Date: Mon, 01 Nov 1999 15:17:42 -0800 From: Jake Burkholder <jake@checker.org> To: Peter Dufault <dufault@hda.com> Cc: freebsd-arch@freebsd.org Subject: Re: Threads goals version III Message-ID: <19991101231743.F30501FD7@io.yi.org> In-Reply-To: Your message of "Mon, 01 Nov 1999 16:53:10 EST." <199911012153.QAA19185@hda.hda.com>
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> Obviously he can't return though. Here's what I would
> argue for in setting a standards document:
>
> main()
> {
> int mainvar;
>
> newthread(t1); /* t1 is a thread and it can access mainvar... */
>
> sub();
> }
>
> sub()
> {
> int subvar;
>
> newthread(t2); /* t2 can access subvar or mainvar, but maybe
> * t1 can't access subvar.
> * Maybe the stack just crossed a page boundary.
> */
>
> while(1)
> keep_stack_context();
> }
>
I think there's confusion here, forking a new thread is not like
forking a new process. A start routine needs to be passed in
and a new stack and context setup, so the thread begins execution
as if start_routine were its "main". See Daniel's
{get,set,make,swap}context diffs.
I supposed that newthread(t1) could mean that there is now
a new thread, which starts executing at the point newthread
was called (in main), only on a new stack, but that doesn't
seem to me to be the way it is commonly implemented.
int global; /* all threads can access this */
main() {
thread t1;
int mainvar;
/* this could also be mmaped or stack based storage */
t.stack = malloc(some memory);
new_thread(&t1, foo, 1, &mainvar);
sub();
wait(t1);
}
sub() {
thread t2;
int subvar;
/* t1 has no access to this at all */
new_thread(&t2, foo, 1, &subvar);
wait(t2);
}
foo(int ac, ...) {
int *p;
va_list ap;
assert(ac == 1);
va_start(ap, ac);
p = va_arg(ap, int *)
va_end(ap);
/* t1 now has access to mainvar through p */
/* t2 now has access to subvar through p */
}
The WHOLE address space is shared. You can pass around
pointers to stack based storage, but a new thread executes
a new start_routine, so it can't access variables from the
scope in which new_thread was called, only what get passed
into start_routine.
At least that is my understanding. Apologies if I
misinterpreted what you were trying to say.
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