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.\"     @(#)qsort.3	6.7 (Berkeley) 6/29/91
.\"
.Dd June 29, 1991
.Dt QSORT 3
.Os
.Sh NAME
.Nm qsort, heapsort
.Nd sort functions
.Sh SYNOPSIS
.Fd #include <stdlib.h>
.Ft void
.Fn qsort "void *base" "size_t nmemb" "size_t size" "int (*compar)(const void *, const void *)"
.Ft int
.Fn heapsort "void *base" "size_t nmemb" "size_t size" "int (*compar)(const void *, const void *)"
.Sh DESCRIPTION
The
.Fn qsort
function is a modified partition-exchange sort, or quicksort.
The
.Fn heapsort
function is a modified selection sort.
.Pp
The
.Fn qsort
and
.Fn heapsort
functions sort an array of
.Fa nmemb
objects, the initial member of which is pointed to by
.Fa base .
The size of each object is specified by
.Fa size .
.Pp
The contents of the array are sorted in ascending order according to
a comparison function pointed to by
.Fa compar ,
which is called with two arguments that point to the objects being
compared.
.Pp
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.
.Pp
The functions
.Fn qsort
and
.Fn heapsort
are
.Em not
stable, that is, if two members compare as equal, their order in
the sorted array is undefined.
.Pp
The
.Fn 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.
.Fn Qsort
takes O N lg N average time.
This implementation uses median selection to avoid the traditional
O N**2 worst-case behavior.
.Pp
The
.Fn 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.
.Fn Heapsort
takes O N lg N worst-case time.
Its
.Em only
advantage over
.Fn qsort
is that it uses no additional memory.
.Sh RETURN VALUES
The
.Fn qsort
function
returns no value.
.Pp
Upon successful completion,
.Fn heapsort
returns 0.
Otherwise, it returns \-1 and the global variable
.Va errno
is set to indicate the error.
.Sh ERRORS
The
.Fn heapsort
function succeeds unless:
.Bl -tag -width Er
.It Bq Er EINVAL
The
.Fa size
argument is zero.
.Sh COMPATIBILITY
Previous versions of
.Fn qsort
did not permit the comparison routine to itself call
.Fn qsort 3 .
This is no longer true.
.Sh SEE ALSO
.Xr sort 1 ,
.Xr radixsort 3
.Rs
.%A Hoare, C.A.R.
.%D 1962
.%T "Quicksort"
.%J "The Computer Journal"
.%V 5:1
.%P pp. 10-15
.Re
.Rs
.%A Williams, J.W.J
.%D 1964
.%T "Heapsort"
.%J "Communications of the ACM"
.%V 7:1
.%P pp. 347-348
.Re
.Rs
.%A Knuth, D.E.
.%D 1968
.%B "The Art of Computer Programming"
.%V Vol. 3
.%T "Sorting and Searching"
.%P pp. 114-123, 145-149
.Re
.Sh STANDARDS
The
.Fn qsort
function
conforms to
.St -ansiC .