QSORT(3) BSD Library Functions Manual QSORT(3)
qsort, heapsort, mergesort -- sort functions
Standard C Library (libc, -lc)
qsort(void *base, size_t nmemb, size_t size, int (*compar)(const void *, const void *));
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 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 nmemb objects, the initial member of
which is pointed to by base. The size of each object is specified by size. mergesort()
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 func-
tion pointed to by compar, which requires two arguments pointing to the objects being com-
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
The functions qsort() and heapsort() are not stable, that is, if two members compare as
equal, their order in the sorted array is undefined. The function mergesort() is stable.
The qsort() function is an implementation of C.A.R. Hoare's ``quicksort'' algorithm, a vari-
ant of partition-exchange sorting; in particular, see D.E. Knuth's Algorithm Q. qsort()
takes O N lg N average time. This implementation uses median selection to avoid its O N**2
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 addi-
tional 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. mergesort() is optimized for data with pre-exist-
ing 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 qsort() function returns no value.
Upon successful completion, heapsort() and mergesort() return 0. Otherwise, they return -1
and the global variable errno is set to indicate the error.
The heapsort() function succeeds unless:
[EINVAL] The size argument is zero, or, the size argument to mergesort() is less
than ``sizeof(void *) / 2''.
[ENOMEM] heapsort() or mergesort() were unable to allocate memory.
Previous versions of qsort() did not permit the comparison routine itself to call qsort().
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,
McIlroy, P.M., "Optimistic Sorting and Information Theoretic Complexity", Proceedings of the
Fourth Annual ACM-SIAM Symposium on Discrete Algorithms, pp. 467-474, 1993.
Bentley, J.L. and McIlroy, M.D., "Engineering a Sort Function", Software-Practice and
Experience, Vol. 23, pp. 1249-1265, 1993.
The qsort() function conforms to ANSI X3.159-1989 (``ANSI C89'').
BSD June 4, 1993 BSD