QSORT(3)

NAME
qsort, qsort_r, heapsort, mergesort - sort functions

LIBRARY
libc

SYNOPSIS
#include <stdlib.h>
void
qsort(void *base, size_t nmemb, size_t size, int *comparconst void *, const void *);
void
qsort_r(void *base, size_t nmemb, size_t size, void *thunk, int *comparvoid *, const void *, const void *);
int
heapsort(void *base, size_t nmemb, size_t size, int *comparconst void *, const void *);
int
mergesort(void *base, size_t nmemb, size_t size, int *comparconst 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 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 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.

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., 1962, Quicksort, The Computer Journal, 5:1, pp. 10-15. Williams, J.W.J, 1964, Heapsort, Communications of the ACM, 7:1, pp. 347-348. Knuth, D.E., 1968, The Art of Computer Programming, Vol. 3, Sorting and Searching, pp. 114-123, 145-149. McIlroy, P.M., Optimistic Sorting and Information Theoretic Complexity, Fourth Annual ACM-SIAM Symposium on Discrete Algorithms, January 1992. Bentley, J.L., 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 isoC.