QSORT(3) | Library Functions Manual | QSORT(3) |

`qsort`

, `heapsort`

,
`mergesort`

— sort
functions

```
#include
<stdlib.h>
```

`void`

`qsort`

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

`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 *)`);

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`. `mergesort`

() 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 int 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 functions
`qsort`

() and
`heapsort`

() are
*not* stable,
that is, if two members compare as equal, their order in the sorted array is
undefined. The function `mergesort`

() is stable.

The
`qsort`

()
function is 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. `qsort`

() takes O N lg N average
time. This implementation uses median selection to avoid its O N**2
worst-case behavior and will fall back to `heapsort`

()
if the recursion depth exceeds 2 lg N.

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. This implementation of `heapsort`

() is
implemented without recursive function calls.

The function
`mergesort`

()
requires additional memory of size `nmemb *`
`size` bytes; it should be used only when space is not
at a premium. `mergesort`

() 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 `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.

#include <stdio.h> #include <stdlib.h> #include <string.h> char *array[] = { "XX", "YYY", "Z" }; #define N (sizeof(array) / sizeof(array[0])) int cmp(const void *a, const void *b) { /* * a and b point to elements of the array. * Cast and dereference to obtain the actual elements, * which are also pointers in this case. */ size_t lena = strlen(*(const char **)a); size_t lenb = strlen(*(const char **)b); /* * Do not subtract the lengths. The difference between values * cannot be represented by an int. */ return lena < lenb ? -1 : lena > lenb; } int main() { size_t i; qsort(array, N, sizeof(array[0]), cmp); for (i = 0; i < N; i++) printf("%s\n", array[i]); }

It is almost always an error to use subtraction to compute the return value of the comparison function.

The `heapsort`

() and
`mergesort`

() functions succeed unless:

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,
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*, pp. 467-464,
January 1993.

Bentley, J.L. and
McIlroy, M.D., Engineering a Sort
Function, *Software - Practice and Experience*,
Vol. 23(11), pp.
1249-1265, November 1993.

Musser, D.,
Introspective Sorting and Selection Algorithms,
*Software - Practice and Experience*,
Vol. 27(8), pp. 983-993,
August 1997.

Previous versions of `qsort`

() did not
permit the comparison routine itself to call
`qsort`

(). This is no longer true.

The `qsort`

() function conforms to
ANSI X3.159-1989
(“ANSI C89”).

A `qsort`

() function first appeared in
Version 2 AT&T UNIX.

February 8, 2020 | OpenBSD-current |