xref: /freebsd/lib/libc/stdlib/qsort.3 (revision 6fd05b64b5b65dd4ba9b86482a0634a5f0b96c29) 
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36.\"     @(#)qsort.3	8.1 (Berkeley) 6/4/93
37.\" $FreeBSD$
38.\"
39.Dd September 30, 2003
40.Dt QSORT 3
41.Os
42.Sh NAME
43.Nm qsort , qsort_r , heapsort , mergesort
44.Nd sort functions
45.Sh LIBRARY
46.Lb libc
47.Sh SYNOPSIS
48.In stdlib.h
49.Ft void
50.Fo qsort
51.Fa "void *base"
52.Fa "size_t nmemb"
53.Fa "size_t size"
54.Fa "int \*[lp]*compar\*[rp]\*[lp]const void *, const void *\*[rp]"
55.Fc
56.Ft void
57.Fo qsort_r
58.Fa "void *base"
59.Fa "size_t nmemb"
60.Fa "size_t size"
61.Fa "void *thunk"
62.Fa "int \*[lp]*compar\*[rp]\*[lp]void *, const void *, const void *\*[rp]"
63.Fc
64.Ft int
65.Fo heapsort
66.Fa "void *base"
67.Fa "size_t nmemb"
68.Fa "size_t size"
69.Fa "int \*[lp]*compar\*[rp]\*[lp]const void *, const void *\*[rp]"
70.Fc
71.Ft int
72.Fo mergesort
73.Fa "void *base"
74.Fa "size_t nmemb"
75.Fa "size_t size"
76.Fa "int \*[lp]*compar\*[rp]\*[lp]const void *, const void *\*[rp]"
77.Fc
78.Sh DESCRIPTION
79The
80.Fn qsort
81function is a modified partition-exchange sort, or quicksort.
82The
83.Fn heapsort
84function is a modified selection sort.
85The
86.Fn mergesort
87function is a modified merge sort with exponential search
88intended for sorting data with pre-existing order.
89.Pp
90The
91.Fn qsort
92and
93.Fn heapsort
94functions sort an array of
95.Fa nmemb
96objects, the initial member of which is pointed to by
97.Fa base .
98The size of each object is specified by
99.Fa size .
100The
101.Fn mergesort
102function
103behaves similarly, but
104.Em requires
105that
106.Fa size
107be greater than
108.Dq "sizeof(void *) / 2" .
109.Pp
110The contents of the array
111.Fa base
112are sorted in ascending order according to
113a comparison function pointed to by
114.Fa compar ,
115which requires two arguments pointing to the objects being
116compared.
117.Pp
118The comparison function must return an integer less than, equal to, or
119greater than zero if the first argument is considered to be respectively
120less than, equal to, or greater than the second.
121.Pp
122The
123.Fn qsort_r
124function behaves identically to
125.Fn qsort ,
126except that it takes an additional argument,
127.Fa thunk ,
128which is passed unchanged as the first argument to function pointed to
129.Fa compar .
130This allows the comparison function to access additional
131data without using global variables, and thus
132.Fn qsort_r
133is suitable for use in functions which must be reentrant.
134.Pp
135The algorithms implemented by
136.Fn qsort ,
137.Fn qsort_r ,
138and
139.Fn heapsort
140are
141.Em not
142stable, that is, if two members compare as equal, their order in
143the sorted array is undefined.
144The
145.Fn mergesort
146algorithm is stable.
147.Pp
148The
149.Fn qsort
150and
151.Fn qsort_r
152functions are an implementation of C.A.R.
153Hoare's
154.Dq quicksort
155algorithm,
156a variant of partition-exchange sorting; in particular, see
157.An D.E. Knuth Ns 's
158.%T "Algorithm Q" .
159.Sy Quicksort
160takes O N lg N average time.
161This implementation uses median selection to avoid its
162O N**2 worst-case behavior.
163.Pp
164The
165.Fn heapsort
166function is an implementation of
167.An "J.W.J. William" Ns 's
168.Dq heapsort
169algorithm,
170a variant of selection sorting; in particular, see
171.An "D.E. Knuth" Ns 's
172.%T "Algorithm H" .
173.Sy Heapsort
174takes O N lg N worst-case time.
175Its
176.Em only
177advantage over
178.Fn qsort
179is that it uses almost no additional memory; while
180.Fn qsort
181does not allocate memory, it is implemented using recursion.
182.Pp
183The function
184.Fn mergesort
185requires additional memory of size
186.Fa nmemb *
187.Fa size
188bytes; it should be used only when space is not at a premium.
189The
190.Fn mergesort
191function
192is optimized for data with pre-existing order; its worst case
193time is O N lg N; its best case is O N.
194.Pp
195Normally,
196.Fn qsort
197is faster than
198.Fn mergesort
199is faster than
200.Fn heapsort .
201Memory availability and pre-existing order in the data can make this
202untrue.
203.Sh RETURN VALUES
204The
205.Fn qsort
206and
207.Fn qsort_r
208functions
209return no value.
210.Pp
211.Rv -std heapsort mergesort
212.Sh ERRORS
213The
214.Fn heapsort
215and
216.Fn mergesort
217functions succeed unless:
218.Bl -tag -width Er
219.It Bq Er EINVAL
220The
221.Fa size
222argument is zero, or,
223the
224.Fa size
225argument to
226.Fn mergesort
227is less than
228.Dq "sizeof(void *) / 2" .
229.It Bq Er ENOMEM
230The
231.Fn heapsort
232or
233.Fn mergesort
234functions
235were unable to allocate memory.
236.El
237.Sh COMPATIBILITY
238Previous versions of
239.Fn qsort
240did not permit the comparison routine itself to call
241.Fn qsort 3 .
242This is no longer true.
243.Sh SEE ALSO
244.Xr sort 1 ,
245.Xr radixsort 3
246.Rs
247.%A Hoare, C.A.R.
248.%D 1962
249.%T "Quicksort"
250.%J "The Computer Journal"
251.%V 5:1
252.%P pp. 10-15
253.Re
254.Rs
255.%A Williams, J.W.J
256.%D 1964
257.%T "Heapsort"
258.%J "Communications of the ACM"
259.%V 7:1
260.%P pp. 347-348
261.Re
262.Rs
263.%A Knuth, D.E.
264.%D 1968
265.%B "The Art of Computer Programming"
266.%V Vol. 3
267.%T "Sorting and Searching"
268.%P pp. 114-123, 145-149
269.Re
270.Rs
271.%A McIlroy, P.M.
272.%T "Optimistic Sorting and Information Theoretic Complexity"
273.%J "Fourth Annual ACM-SIAM Symposium on Discrete Algorithms"
274.%V January 1992
275.Re
276.Rs
277.%A Bentley, J.L.
278.%A McIlroy, M.D.
279.%T "Engineering a Sort Function"
280.%J "Software--Practice and Experience"
281.%V Vol. 23(11)
282.%P pp. 1249-1265
283.%D November\ 1993
284.Re
285.Sh STANDARDS
286The
287.Fn qsort
288function
289conforms to
290.St -isoC .
291