Fork and returning twice

Question

I am working on a project that requires implementation of a fork() in unix. I read freeBSD and openBSD source code but it is really hard to understand. Can someone please Explain the returning twice concept? I understand that one return is pid of a child, and that gets returned to parent and other one is zero and it gets returned to a child process. But I cannot wrap my head around how to implement this notion of returning twice... how can I return twice? Thanks everyone in advance.

Answer 1

When you call fork, it returns "twice" in that the fork spawns two processes, which each return.

So, if you're implementing fork, you have to create a second process without ending the first. Then the return-twice behavior will happen naturally: each of the two distinct processes will continue execution, only differing in the value they return (the child giving zero, and the parent giving the child's PID).