qsort

Sort functions

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Contents

Interface
Description
Return Values
Errors
Compatibility
Page Comments

Interface

#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 * ))

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. 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 <span class="Em">not</span> 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 sorting; in particular, see D.E. Knuth's "Algorithm Q". Quicksort takes $O(N \log N)$ 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 $O(N \log N)$ 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 $O(N \log N)$ ; its best case is

Normally, qsort is faster than mergesort, which is faster than heapsort. Memory availability and pre-existing order in the data can make this untrue.

Example 1

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

int main ()
{
  char strings[4][20] = {"apples", "grapes", "strawberries", "bananas"};
 
  // sort the strings
  qsort(strings, 4, 20, (int(*)(const void*, const void*))strcmp);
 
  // display the strings in ascending lexicographic order
  for (int i = 0; i < 4; ++i)
    printf("%s\n", strings[i]);
 
  return 0;
}

Output:

apples
bananas
grapes
strawberries

Return Values

The qsort and qsort_r functions return no value.

Errors

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.

Compatibility

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

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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 ))