QSORT(3) Library Functions Manual QSORT(3)

heapsort, heapsort_b, mergesort, mergesort_b, qsort, qsort_b, qsort_r — sort functions

#include <stdlib.h>

int
heapsort(void *base, size_t nel, size_t width, int (*compar)(const void *, const void *));

int
heapsort_b(void *base, size_t nel, size_t width, int (^compar)(const void *, const void *));

int
mergesort(void *base, size_t nel, size_t width, int (*compar)(const void *, const void *));

int
mergesort_b(void *base, size_t nel, size_t width, int (^compar)(const void *, const void *));

void
qsort(void *base, size_t nel, size_t width, int (*compar)(const void *, const void *));

void
qsort_b(void *base, size_t nel, size_t width, int (^compar)(const void *, const void *));

void
qsort_r(void *base, size_t nel, size_t width, void *thunk, int (*compar)(void *, const void *, const void *));

The () function is a modified partition-exchange sort, or quicksort. The heapsort() function is a modified selection sort. The mergesort() function is a modified merge sort with exponential search, intended for sorting data with pre-existing order.

The () and heapsort() functions sort an array of nel objects, the initial member of which is pointed to by base. The size of each object is specified by width. The mergesort() function behaves similarly, but that width be greater than or equal to “sizeof(void *) / 2”.

The contents of the array base are sorted in ascending order according to a comparison function pointed to by compar, 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 () function behaves identically to qsort(), except that it takes an additional argument, thunk, which is passed unchanged as the first argument to function pointed to compar. This allows the comparison function to access additional data without using global variables, and thus qsort_r() is suitable for use in functions which must be reentrant.

The algorithms implemented by (), qsort_r(), and heapsort() are stable; that is, if two members compare as equal, their order in the sorted array is undefined. The mergesort() algorithm is stable.

The () and qsort_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's Algorithm Q. takes O N lg N average time. This implementation uses median selection to avoid its O N**2 worst-case behavior.

The () 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. takes O N lg N worst-case time. Its advantage over qsort() is that it uses almost no additional memory; while qsort() does not allocate memory, it is implemented using recursion.

The function () requires additional memory of size nel * width bytes; it should be used only when space is not at a premium. The 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.

Normally, () is faster than mergesort() which is faster than heapsort(). Memory availability and pre-existing order in the data can make this untrue.

The (), (), and () routines are like the corresponding routines without the _b suffix, expect that the compar callback is a block pointer instead of a function pointer.

The qsort(), qsort_b() and qsort_r() functions return no value.


The heapsort(), heapsort_b(), mergesort(), and mergesort_b() functions return the value 0 if successful; otherwise the value -1 is returned and the global variable errno is set to indicate the error.

Previous versions of qsort() did not permit the comparison routine itself to call qsort(3). This is no longer true.

The heapsort(), heapsort_b(), mergesort(), and mergesort_b() functions succeed unless:

[]
The width argument is zero, or, the width argument to mergesort() or mergesort_b() is less than “sizeof(void *) / 2”.
[]
The heapsort(), heapsort_b(), mergesort(), or mergesort_b() functions 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, January 1992.

Bentley, J.L. and McIlroy, M.D., Engineering a Sort Function, Software--Practice and Experience, Vol. 23(11), pp. 1249-1265, November 1993.

The qsort() function conforms to ISO/IEC 9899:1990 (“ISO C90”).

September 30, 2003 macOS 15.0