qsort, qsort_r, heapsort, mergesort  sort functions
Standard C Library (libc, lc)
#include <stdlib.h>
void
qsort(void *base, size_t nmemb, size_t size,
int (*compar)(const void *, const void *));
void
qsort_r(void *base, size_t nmemb, size_t size, void *thunk,
int (*compar)(void *, const void *, const void *));
int
heapsort(void *base, size_t nmemb, size_t size,
int (*compar)(const void *, const void *));
int
mergesort(void *base, size_t nmemb, size_t size,
int (*compar)(const void *, const void *));
The qsort() function is a modified partitionexchange 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 preexisting 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. 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 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 partitionexchange sorting;
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 worstcase 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 worstcase 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 preexisting 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 preexisting order in the data can make this
untrue.
The qsort() and qsort_r() functions return no value.
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:
[EINVAL] The size argument is zero, or, the size argument to
mergesort() is less than ``sizeof(void *) / 2''.
[ENOMEM] The heapsort() or mergesort() 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.
sort(1), radixsort(3)
Hoare, C.A.R., "Quicksort", The Computer Journal, 5:1, pp. 1015, 1962.
Williams, J.W.J, "Heapsort", Communications of the ACM, 7:1, pp. 347348,
1964.
Knuth, D.E., "Sorting and Searching", The Art of Computer Programming,
Vol. 3, pp. 114123, 145149, 1968.
McIlroy, P.M., "Optimistic Sorting and Information Theoretic Complexity",
Fourth Annual ACMSIAM Symposium on Discrete Algorithms, January 1992.
Bentley, J.L. and McIlroy, M.D., "Engineering a Sort Function",
SoftwarePractice and Experience, Vol. 23(11), pp. 12491265,
November 1993.
The qsort() function conforms to ISO/IEC 9899:1990 (``ISO C89'').
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