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IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE .\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL .\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS .\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) .\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT .\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY .\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF .\" SUCH DAMAGE. .\" .\" @(#)qsort.3 8.1 (Berkeley) 6/4/93 .\" $FreeBSD: src/lib/libc/stdlib/qsort.3,v 1.15 2004/07/02 23:52:12 ru Exp $ .\" .Dd September 30, 2003 .Dt QSORT 3 .Os .Sh NAME .Nm qsort , qsort_r , heapsort , mergesort .Nd sort functions .Sh LIBRARY .Lb libc .Sh SYNOPSIS .In stdlib.h .Ft void .Fo qsort .Fa "void *base" .Fa "size_t nmemb" .Fa "size_t size" .Fa "int \*[lp]*compar\*[rp]\*[lp]const void *, const void *\*[rp]" .Fc .Ft void .Fo qsort_r .Fa "void *base" .Fa "size_t nmemb" .Fa "size_t size" .Fa "void *thunk" .Fa "int \*[lp]*compar\*[rp]\*[lp]void *, const void *, const void *\*[rp]" .Fc .Ft int .Fo heapsort .Fa "void *base" .Fa "size_t nmemb" .Fa "size_t size" .Fa "int \*[lp]*compar\*[rp]\*[lp]const void *, const void *\*[rp]" .Fc .Ft int .Fo mergesort .Fa "void *base" .Fa "size_t nmemb" .Fa "size_t size" .Fa "int \*[lp]*compar\*[rp]\*[lp]const void *, const void *\*[rp]" .Fc .Sh DESCRIPTION The .Fn qsort function is a modified partition-exchange sort, or quicksort. The .Fn heapsort function is a modified selection sort. The .Fn mergesort function is a modified merge sort with exponential search intended for sorting data with pre-existing order. .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 . The .Fn mergesort function behaves similarly, but .Em requires that .Fa size be greater than .Dq "sizeof(void *) / 2" . .Pp The contents of the array .Fa base are sorted in ascending order according to a comparison function pointed to by .Fa compar , which requires two arguments pointing 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 .Fn qsort_r function behaves identically to .Fn qsort , except that it takes an additional argument, .Fa thunk , which is passed unchanged as the first argument to function pointed to .Fa compar . This allows the comparison function to access additional data without using global variables, and thus .Fn qsort_r is suitable for use in functions which must be reentrant. .Pp The algorithms implemented by .Fn qsort , .Fn qsort_r , and .Fn heapsort are .Em not stable, that is, if two members compare as equal, their order in the sorted array is undefined. The .Fn mergesort algorithm is stable. .Pp The .Fn qsort and .Fn qsort_r functions are an implementation of C.A.R. Hoare's .Dq quicksort algorithm, a variant of partition-exchange sorting; in particular, see .An D.E. Knuth Ns 's .%T "Algorithm Q" . .Sy Quicksort takes O N lg N average time. This implementation uses median selection to avoid its O N**2 worst-case behavior. .Pp The .Fn heapsort function is an implementation of .An "J.W.J. William" Ns 's .Dq heapsort algorithm, a variant of selection sorting; in particular, see .An "D.E. Knuth" Ns 's .%T "Algorithm H" . .Sy Heapsort takes O N lg N worst-case time. Its .Em only advantage over .Fn qsort is that it uses almost no additional memory; while .Fn qsort does not allocate memory, it is implemented using recursion. .Pp The function .Fn mergesort requires additional memory of size .Fa nmemb * .Fa size bytes; it should be used only when space is not at a premium. The .Fn mergesort function is optimized for data with pre-existing order; its worst case time is O N lg N; its best case is O N. .Pp Normally, .Fn qsort is faster than .Fn mergesort is faster than .Fn heapsort . Memory availability and pre-existing order in the data can make this untrue. .Sh RETURN VALUES The .Fn qsort and .Fn qsort_r functions return no value. .Pp .Rv -std heapsort mergesort .Sh ERRORS The .Fn heapsort and .Fn mergesort functions succeed unless: .Bl -tag -width Er .It Bq Er EINVAL The .Fa size argument is zero, or, the .Fa size argument to .Fn mergesort is less than .Dq "sizeof(void *) / 2" . .It Bq Er ENOMEM The .Fn heapsort or .Fn mergesort functions were unable to allocate memory. .El .Sh COMPATIBILITY Previous versions of .Fn qsort did not permit the comparison routine itself to 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 .Rs .%A McIlroy, P.M. .%T "Optimistic Sorting and Information Theoretic Complexity" .%J "Fourth Annual ACM-SIAM Symposium on Discrete Algorithms" .%V January 1992 .Re .Rs .%A Bentley, J.L. .%A McIlroy, M.D. .%T "Engineering a Sort Function" .%J "Software--Practice and Experience" .%V Vol. 23(11) .%P pp. 1249-1265 .%D November\ 1993 .Re .Sh STANDARDS The .Fn qsort function conforms to .St -isoC .