580b4d185b
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327 lines
7.5 KiB
Groff
327 lines
7.5 KiB
Groff
.\" Copyright (c) 1990, 1991, 1993
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.\" The Regents of the University of California. All rights reserved.
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.\"
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.\" This code is derived from software contributed to Berkeley by
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.\" the American National Standards Committee X3, on Information
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.\" Processing Systems.
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.\"
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.\" Redistribution and use in source and binary forms, with or without
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.\" modification, are permitted provided that the following conditions
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.\" are met:
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.\" 1. Redistributions of source code must retain the above copyright
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.\" notice, this list of conditions and the following disclaimer.
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.\" 2. Redistributions in binary form must reproduce the above copyright
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.\" notice, this list of conditions and the following disclaimer in the
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.\" documentation and/or other materials provided with the distribution.
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.\" 3. Neither the name of the University nor the names of its contributors
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.\" may be used to endorse or promote products derived from this software
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.\" without specific prior written permission.
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.\"
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.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
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.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
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.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
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.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
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.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
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.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
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.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
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.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
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.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
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.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
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.\" SUCH DAMAGE.
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.\"
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.\" @(#)qsort.3 8.1 (Berkeley) 6/4/93
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.\" $FreeBSD$
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.\"
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.Dd February 20, 2013
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.Dt QSORT 3
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.Os
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.Sh NAME
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.Nm qsort , qsort_r , heapsort , mergesort
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.Nd sort functions
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.Sh LIBRARY
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.Lb libc
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.Sh SYNOPSIS
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.In stdlib.h
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.Ft void
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.Fo qsort
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.Fa "void *base"
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.Fa "size_t nmemb"
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.Fa "size_t size"
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.Fa "int \*[lp]*compar\*[rp]\*[lp]const void *, const void *\*[rp]"
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.Fc
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.Ft void
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.Fo qsort_r
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.Fa "void *base"
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.Fa "size_t nmemb"
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.Fa "size_t size"
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.Fa "void *thunk"
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.Fa "int \*[lp]*compar\*[rp]\*[lp]void *, const void *, const void *\*[rp]"
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.Fc
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.Ft int
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.Fo heapsort
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.Fa "void *base"
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.Fa "size_t nmemb"
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.Fa "size_t size"
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.Fa "int \*[lp]*compar\*[rp]\*[lp]const void *, const void *\*[rp]"
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.Fc
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.Ft int
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.Fo mergesort
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.Fa "void *base"
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.Fa "size_t nmemb"
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.Fa "size_t size"
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.Fa "int \*[lp]*compar\*[rp]\*[lp]const void *, const void *\*[rp]"
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.Fc
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.Sh DESCRIPTION
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The
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.Fn qsort
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function is a modified partition-exchange sort, or quicksort.
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The
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.Fn heapsort
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function is a modified selection sort.
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The
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.Fn mergesort
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function is a modified merge sort with exponential search
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intended for sorting data with pre-existing order.
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.Pp
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The
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.Fn qsort
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and
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.Fn heapsort
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functions sort an array of
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.Fa nmemb
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objects, the initial member of which is pointed to by
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.Fa base .
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The size of each object is specified by
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.Fa size .
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The
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.Fn mergesort
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function
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behaves similarly, but
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.Em requires
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that
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.Fa size
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be greater than
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.Dq "sizeof(void *) / 2" .
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.Pp
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The contents of the array
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.Fa base
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are sorted in ascending order according to
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a comparison function pointed to by
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.Fa compar ,
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which requires two arguments pointing to the objects being
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compared.
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.Pp
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The comparison function must return an integer less than, equal to, or
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greater than zero if the first argument is considered to be respectively
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less than, equal to, or greater than the second.
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.Pp
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The
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.Fn qsort_r
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function behaves identically to
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.Fn qsort ,
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except that it takes an additional argument,
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.Fa thunk ,
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which is passed unchanged as the first argument to function pointed to
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.Fa compar .
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This allows the comparison function to access additional
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data without using global variables, and thus
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.Fn qsort_r
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is suitable for use in functions which must be reentrant.
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.Pp
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The algorithms implemented by
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.Fn qsort ,
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.Fn qsort_r ,
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and
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.Fn heapsort
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are
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.Em not
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stable, that is, if two members compare as equal, their order in
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the sorted array is undefined.
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The
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.Fn mergesort
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algorithm is stable.
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.Pp
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The
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.Fn qsort
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and
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.Fn qsort_r
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functions are an implementation of C.A.R.
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Hoare's
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.Dq quicksort
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algorithm,
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a variant of partition-exchange sorting; in particular, see
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.An D.E. Knuth Ns 's
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.%T "Algorithm Q" .
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.Sy Quicksort
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takes O N lg N average time.
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This implementation uses median selection to avoid its
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O N**2 worst-case behavior.
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.Pp
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The
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.Fn heapsort
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function is an implementation of
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.An "J.W.J. William" Ns 's
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.Dq heapsort
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algorithm,
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a variant of selection sorting; in particular, see
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.An "D.E. Knuth" Ns 's
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.%T "Algorithm H" .
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.Sy Heapsort
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takes O N lg N worst-case time.
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Its
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.Em only
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advantage over
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.Fn qsort
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is that it uses almost no additional memory; while
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.Fn qsort
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does not allocate memory, it is implemented using recursion.
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.Pp
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The function
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.Fn mergesort
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requires additional memory of size
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.Fa nmemb *
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.Fa size
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bytes; it should be used only when space is not at a premium.
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The
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.Fn mergesort
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function
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is optimized for data with pre-existing order; its worst case
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time is O N lg N; its best case is O N.
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.Pp
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Normally,
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.Fn qsort
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is faster than
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.Fn mergesort
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is faster than
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.Fn heapsort .
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Memory availability and pre-existing order in the data can make this
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untrue.
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.Sh RETURN VALUES
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The
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.Fn qsort
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and
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.Fn qsort_r
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functions
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return no value.
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.Pp
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.Rv -std heapsort mergesort
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.Sh EXAMPLES
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A sample program that sorts an array of
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.Vt int
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values in place using
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.Fn qsort ,
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and then prints the sorted array to standard output is:
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.Bd -literal
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#include <stdio.h>
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#include <stdlib.h>
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/*
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* Custom comparison function that can compare 'int' values through pointers
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* passed by qsort(3).
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*/
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static int
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int_compare(const void *p1, const void *p2)
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{
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int left = *(const int *)p1;
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int right = *(const int *)p2;
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return ((left > right) - (left < right));
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}
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/*
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* Sort an array of 'int' values and print it to standard output.
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*/
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int
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main(void)
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{
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int int_array[] = { 4, 5, 9, 3, 0, 1, 7, 2, 8, 6 };
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const size_t array_size = sizeof(int_array) / sizeof(int_array[0]);
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size_t k;
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qsort(&int_array, array_size, sizeof(int_array[0]), int_compare);
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for (k = 0; k < array_size; k++)
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printf(" %d", int_array[k]);
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puts("");
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return (EXIT_SUCCESS);
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}
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.Ed
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.Sh COMPATIBILITY
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Previous versions of
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.Fn qsort
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did not permit the comparison routine itself to call
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.Fn qsort 3 .
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This is no longer true.
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.Sh ERRORS
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The
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.Fn heapsort
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and
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.Fn mergesort
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functions succeed unless:
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.Bl -tag -width Er
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.It Bq Er EINVAL
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The
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.Fa size
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argument is zero, or,
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the
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.Fa size
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argument to
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.Fn mergesort
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is less than
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.Dq "sizeof(void *) / 2" .
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.It Bq Er ENOMEM
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The
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.Fn heapsort
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or
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.Fn mergesort
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functions
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were unable to allocate memory.
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.El
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.Sh SEE ALSO
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.Xr sort 1 ,
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.Xr radixsort 3
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.Rs
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.%A Hoare, C.A.R.
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.%D 1962
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.%T "Quicksort"
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.%J "The Computer Journal"
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.%V 5:1
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.%P pp. 10-15
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.Re
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.Rs
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.%A Williams, J.W.J
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.%D 1964
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.%T "Heapsort"
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.%J "Communications of the ACM"
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.%V 7:1
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.%P pp. 347-348
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.Re
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.Rs
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.%A Knuth, D.E.
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.%D 1968
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.%B "The Art of Computer Programming"
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.%V Vol. 3
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.%T "Sorting and Searching"
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.%P pp. 114-123, 145-149
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.Re
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.Rs
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.%A McIlroy, P.M.
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.%T "Optimistic Sorting and Information Theoretic Complexity"
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.%J "Fourth Annual ACM-SIAM Symposium on Discrete Algorithms"
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.%V January 1992
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.Re
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.Rs
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.%A Bentley, J.L.
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.%A McIlroy, M.D.
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.%T "Engineering a Sort Function"
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.%J "Software--Practice and Experience"
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.%V Vol. 23(11)
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.%P pp. 1249-1265
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.%D November\ 1993
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.Re
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.Sh STANDARDS
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The
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.Fn qsort
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function
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conforms to
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.St -isoC .
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