Confusion about init_task (pid 0 or pid 1?)

Question

I'm playing with the Linux kernel, and one thing that I don't understand is the pid of the init_task task.

As far as I know, there are two special pids: pid 0 for the idle/swapper task, and pid 1 for the init task.

Every online resource (e.g. one, two) I could find say that the init_task task represents the swapper task, i.e. it should have pid 0.

But when I print all the pids using the for_each_process macro, which starts from init_task, I get pid 1 as the first process. I don't get pid 0 at all. Which means that init_task has pid 1, and that it's the init task (?!).

Please help me resolve this confusion.

P.S. the kernel version is 2.4.

Answer 1

The reason for my confusion was the tricky definition of the for_each_task macro:

#define for_each_task(p) \
        for (p = &init_task ; (p = p->next_task) != &init_task ; )

Even though it seems that p starts from init_task, it actually starts from init_task.next_task because of the assignment in the condition.

So for_each_task(p) { /* ... */ } could be rewritten as:

p = init_task.next_task;
while(p != &init_task)
{
    /* ... */
    p = p->next_task;
}

As it can be seen, the swapper process is not part of the iteration.