# FreeBSD Manual Pages

QSORT(3) BSD Library Functions Manual QSORT(3)NAMEqsort,qsort_r,heapsort,mergesort-- sort functionsLIBRARYStandard C Library (libc, -lc)SYNOPSIS#include<stdlib.h>voidqsort(void*base,size_tnmemb,size_tsize,int(*compar)(constvoid*,constvoid*));voidqsort_r(void*base,size_tnmemb,size_tsize,void*thunk,int(*compar)(void*,constvoid*,constvoid*));intheapsort(void*base,size_tnmemb,size_tsize,int(*compar)(constvoid*,constvoid*));intmergesort(void*base,size_tnmemb,size_tsize,int(*compar)(constvoid*,constvoid*));DESCRIPTIONTheqsort() 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. Themergesort() function 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. Theqsort_r() function behaves identically toqsort(), except that it takes an additional argument,thunk, which is passed unchanged as the first argument to function pointed tocompar. This allows the comparison function to access additional data without using global variables, and thusqsort_r() is suitable for use in functions which must be reentrant. The algorithms implemented byqsort(),qsort_r(), andheapsort() arenotstable, that is, if two members compare as equal, their order in the sorted array is undefined. Themergesort() algorithm is stable. Theqsort() andqsort_r() functions are an implementation of C.A.R. Hoare's "quicksort" algorithm, a variant of partition-exchange sorting; in particular, see D.E. Knuth'sAlgorithmQ.Quicksorttakes O N lg N average time. This implementation 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'sAlgorithmH.Heapsorttakes O N lg N worst-case time. Itsonlyadvantage overqsort() is that it uses almost no additional memory; whileqsort() does not allocate memory, it is implemented using recur- sion. The functionmergesort() requires additional memory of sizenmemb*sizebytes; it should be used only when space is not at a premium. Themergesort() function 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() is faster thanheapsort(). Memory availability and pre-existing order in the data can make this un- true.RETURN VALUESTheqsort() andqsort_r() functions return no value. Theheapsort() andmergesort() functions return the value 0 if success- ful; otherwise the value -1 is returned and the global variableerrnois set to indicate the error.EXAMPLESA sample program that sorts an array ofintvalues in place usingqsort(), and then prints the sorted array to standard output is: #include <stdio.h> #include <stdlib.h> /* * Custom comparison function that can compare 'int' values through pointers * passed by qsort(3). */ static int int_compare(const void *p1, const void *p2) { int left = *(const int *)p1; int right = *(const int *)p2; return ((left > right) - (left < right)); } /* * Sort an array of 'int' values and print it to standard output. */ int main(void) { int int_array[] = { 4, 5, 9, 3, 0, 1, 7, 2, 8, 6 }; const size_t array_size = sizeof(int_array) / sizeof(int_array[0]); size_t k; qsort(&int_array, array_size, sizeof(int_array[0]), int_compare); for (k = 0; k < array_size; k++) printf(" %d", int_array[k]); puts(""); return (EXIT_SUCCESS); }COMPATIBILITYPrevious versions ofqsort() did not permit the comparison routine itself to callqsort(3). This is no longer true.ERRORSTheheapsort() andmergesort() functions succeed unless: [EINVAL] Thesizeargument is zero, or, thesizeargument tomergesort() is less than "sizeof(void *) / 2". [ENOMEM] Theheapsort() ormergesort() functions were unable to allocate memory.SEE ALSOsort(1), radixsort(3) Hoare, C.A.R., "Quicksort",TheComputerJournal, 5:1, pp. 10-15, 1962. Williams, J.W.J, "Heapsort",CommunicationsoftheACM, 7:1, pp. 347-348, 1964. Knuth, D.E., "Sorting and Searching",TheArtofComputerProgramming, Vol. 3, pp. 114-123, 145-149, 1968. McIlroy, P.M., "Optimistic Sorting and Information Theoretic Complexity",FourthAnnualACM-SIAMSymposiumonDiscreteAlgorithms, January 1992. Bentley, J.L. and McIlroy, M.D., "Engineering a Sort Function",Software--PracticeandExperience, Vol. 23(11), pp. 1249-1265, November 1993.STANDARDSTheqsort() function conforms to ISO/IEC 9899:1990 ("ISO C90"). BSD February 20, 2013 BSD

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