QSORT(3) BSD Library Functions Manual QSORT(3)
heapsort, heapsort_b, mergesort, mergesort_b, qsort, qsort_b, qsort_r -- sort functions
heapsort(void *base, size_t nel, size_t width, int (*compar)(const void *, const void *));
heapsort_b(void *base, size_t nel, size_t width, int (^compar)(const void *, const void *));
mergesort(void *base, size_t nel, size_t width, int (*compar)(const void *, const void *));
mergesort_b(void *base, size_t nel, size_t width, int (^compar)(const void *, const void *));
qsort(void *base, size_t nel, size_t width, int (*compar)(const void *, const void *));
qsort_b(void *base, size_t nel, size_t width, int (^compar)(const void *, const void *));
qsort_r(void *base, size_t nel, size_t width, void *thunk, int (*compar)(void *, const void *, const void *));
The qsort() 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 qsort() 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 requires 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 respec-
tively less than, equal to, or greater than the second.
The qsort_r() 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(), qsort_r(), and heapsort() are not stable; that is, if two members compare as equal, their order in the
sorted array is undefined. The mergesort() algorithm is stable.
The qsort() and qsort_r() functions are an implementation of C.A.R. Hoare's ``quicksort'' algorithm, a variant of partition-exchange sort-
ing; in particular, see D.E. Knuth's Algorithm Q. Quicksort takes O N lg N average time. This implementation uses median selection to avoid
its O N**2 worst-case behavior.
The heapsort() 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. Its only advantage over qsort() is that it uses almost no additional
memory; while qsort() does not allocate memory, it is implemented using recursion.
The function mergesort() 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, qsort() is faster than mergesort(), which is faster than heapsort(). Memory availability and pre-existing order in the data can
make this untrue.
The heapsort_b(), mergesort_b(), and qsort_b() 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.
The heapsort(), heapsort_b(), mergesort() and mergesort_b() functions succeed unless:
[EINVAL] The width argument is zero, or, the width argument to mergesort() or mergesort_b() is less than ``sizeof(void *) / 2''.
[ENOMEM] The heapsort(), heapsort_b(), mergesort() and mergesort_b() functions were unable to allocate memory.
Previous versions of qsort() did not permit the comparison routine itself to call qsort(3). This is no longer true.
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
Bentley, J.L. and McIlroy, M.D., "Engineering a Sort Function", Software--Practice and Experience, Vol. 23(11), pp. 1249-1265,
The qsort() function conforms to ISO/IEC 9899:1990 (``ISO C90'').
September 30, 2003 BSD