Pointers

A pointer stores a memory address. & gives the address of a variable, and * reads or writes the value at an address.

#include <stdio.h>

void swap(int *a, int *b) {
  int t = *a;
  *a = *b;
  *b = t;
}

int main(void) {
  int x = 1, y = 2;
  swap(&x, &y);
  printf("%d %d\n", x, y);

  int arr[] = {10, 20, 30};
  int *p = arr;
  printf("%d %d\n", *p, *(p + 2));
  return 0;
}

2 1
10 30

C passes arguments by value, so swap needs pointers to change the caller's variables. Adding 2 to a pointer moves it forward by two elements, not two bytes.