How does fork() work?

Question

Im really new to forking, what is the pid doing in this code? Can someone please explain what comes out at line X and line Y ?

#include <sys/types.h>
#include <stdio.h>
#include <unistd.h>
#define SIZE 5
int nums[SIZE] = {0,1,2,3,4};
int main()
{
    int i;
    pid_t pid;
    pid = fork();
    if (pid == 0) {
        for (i = 0; i < SIZE; i++) {
            nums[i] *= -i;
            printf("CHILD: %d ",nums[i]); /* LINE X */
        }
    }
    else if (pid > 0) {
        wait(NULL);
        for (i = 0; i < SIZE; i++)
            printf("PARENT: %d ",nums[i]); /* LINE Y */
    }
    return 0;
}

Answer 1

fork() duplicates the process, so after calling fork there are actually 2 instances of your program running.

How do you know which process is the original (parent) one, and which is the new (child) one?

In the parent process, the PID of the child process (which will be a positive integer) is returned from fork(). That's why the if (pid > 0) { /* PARENT */ } code works. In the child process, fork() just returns 0.

Thus, because of the if (pid > 0) check, the parent process and the child process will produce different output, which you can see here (as provided by @jxh in the comments).