Re: how fork returns value

[Lal <learner.kernel@xxxxxxxxx>]

On Mon, Oct 6, 2008 at 12:03 PM, Mitul Modi <mituld.modi@xxxxxxxxx> wrote:
> hi Lal,
>
> thanks for the analysis and clearing the doubt. so, parent process returns
> while child is added in to runqueue. right?
>

Depends. For example if CLONE_VM is not set, then kernel runs child
process first in anticipation of an exec to avoid COW overhead.
If CLONE_VM is set then kernel puts the child process at the end of
runqueue. The parent process continues through ret_from_intr where
kernel may again decide to schedule it out and run another process
(may be child process or some other).

Regarding CLONE_PARENT_SETTID discussion above, I would like to add
that child process pid is returned to a parent process user space
pointer only if CLONE_PARENT_SETTID is set; otherwise parent process
also returns child process pid through %eax. This mechanism is
required to cater different application requirements through a single
do_fork function.

For example, pthread_create returns thread handle (which internally
may have many other variables including child pid) through an
argument, therefore pthread_create calls clone system call with
CLONE_PARENT_SETTID set, while fork() returns child pid through fork
return value, therefore clone system call is called without
CLONE_PARENT_SETTID.

-Lal

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