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

NAME
     qsort, qsort_r, heapsort, mergesort -- 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	*));

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

     int
     heapsort(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	*));

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 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 mem-
     ory; 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.

RETURN VALUES
     The qsort() and qsort_r() functions return	no value.

     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.

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	can compare '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 };
	    const 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
     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'').

FreeBSD	10.1		       February	20, 2013		  FreeBSD 10.1

NAME | LIBRARY | SYNOPSIS | DESCRIPTION | RETURN VALUES | EXAMPLES | COMPATIBILITY | ERRORS | SEE ALSO | STANDARDS

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