qsort is a C standard library function that can sort arrays of any type in ascending order.

It works by using a comparison function, specified by the user, to determine the relative order of elements in the array. This comparison function must take two const void* arguments and return an integer less than, equal to, or greater than zero, depending on whether the first argument is considered less than, equal to, or greater than the second argument, respectively.

Here’s an example of using qsort to sort an array of strings alphabetically:

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

int cmpstr(const void* p1, const void* p2) {
  // Cast the arguments from `const void*` to `const char*`
  const char* s1 = *(const char* const*) p1;
  const char* s2 = *(const char* const*) p2;

  return strcmp(s1, s2);
}

int main() {
  char* strings[] = {"hello", "world", "foo", "bar"};
  int n = sizeof(strings) / sizeof(strings[0]);

  qsort(strings, n, sizeof(char*), cmpstr);

  for (int i = 0; i < n; i++) {
    printf("%s\n", strings[i]);
  }

  return 0;
}

In the comparison function cmpstr, the arguments p1 and p2 are of type const void*, which means that they are pointers to constant data of any type. To compare the strings, we need to cast these arguments from const void* to const char*. The *(const char* const*) syntax is used to dereference the const void* argument and cast it to a const char* pointer. This double indirection is necessary because the array strings is actually an array of pointers to strings, not an array of strings.

After casting the arguments to const char*, we can use the strcmp function to compare the two strings and return the result. This is passed to qsort as the comparison function, and qsort uses it to sort the array of strings in ascending order.