Re: linuxthreads and tid testing

Eric Paire (
Mon, 02 Aug 1999 08:28:10 +0200

> On Fri, 30 Jul 1999, Eric Paire wrote:
> > Don't you think that this should be the purpose of the CLONE_PID flag ?
> > With this flag, all system calls returning the current pid would return
> > the pid of the last clone() call without this flag set, except getpid()
> > (in order to get a standard way to get the actual kernel pid of a
> > CLONE_PID thread, and because this could be hidden by the libpthread).
> Doesn't this strike you as a bit ugly? If EVERYTHING is using the same
> pid, then why not make getpid() use it as well, and then add a system call
> to get the thread ID. It's NOT an appreciable amount of extra kernel
> code, and it makes everything a lot more consistent. Keeping the kernel
> small is an admirable goal, but not if it means inventing completely
> pointless and arbitrary behavior to eliminate a few lines of kernel code.
> Sometimes the kernel is the right place for implementing certain behavior,
> especially when that behavior changes the common conception of how the
> kernel performs its tasks.
I don't think that Linus Torvalds is ready to see the notion of thread id
to invade the kernel (and I think he is right). The PID should be enough
for what we want to do in user space. I just think that we should be able
to hide from a user space program the fact that the POSIX semantics wants
to attribute 1 PID to all threads to a program. Maybe, this should all be
done in user space (comments Ulrich ?) by wrapping all system calls returning
PID in their data. The main drawback I see for this is that each time a
new system call return PID, the libpthread should also be aligned to wrap
it. This is the reason why I think it is better to do this wrapping inside
the kernel.

BTW, If the fact that getpid() is a CLONE_PID rule exception hurts you,
then a simple way to avoid such is to return 2 pids (as for fork):
1) the standard one (which is the one potentially fixed by CLONE_PID), and
2) the effective one (as seen by the kernel).

+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+ Eric PAIRE
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