QSORT(3) FreeBSD Library Functions Manual QSORT(3)

qsort, qsort_b, qsort_r, heapsort, heapsort_b, mergesort, mergesort_b — sort functions

Standard C Library (libc, -lc)

#include <stdlib.h>

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

void
qsort_b(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 *thunk);

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

int
heapsort_b(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 *));

int
mergesort_b(void *base, size_t nmemb, size_t size, int (^compar)(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);

The () 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 () 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 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 () function behaves identically to qsort(), except that it takes an additional argument, thunk, which is passed unchanged as the last 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 () function behaves identically to qsort(), except that it takes a block, rather than a function pointer.

The algorithms implemented by (), qsort_r(), and heapsort() are stable, that is, if two members compare as equal, their order in the sorted array is undefined. The () function behaves identically to heapsort(), except that it takes a block, rather than a function pointer. The mergesort() algorithm is stable. The () function behaves identically to mergesort(), except that it takes a block, rather than a function pointer.

The () 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. takes O N lg N average time. This implementation uses median selection to avoid its O N**2 worst-case behavior.

The () 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. takes O N lg N worst-case time. Its advantage over qsort() is that it uses almost no additional memory; while qsort() does not allocate memory, it is implemented using recursion.

The function () 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, () is faster than mergesort() is faster than heapsort(). Memory availability and pre-existing order in the data can make this untrue.

The () function behaves the same as qsort_r(), except that if nmemb or size are greater than RSIZE_MAX, or nmemb is not zero and compar is NULL or size is zero, 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.

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.

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);
}

The order of arguments for the comparison function used with qsort_r() is historically different from the one used by qsort_s() and the GNU libc implementation of qsort_r(). However, as of FreeBSD 14.0, the qsort_r() has been updated so that the thunk parameter appears last to match IEEE Std 1003.1-2024 (“POSIX.1”). Both the historical and the updated interfaces are now accepted via C11 generic selection and C++ polymorphism, but the updated interface is recommended for portable applications.

qsort_s() is part of the 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.

The heapsort() and mergesort() functions succeed unless:

[]
The size argument is zero, or, the size argument to mergesort() is less than “sizeof(void *) / 2”.
[]
The heapsort() or mergesort() functions were unable to allocate memory.

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.

The qsort() function conforms to ISO/IEC 9899:1990 (“ISO C90”). The qsort_r() function conforms to IEEE Std 1003.1-2024 (“POSIX.1”). The qsort_s() function conforms to ISO/IEC 9899:2011 (“ISO C11”) K.3.6.3.2.

The variants of these functions that take blocks as arguments first appeared in Mac OS X. This implementation was created by David Chisnall.

In FreeBSD 14.0, the prototype of qsort_r() was updated to match IEEE Std 1003.1-2024 (“POSIX.1”).

October 27, 2024 FreeBSD 15.0-CURRENT