QSORT(3)

QSORT(3) | Library Functions Manual | QSORT(3)

# NAME

`qsort`, `heapsort`, `mergesort`, `qsort_r`, `heapsort_r`, `mergesort_r` — sort functions

# LIBRARY

Standard C Library (libc, -lc)

# SYNOPSIS

`#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, int (*compar)(const void *, const void *, void *), void *cookie);

int
`heapsort`(void *base, size_t nmemb, size_t size, int (*compar)(const void *, const void *));

int
`heapsort_r`(void *base, size_t nmemb, size_t size, int (*compar)(const void *, const void *, void *), void *cookie);

int
`mergesort`(void *base, size_t nmemb, size_t size, int (*compar)(const void *, const void *));

int
`mergesort_r`(void *base, size_t nmemb, size_t size, int (*compar)(const void *, const void *, void *), void *cookie);

# DESCRIPTION

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 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 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_r`(), `heapsort_r`(), and `mergesort_r`() functions pass an additional cookie argument through to the comparison function.

The `qsort`() function is 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. `qsort`() 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 nmemb * size bytes; it should be used only when space is not at a premium. `mergesort`() 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.

# RETURN VALUES

The `qsort`() and `qsort_r`() functions return no value.

Upon successful completion, `heapsort`(), `mergesort`() `heapsort_r`(), and `mergesort_r`() return 0. Otherwise, they return -1 and the global variable errno is set to indicate the error.

# COMPATIBILITY

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

# ERRORS

The `heapsort`(), `mergesort`() `heapsort_r`(), and `heapsort_r`() functions succeed unless:

[`EINVAL`]
  The size argument is zero, or, the size argument to `mergesort`() or `mergesort_r`() is less than “sizeof(void *) / 2”.
[`ENOMEM`]
  `heapsort`(), `heapsort_r`(), `mergesort`(), or `mergesort_r`() were unable to allocate memory.

# SEE ALSO

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, 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.

# STANDARDS

The `qsort`() function conforms to ANSI X3.159-1989 (“ANSI C89”).

# HISTORY

The `qsort_r`(), `heapsort_r`(), and `mergesort_r`() functions were added in NetBSD 11.0.

The `qsort_r`() function conforms to IEEE Std 1003.1-2024 (“POSIX.1”).

July 20, 2025 | NetBSD 11.0