Question

I can’t find any information regarding what sorting algorithm C qsort function uses.

Is it quicksort? It is not mentioned in man.

Answer 1

The implementation of qsort is not specified: an implementation may use any sorting algorithm. Interestingly, the sort does not need to be stable, and there is no complexity requirement.

The entire specification of qsort (C11 §7.22.5.2) is as follows:

The qsort function

Synopsis

#include <stdlib.h>
void qsort(void *base, size_t nmemb, size_t size,
     int (*compar)(const void *, const void *));

Description

The qsort function sorts an array of nmemb objects, the initial element of which is
pointed to by base. The size of each object is specified by size.

The contents of the array are sorted into ascending order according to a comparison function pointed to by compar, which is called with two arguments that point to the objects being compared. The function shall 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.

If two elements compare as equal, their order in the resulting sorted array is unspecified.

Returns

The qsort function returns no value.