Fork implementation

Question

How is fork system call code written . I want to know some details how a function can return two different values and that to two different processes . In short want to know how fork system call is implemented?

Answer 1

Carl's answer was great. I'd like to add that in many operating systems return values are passed in one of the registers. In x86 architecture this register might be eax, In ARM architecture this register might be R0, etc.

Each process also have a Process Control Block (PCB), which store values of registers at the time some interrupt, syscall, or exception happened and control was passed to the OS. The next time the process scheduled, the values of the registers are restored from PCB.

Now, when fork() happens, OS can do:

 child_process->PCB[return_value_register] = 0;
 parrent_process->PCB[return_value_register] = child_pid;

So, when the processes are rescheduled, each of them see a different return value.

As an example, you can see xv6's implementation of fork. In there, the parent process is still in running state, so it returns parent's return value using simple return statement. But it sets value of EAX register for child process to 0, so when child process is scheduled it sees 0 as return value:

// Clear %eax so that fork returns 0 in the child.
np->tf->eax = 0;

Note that return 0 will also compile to something like "mov eax, 0".

Update: I just implemented fork() for a hobby OS I am doing. You can see the source code here.