The **`qsort()`** is a C library function that uses a quick sort algorithm to sort an array. Here is how it is declared in C: 

> A **void pointer** is a pointer that can point to any datatype.

The most interesting part of the syntax above is the `comparator` function. It is called by `qsort()`, multiple times, to compare two elements. Since the comparator function is user-defined, it can be used to define the logic for comparing elements of any datatype (i.e. **structs**). 

Consider the declaration of `comparator` below:
```c
int comparator(const void* p1, const void* p2);
```
The function returns an integer which based on the following criteria:

# Example
Consider the code snippet below, which sorts an array in ascending order:

#include <stdlib.h> // needed to use qsort()

int arr[] = {20, 15, 36, -8, 2, 7};

int comparator (const void * p1, const void * p2)
{
  return (*(int*)p1 - *(int*)p2);
}

// driver code
int main ()
{
  printf("The unsorted array is: \n");
  for(int i = 0; i < 6; i++)
  {   
    printf("%d ", arr[i]);
  }  
  qsort(arr, 6, sizeof(int), comparator);
  printf("\nThe sorted array is: \n");
  for(int i = 0; i < 6; i++)
  {   
    printf("%d ", arr[i]);
  }
}

# Explanation
- **Lines 5-7**: The `comparator` function takes two void pointers, `p1` and `p2`, as arguments and returns their difference; this is how `qsort()` determines which element is smaller, or larger, than the other. Note how the pointers are **dereferenced** in **line 7** -- more information on dereferencing C pointers can be found [here](https://www.educative.io/answers/what-is-a-pointer-in-c).

What is the qsort() function in C?

A C library function that uses quick sort to sort arrays with a user-defined comparator function.