# heapsort man page

**heapsort**, **mergesort** — sort functions

## Library

library “libbsd”

## Synopsis

**#include <bsd/stdlib.h>**

`int` **heapsort**(`void *base`, `size_t nmemb`, `size_t size`, `int (*compar)(const void *, const void *)`);

`int` **mergesort**(`void *base`, `size_t nmemb`, `size_t size`, `int (*compar)(const void *, const void *)`);

## Description

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 **heapsort**() function sorts 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 algorithm implemented by **heapsort**() is *not* stable, that is, if two members compare as equal, their order in the sorted array is undefined. The **mergesort**() algorithm is stable.

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 `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 **heapsort**() and **mergesort**() functions return the value 0 if successful; otherwise the value -1 is returned and the global variable `errno` is set to indicate the error.

## 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

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.

## Referenced By

The man page mergesort(3) is an alias of heapsort(3).