How is it possible for fork() to return two values?

Question

Since a function in C returns only one value, all the time, how can fork(), which is also a function, return two values?

Answer 1

The key insight here is to think about the fact that after a fork() you have really two copies of your program. These are two processes, running the same exact copy of your code, and the execution pointer is exactly at the same line of code i.e. fork(), poised to return.

The OS arranges for the fork() to return in the parent process with the pid of the child, and for it to return in the child process with zero (if things go well).

That is why they say that fork() returns twice. Once in each process.