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

     heapsort, mergesort, qsort, qsort_r -- sort functions

     Standard C Library (libc, -lc)

     #include <stdlib.h>

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

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

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

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

     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 ``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; memory;
     ory; while qsort() does not allocate memory, it is implemented using

     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 qsort() and qsort_r() functions return no value.

     The heapsort() and mergesort() 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.

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

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

     [ENOMEM]           The heapsort() or mergesort() functions were unable to
                        allocate memory.

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

     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'').

BSD                           September 30, 2003                           BSD