Skip site navigation (1)Skip section navigation (2)

FreeBSD Manual Pages


home | help
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>

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

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

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

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

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

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

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

     #define __STDC_WANT_LIB_EXT1__ 1

     qsort_s(void *base, rsize_t nmemb,	rsize_t	size,
	 int (*compar)(const void *, const void	*, void	*), void *thunk);

     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() func-
     tion 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 recur-

     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 un-

     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 re-

     The qsort() and qsort_r() functions return	no value.  The qsort_s() func-
     tion returns zero on success, non-zero on error.

     The heapsort() and	mergesort() functions return the value 0 if success-
     ful; 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 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]);
	     return (EXIT_SUCCESS);

     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 plat-

     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:

     [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.

     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,

     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").
     qsort_s() conforms	to ISO/IEC 9899:2011 ("ISO C11") K.

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

FreeBSD	13.0		       January 20, 2020			  FreeBSD 13.0


Want to link to this manual page? Use this URL:

home | help