heapsort man page
heapsort, mergesort — sort functions
heapsort(void *base, size_t nmemb, size_t size, int (*compar)(const void *, const void *));
mergesort(void *base, size_t nmemb, size_t size, int (*compar)(const void *, const void *));
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 heapsort() function sorts an array of nmemb objects, the initial member of which is pointed to by base. The size of each object is specified by size. The mergesort() function behaves similarly, but requires that size be greater than “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 algorithm implemented by heapsort() is not stable, that is, if two members compare as equal, their order in the sorted array is undefined. The mergesort() algorithm is stable.
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 nmemb * size 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() is faster than heapsort(). Memory availability and pre-existing order in the data can make this untrue.
The heapsort() and mergesort() 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() and mergesort() functions succeed unless:
The size argument is zero, or, the size argument to mergesort() is less than “sizeof(void *) / 2”.
The heapsort() or mergesort() functions were unable to allocate memory.
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 man page mergesort(3) is an alias of heapsort(3).