QSORT(3) BSD Programmer's Manual QSORT(3)

qsort,heapsort,mergesort- sort functions

#include <stdlib.h>voidqsort(void *base,size_t nmemb,size_t size,int (*compar)(const void*,const void*));intheapsort(void *base,size_t nmemb,size_t size,int (*compar)(const void*,const void*));intmergesort(void *base,size_t nmemb,size_t size,int (*compar)(const void*,const void*));

Theqsort() function is a modified partition-exchange sort, or quicksort. Theheapsort() function is a modified selection sort. Themergesort() function is a modified merge sort with exponential search intended for sorting data with pre-existing order. Theqsort() andheapsort() functions sort an array ofnmembobjects, the initial member of which is pointed to bybase. The size of each object is specified bysize.mergesort() behaves similarly, butrequiresthatsizebe greater than "sizeof(void *) / 2". The contents of the arraybaseare sorted in ascending order according to a comparison function pointed to bycompar, which requires two arguments pointing to the objects being compared. 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. The functionsqsort() andheapsort() arenotstable, that is, if two members compare as equal, their order in the sorted array is undefined. The functionmergesort() is stable. Theqsort() 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.qsort() takes O N lg N average time. This im- plementation uses median selection to avoid its O N**2 worst-case behavior. Theheapsort() 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.heapsort() takes O N lg N worst-case time. This implementation ofheapsort() is implemented without recursive function calls. The functionmergesort() requires additional memory of sizenmemb * sizebytes; it should be used only when space is not at a premium.mergesort() is optimized for data with pre-existing order; its worst case time is O N lg N; its best case is O N. Normally,qsort() is faster thanmergesort(), which is faster thanheap-sort(). Memory availability and pre-existing order in the data can make this untrue.

Theqsort() function returns no value. Upon successful completion,heapsort() andmergesort() return 0. Other- wise, they return -1 and the global variableerrnois set to indicate the error.

Theheapsort() andmergesort() functions succeed unless: [EINVAL] Thesizeargument is zero, or thesizeargument tomer-gesort() is less than "sizeof(void *) / 2". [ENOMEM]heapsort() ormergesort() were unable to allocate memory.

sort(1), radixsort(3) Hoare, C.A.R., "Quicksort",The Computer Journal, 5:1, pp. 10-15, 1962. Williams, J.W.J, "Heapsort",Communications of the ACM, 7:1, pp. 347-348, 1964. Knuth, D.E., "Sorting and Searching",The Art of Computer Programming, Vol. 3, pp. 114-123, 145-149, 1968. McIlroy, P.M., "Optimistic Sorting and Information Theoretic Complexity",Fourth Annual ACM-SIAM Symposium on Discrete Algorithms, pp. 467-464, January 1993. Bentley, J.L. and McIlroy, M.D., "Engineering a Sort Function",Software-Practice and Experience, Vol. 23(11), pp. 1249-1265, November 1993.

Previous versions ofqsort() did not permit the comparison routine itself to callqsort(). This is no longer true. Theqsort() function conforms to ANSI X3.159-1989 ("ANSI C"). MirOS BSD #10-current June 4, 1993 1

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