What exactly does fork return?

Question

On success, the PID of the child process is returned in the parent’s thread of execution, and a 0 is returned in the child’s thread of e

Answer 1

Once fork is executed, you have two processes. The call returns different values to each process.

If you do something like this

int f;
f = fork();
if (f == 0) {
  printf("I am the child\n");
} else {
  printf("I am the parent and the childs pid is %d\n",f);

}

You will see both the messages printed. They're being printed by two separate processes. This is they way you can differentiate between the two processes created.