Re: how fork returns value

[Rene Herman <rene.herman@xxxxxxxxxxxx>]

On 07-10-08 17:55, Peter Teoh wrote:

> thank you for the answer.   next question (out of curiosity, sorry if
> it is overstretching the OP) is in architecture without any MMU,
> what does syscall like fork() get translate into?

Please note that this is not something generic for a (fork-implementing) 
OS on a non-MMU architecture or anything. It's perfectly possible to 
implement fork() with all the expected semantics simply by actually 
physically copying the address space and for that you don't need an MMU.

For example MINIX, originally written for the Intel 8086 (a CPU without 
MMU), did/does. Heck, all "original" UNIX implementations did ...

> the fact that fork() does not exists on non-MMU is discussed
> everywhere, but then how is it replaced?   For example, the following
> seemed to imply fork() inside application should continue to compile
> (but should not give error when run, right?):
>
> http://mailman.uclinux.org/pipermail/uclinux-dev/2003-September/020871.html
> http://mailman.uclinux.org/pipermail/uclinux-dev/2003-September/021010.html
>
> is it just a passthrough?

... but uclinux doesn't it seems.

I've never really concerned myself with uclinux but I assume this was 
done mostly as a forced optimization: actually copying the address space 
is ofcourse rather slow and given that the vast majority of fork() calls 
is followed by a rather immediate exec() (which immediately retires that 
address space again) this will often be totally in vain. In that sense, 
I suppose it makes sense to force ports of fork() using applications ...

... to, generally, vfork() using applications. vfork() is a "fake fork" 
(or at least, can/will be implemented as such) which historically exists 
to avoid that exact copy -- see the man page.

As said, never really concerned myself with uclinux, but no, I don't 
believe this will be an automatic translation thing (ie, still compile) 
but will have to be done manually.

> And does   "Task-struct" even exists in non-MMU architecture?

Again with the slight disclaimer towards the specifics of uclinux, yes, 
definitely. The task_struct is the embodiment of the schedulable entity 
(the process, if we ignore threads) and if uclinux were a single-tasking 
OS it would be very overly complex for its purpose.

Rene.

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