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QSORT(3)                 BSD Library Functions Manual                 QSORT(3)

NAME
     heapsort, heapsort_b, mergesort, mergesort_b, qsort, qsort_b, qsort_r -- sort functions

SYNOPSIS
     #include <stdlib.h>

     int
     heapsort(void *base, size_t nel, size_t width, int (*compar)(const void *, const void *));

     int
     heapsort_b(void *base, size_t nel, size_t width, int (^compar)(const void *, const void *));

     int
     mergesort(void *base, size_t nel, size_t width, int (*compar)(const void *, const void *));

     int
     mergesort_b(void *base, size_t nel, size_t width, int (^compar)(const void *, const void *));

     void
     qsort(void *base, size_t nel, size_t width, int (*compar)(const void *, const void *));

     void
     qsort_b(void *base, size_t nel, size_t width, int (^compar)(const void *, const void *));

     void
     qsort_r(void *base, size_t nel, size_t width, void *thunk,
         int (*compar)(void *, 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 nel objects, the initial member of which is
     pointed to by base.  The size of each object is specified by width.  The mergesort() function behaves
     similarly, but requires that width be greater than or equal to ``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 mem-bers members
     bers compare as equal, their order in the sorted array is undefined.  The mergesort() algorithm is sta-ble. stable.
     ble.

     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 behav-ior. behavior.
     ior.

     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 nel * width 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() which is faster than heapsort().  Memory availability and
     pre-existing order in the data can make this untrue.

     The heapsort_b(), mergesort_b(), and qsort_b() routines are like the corresponding routines without the
     _b suffix, expect that the compar callback is a block pointer instead of a function pointer.

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

     The heapsort(), heapsort_b(), mergesort(), and mergesort_b() functions return the value 0 if success-ful; successful;
     ful; otherwise the value -1 is returned and the global variable errno is set to indicate the error.

COMPATIBILITY
     Previous versions of qsort() did not permit the comparison routine itself to call qsort(3).  This is no
     longer true.

ERRORS
     The heapsort(), heapsort_b(), mergesort(), and mergesort_b() functions succeed unless:

     [EINVAL]           The width argument is zero, or, the width argument to mergesort() or mergesort_b()
                        is less than ``sizeof(void *) / 2''.

     [ENOMEM]           The heapsort(), heapsort_b(), mergesort(), or mergesort_b() 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'').

BSD                           September 30, 2003                           BSD

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