXAMPLES
A sample program that sorts an array of int values in place using
qsort(), and then prints the sorted array to standard output is:
#include <stdio.h>
#include <stdlib.h>
/*
* Custom comparison function that compares 'int' values through pointers
* passed by qsort(3).
*/
static int
int_compare(const void *p1, const void *p2)
{
int left = *(const int *)p1;
int right = *(const int *)p2;
return ((left > right) - (left < right));
}
/*
* Sort an array of 'int' values and print it to standard output.
*/
int
main(void)
{
int int_array[] = { 4, 5, 9, 3, 0, 1, 7, 2, 8, 6 };
size_t array_size = sizeof(int_array) / sizeof(int_array[0]);
size_t k;
qsort(&int_array, array_size, sizeof(int_array[0]), int_compare);
for (k = 0; k < array_size; k++)
printf(" %d", int_array[k]);
puts("");
return (EXIT_SUCCESS);
}
COMPATIBILITY
The order of arguments for the comparison function used with qsort_r() is
different from the one used by qsort_s(), and the GNU libc implementation
of qsort_r(). When porting software written for GNU libc, it is usually
possible to replace qsort_r() with qsort_s() to work around this problem.
qsort_s() is part of the optional Annex K portion of ISO/IEC 9899:2011
("ISO C11") and may not be portable to other standards-conforming
platforms.
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.
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").
qsort_s() conforms to ISO/IEC 9899:2011 ("ISO C11") K.3.6.3.2.
HISTORY
The variants of these functions that take blocks as arguments first
appeared in Mac OS X. This implementation was created by David Chisnall.
FreeBSD 13.1-RELEASE-p6 January 20, 2020 FreeBSD 13.1-RELEASE-p6
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