QSORT(3C) Standard C Library Functions QSORT(3C)

qsort, qsort_r
quick sort

#include <stdlib.h>

void
qsort(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, int (*compar_arg)(const void *, const void *, void *), void *arg);

The qsort() function is an implementation of the quick-sort algorithm. It sorts a table of data in place. The contents of the table are sorted in ascending order according to the user-supplied comparison function.

The Ibase argument points to the element at the base of the table. The nel argument is the number of elements in the table. The width argument specifies the size of each element in bytes. The compar arguments that point to the elements being compared. The comparison function need not compare every byte, so arbitrary data may be contained in the elements in addition to the values being compared.

The function must return an integer less than, equal to, or greater than zero to indicate if the first argument is to be considered less than, equal to, or greater than the second argument.

The contents of the table are sorted in ascending order according to the user supplied comparison function. The relative order in the output of two items that compare as equal is unpredictable.

The qsort_r() function behaves similarly to the qsort() function, except that its comparison function, compar_arg(), takes an extra argument which is the qsort_r() argument arg. This allows one to avoid global data in the comparison function, unlike with the qsort() function.

The qsort() and qsort_r() function safely allows concurrent access by multiple threads to disjoint data, such as overlapping subtrees or tables.

Example 1 Program sorts.

The following program sorts a simple array:

#include <stdlib.h>
#include <stdio.h>

static int
intcompare(const void *p1, const void *p2)
{
    int i = *((int *)p1);
    int j = *((int *)p2);

    if (i > j)
        return (1);
    if (i < j)
        return (-1);
    return (0);
}

int
main()
{
    int i;
    int a[10] = { 9, 8, 7, 6, 5, 4, 3, 2, 1, 0 };
    size_t nelems = sizeof (a) / sizeof (int);

    qsort((void *)a, nelems, sizeof (int), intcompare);

    for (i = 0; i < nelems; i++) {
        (void) printf("%d ", a[i]);
    }

    (void) printf("\n");
    return (0);
}

Standard

MT-Safe

sort(1), bsearch(3C), lsearch(3C), string(3C), attributes(7), standards(7)
October 11, 2020 OmniOS