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Manual Reference Pages  -  QSORT (3)


qsort, qsort_r, heapsort, mergesort - sort functions


Return Values
See Also


.Lb libc


.In stdlib.h void
.Fo qsort void *base size_t nmemb size_t size int lp]*comparrp]lp]const void *, const void *rp]
.Fc void
.Fo qsort_r void *base size_t nmemb size_t size void *thunk int lp]*comparrp]lp]void *, const void *, const void *rp]
.Fc int
.Fo heapsort void *base size_t nmemb size_t size int lp]*comparrp]lp]const void *, const void *rp]
.Fc int
.Fo mergesort void *base size_t nmemb size_t size int lp]*comparrp]lp]const void *, const void *rp]


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 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 partition-exchange sorting; in particular, see
.An D.E. Knuth Ns ’s Algorithm Q. Quicksort 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
.An J.W.J. William Ns ’s "heapsort" algorithm, a variant of selection sorting; in particular, see
.An D.E. Knuth Ns ’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 qsort and qsort_r functions return no value.

.Rv -std heapsort mergesort


A sample program that sorts an array of
.Vt int values in place using qsort, and then prints the sorted array to standard output is:
#include <stdio.h>
#include <stdlib.h>

/* * Custom comparison function that can compare ’int’ values through pointers * passed by qsort(3). */ static int int_compare(const void *p1, const void *p2) { int left = *(const int *)p1; int right = *(const int *)p2;

return ((left > right) - (left < right)); }

/* * Sort an array of ’int’ values and print it to standard output. */ int main(void) { int int_array[] = { 4, 5, 9, 3, 0, 1, 7, 2, 8, 6 }; const size_t array_size = sizeof(int_array) / sizeof(int_array[0]); size_t k;

qsort(&int_array, array_size, sizeof(int_array[0]), int_compare); for (k = 0; k < array_size; k++) printf(" %d", int_array[k]); puts(""); return (EXIT_SUCCESS); }


Previous versions of qsort did not permit the comparison routine itself to call qsort 3. This is no longer true.


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.


sort(1), radixsort(3)
.Rs Quicksort
.Rs Heapsort
.Rs Sorting and Searching
.Rs Optimistic Sorting and Information Theoretic Complexity
.Rs Engineering a Sort Function


The qsort function conforms to -isoC.
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