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1.\" Copyright (c) 1990, 1991, 1993
2.\" The Regents of the University of California. All rights reserved.
3.\"
4.\" This code is derived from software contributed to Berkeley by
5.\" the American National Standards Committee X3, on Information
6.\" Processing Systems.
7.\"
8.\" Redistribution and use in source and binary forms, with or without
9.\" modification, are permitted provided that the following conditions
10.\" are met:
11.\" 1. Redistributions of source code must retain the above copyright
12.\" notice, this list of conditions and the following disclaimer.
13.\" 2. Redistributions in binary form must reproduce the above copyright
14.\" notice, this list of conditions and the following disclaimer in the
15.\" documentation and/or other materials provided with the distribution.
16.\" 3. Neither the name of the University nor the names of its contributors
17.\" may be used to endorse or promote products derived from this software
18.\" without specific prior written permission.
19.\"
20.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
21.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
22.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
23.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
24.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
25.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
26.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
27.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
28.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
29.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
30.\" SUCH DAMAGE.
31.\"
32.\" $OpenBSD: qsort.3,v 1.27 2020/02/08 01:09:57 jsg Exp $
33.\"
34.Dd $Mdocdate: February 8 2020 $
35.Dt QSORT 3
36.Os
37.Sh NAME
38.Nm qsort ,
39.Nm heapsort ,
40.Nm mergesort
41.Nd sort functions
42.Sh SYNOPSIS
43.In stdlib.h
44.Ft void
45.Fn qsort "void *base" "size_t nmemb" "size_t size" "int (*compar)(const void *, const void *)"
46.Ft int
47.Fn heapsort "void *base" "size_t nmemb" "size_t size" "int (*compar)(const void *, const void *)"
48.Ft int
49.Fn mergesort "void *base" "size_t nmemb" "size_t size" "int (*compar)(const void *, const void *)"
50.Sh DESCRIPTION
51The
52.Fn qsort
53function is a modified partition-exchange sort, or quicksort.
54The
55.Fn heapsort
56function is a modified selection sort.
57The
58.Fn mergesort
59function is a modified merge sort with exponential search
60intended for sorting data with pre-existing order.
61.Pp
62The
63.Fn qsort
64and
65.Fn heapsort
66functions sort an array of
67.Fa nmemb
68objects, the initial member of which is pointed to by
69.Fa base .
70The size of each object is specified by
71.Fa size .
72.Fn mergesort
73behaves similarly, but
74.Em requires
75that
76.Fa size
77be greater than
78.Dq "sizeof(void *) / 2" .
79.Pp
80The contents of the array
81.Fa base
82are sorted in ascending order according to
83a comparison function pointed to by
84.Fa compar ,
85which requires two arguments pointing to the objects being
86compared.
87.Pp
88The comparison function must return an int less than, equal to, or
89greater than zero if the first argument is considered to be respectively
90less than, equal to, or greater than the second.
91.Pp
92The functions
93.Fn qsort
94and
95.Fn heapsort
96are
97.Em not
98stable, that is, if two members compare as equal, their order in
99the sorted array is undefined.
100The function
101.Fn mergesort
102is stable.
103.Pp
104The
105.Fn qsort
106function is an implementation of C.A.R. Hoare's
107.Dq quicksort
108algorithm,
109a variant of partition-exchange sorting; in particular, see D.E. Knuth's
110Algorithm Q.
111.Fn qsort
112takes O N lg N average time.
113This implementation uses median selection to avoid its
114O N**2 worst-case behavior and will fall back to
115.Fn heapsort
116if the recursion depth exceeds 2 lg N.
117.Pp
118The
119.Fn heapsort
120function is an implementation of J.W.J. William's
121.Dq heapsort
122algorithm,
123a variant of selection sorting; in particular, see D.E. Knuth's Algorithm H.
124.Fn heapsort
125takes O N lg N worst-case time.
126This implementation of
127.Fn heapsort
128is implemented without recursive function calls.
129.Pp
130The function
131.Fn mergesort
132requires additional memory of size
133.Fa nmemb *
134.Fa size
135bytes; it should be used only when space is not at a premium.
136.Fn mergesort
137is optimized for data with pre-existing order; its worst case
138time is O N lg N; its best case is O N.
139.Pp
140Normally,
141.Fn qsort
142is faster than
143.Fn mergesort ,
144which is faster than
145.Fn heapsort .
146Memory availability and pre-existing order in the data can make this untrue.
147.Sh RETURN VALUES
148.Rv -std heapsort mergesort
149.Sh EXAMPLES
150.Bd -literal
151#include <stdio.h>
152#include <stdlib.h>
153#include <string.h>
154
155char *array[] = { "XX", "YYY", "Z" };
156#define N (sizeof(array) / sizeof(array[0]))
157
158int
159cmp(const void *a, const void *b)
160{
161 /*
162 * a and b point to elements of the array.
163 * Cast and dereference to obtain the actual elements,
164 * which are also pointers in this case.
165 */
166 size_t lena = strlen(*(const char **)a);
167 size_t lenb = strlen(*(const char **)b);
168 /*
169 * Do not subtract the lengths. The difference between values
170 * cannot be represented by an int.
171 */
172 return lena < lenb ? -1 : lena > lenb;
173}
174
175int
176main()
177{
178 size_t i;
179
180 qsort(array, N, sizeof(array[0]), cmp);
181 for (i = 0; i < N; i++)
182 printf("%s\en", array[i]);
183}
184.Ed
185.Pp
186It is almost always an error to use subtraction to compute the return value
187of the comparison function.
188.Sh ERRORS
189The
190.Fn heapsort
191and
192.Fn mergesort
193functions succeed unless:
194.Bl -tag -width Er
195.It Bq Er EINVAL
196The
197.Fa size
198argument is zero, or the
199.Fa size
200argument to
201.Fn mergesort
202is less than
203.Dq "sizeof(void *) / 2" .
204.It Bq Er ENOMEM
205.Fn heapsort
206or
207.Fn mergesort
208were unable to allocate memory.
209.El
210.Sh SEE ALSO
211.Xr sort 1 ,
212.Xr radixsort 3
213.Rs
214.%A Hoare, C.A.R.
215.%D 1962
216.%T "Quicksort"
217.%J "The Computer Journal"
218.%V 5:1
219.%P pp. 10-15
220.Re
221.Rs
222.%A Williams, J.W.J
223.%D 1964
224.%T "Heapsort"
225.%J "Communications of the ACM"
226.%V 7:1
227.%P pp. 347\-348
228.Re
229.Rs
230.%A Knuth, D.E.
231.%D 1968
232.%B "The Art of Computer Programming"
233.%V Vol. 3
234.%T "Sorting and Searching"
235.%P pp. 114\-123, 145\-149
236.Re
237.Rs
238.%A McIlroy, P.M.
239.%T "Optimistic Sorting and Information Theoretic Complexity"
240.%J "Fourth Annual ACM-SIAM Symposium on Discrete Algorithms"
241.%P pp. 467\-464
242.%D January 1993
243.Re
244.Rs
245.%A Bentley, J.L.
246.%A McIlroy, M.D.
247.%T "Engineering a Sort Function"
248.%J "Software \- Practice and Experience"
249.%V Vol. 23(11)
250.%P pp. 1249\-1265
251.%D November 1993
252.Re
253.Rs
254.%A Musser, D.
255.%T "Introspective Sorting and Selection Algorithms"
256.%J "Software \- Practice and Experience"
257.%V Vol. 27(8)
258.%P pp. 983\-993
259.%D August 1997
260.Re
261.Sh STANDARDS
262Previous versions of
263.Fn qsort
264did not permit the comparison routine itself to call
265.Fn qsort .
266This is no longer true.
267.Pp
268The
269.Fn qsort
270function conforms to
271.St -ansiC .
272.Sh HISTORY
273A
274.Fn qsort
275function first appeared in
276.At v2 .