c - How does fork() know when to return 0?

Question

Take the following example:

int main(void)
{
     pid_t  pid;

     pid = fork();
     if (pid == 0) 
          ChildProcess();
     else 
          ParentProcess();
}

So correct me if I am wrong, once fork() executes a child process is created. Now going by this answer fork() returns twice. That is once for the parent process and once for the child process.

Which means that two separate processes come into existence DURING the fork call and not after it ending.

Now I don't get it how it understands how to return 0 for the child process and the correct PID for the parent process.

This where it gets really confusing. This answer states that fork() works by copying the context information of the process and manually setting the return value to 0.

First am I right in saying that the return to any function is placed in a single register? Since in a single processor environment a process can call only one subroutine that returns only one value (correct me if I am wrong here).

Let's say I call a function foo() inside a routine and that function returns a value, that value will be stored in a register say BAR. Each time a function wants to return a value it will use a particular processor register. So if I am able to manually change the return value in the process block I am able to change the value returned to the function right?

So am I correct in thinking that is how fork() works?

Answer 1

How it works is largely irrelevant - as a developer working at a certain level (ie, coding to the UNIX APIs), you really only need to know that it works.

Having said that however, and recognising that curiosity or a need to understand at some depth is generally a good trait to have, there are any number of ways that this could be done.

First off, your contention that a function can only return one value is correct as far as it goes but you need to remember that, after the process split, there are actually two instances of the function running, one in each process. They're mostly independent of each other and can follow different code paths. The following diagram may help in understanding this:

Process 314159 | Process 271828
-------------- | --------------
runs for a bit |
calls fork     |
               | comes into existence
returns 271828 | returns 0

You can hopefully see there that a single instance of fork can only return one value (as per any other C function) but there are actually multiple instances running, which is why it's said to return multiple values in the documentation.

---

Here's one possibility on how it could work.

When the fork() function starts running, it stores the current process ID (PID).

Then, when it comes time to return, if the PID is the same as that stored, it's the parent. Otherwise it's the child. Pseudo-code follows:

def fork():
    saved_pid = getpid()

    # Magic here, returns PID of other process or -1 on failure.

    other_pid = split_proc_into_two();

    if other_pid == -1:        # fork failed -> return -1
        return -1

    if saved_pid == getpid():  # pid same, parent -> return child PID
        return other_pid

    return 0                   # pid changed, child, return zero

Note that there's a lot of magic in the split_proc_into_two() call and it almost certainly won't work that way at all under the covers^(a). It's just to illustrate the concepts around it, which is basically:

• get the original PID before the split, which will remain identical for both processes after they split.
• do the split.
• get the current PID after the split, which will be different in the two processes.

You may also want to take a look at this answer, it explains the fork/exec philosophy.

---

^(a) It's almost certainly more complex than I've explained. For example, in MINIX, the call to fork ends up running in the kernel, which has access to the entire process tree.

It simply copies the parent process structure into a free slot for the child, along the lines of:

sptr = (char *) proc_addr (k1); // parent pointer
chld = (char *) proc_addr (k2); // child pointer
dptr = chld;
bytes = sizeof (struct proc);   // bytes to copy
while (bytes--)                 // copy the structure
    *dptr++ = *sptr++;

Then it makes slight modifications to the child structure to ensure it will be suitable, including the line:

chld->p_reg[RET_REG] = 0;       // make sure child receives zero

So, basically identical to the scheme I posited, but using data modifications rather than code path selection to decide what to return to the caller - in other words, you'd see something like:

return rpc->p_reg[RET_REG];

at the end of fork() so that the correct value gets returned depending on whether it's the parent or child process.