# FreeBSD Manual Pages

QSORT(3) FreeBSD Library Functions Manual QSORT(3)NAMEqsort,qsort_b,qsort_r,heapsort,heapsort_b,mergesort,mergesort_b-- sort functionsLIBRARYStandard C Library (libc, -lc)SYNOPSIS#include<stdlib.h>voidqsort(void*base,size_tnmemb,size_tsize,int(*compar)(constvoid*,constvoid*));voidqsort_b(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*));intheapsort_b(void*base,size_tnmemb,size_tsize,int(^compar)(constvoid*,constvoid*));intmergesort(void*base,size_tnmemb,size_tsize,int(*compar)(constvoid*,constvoid*));intmergesort_b(void*base,size_tnmemb,size_tsize,int(^compar)(constvoid*,constvoid*));#define__STDC_WANT_LIB_EXT1__1errno_tqsort_s(void*base,rsize_tnmemb,rsize_tsize,int(*compar)(constvoid*,constvoid*,void*),void*thunk);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. Theqsort_b() function behaves identically toqsort(), except that it takes a block, rather than a function pointer. The algorithms implemented byqsort(),qsort_r(), andheapsort() arenotstable, that is, if two members compare as equal, their order in the sorted array is undefined. Theheapsort_b() function behaves identically toheapsort(), except that it takes a block, rather than a function pointer. Themergesort() algorithm is stable. Themergesort_b() func- tion behaves identically tomergesort(), except that it takes a block, rather than a function pointer. 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. Theqsort_s() function behaves the same asqsort_r(), except that:-The order of arguments is different-The order of arguments tocomparis different-ifnmemborsizeare greater than RSIZE_MAX, ornmembis not zero andcomparis NULL, then the runtime-constraint handler is called, andqsort_s() returns an error. Note that the handler is called beforeqsort_s() returns the error, and the handler function might not re- turn.RETURN VALUESTheqsort() andqsort_r() functions return no value. Theqsort_s() func- tion returns zero on success, non-zero on error. 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 compares '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 }; 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); }COMPATIBILITYThe order of arguments for the comparison function used withqsort_r() is different from the one used byqsort_s(), and the GNU libc implementation ofqsort_r(). When porting software written for GNU libc, it is usually possible to replaceqsort_r() withqsort_s() to work around this problem.qsort_s() is part of theoptionalAnnex K portion of ISO/IEC 9899:2011 ("ISO C11") and may not be portable to other standards-conforming plat- forms. Previous 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").qsort_s() conforms to ISO/IEC 9899:2011 ("ISO C11") K.3.6.3.2.HISTORYThe variants of these functions that take blocks as arguments first ap- peared in Mac OS X. This implementation was created by David Chisnall. FreeBSD 13.0 January 20, 2020 FreeBSD 13.0

NAME | LIBRARY | SYNOPSIS | DESCRIPTION | RETURN VALUES | EXAMPLES | COMPATIBILITY | ERRORS | SEE ALSO | STANDARDS | HISTORY

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