man pages section 3: Basic Library Functions
Updated: July 2014

## qsort(3C)

### Name

qsort - quick sort

### Synopsis

```#include <stdlib.h>

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

### Description

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 base 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 argument is the name of the comparison function, which is called with two arguments that point to the elements 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.

### Usage

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

### Examples

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 = { 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);
}```

### Attributes

See attributes(5) for descriptions of the following attributes:

ATTRIBUTE TYPE
ATTRIBUTE VALUE
Interface Stability
Committed
MT-Level
MT-Safe
Standard

### Notes

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 relative order in the output of two items that compare as equal is unpredictable.