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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
     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.

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

     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 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 11.0-PRERELEASE        February 20, 2013       FreeBSD 11.0-PRERELEASE

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

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