shared variables in parfor()

Hello,

I naively thought that the following code would print a few zeros, which
would then follow by all ones:

my(q=0); parfor(i=1,10^6, print(q); 0, R, q=1)

but it constantly prints zeros. So, the value of q is not shared between
threads.
What is the right way to create a shared variable (with pthreads if that
matters)?

Regards,
Max