Question

I’m modeling an algorithm in Spin.
I have a process that has several channels and at some point, I know a message is going to come but don’t know from which channel. So want to wait (block) the process until it a message comes from any of the channels. how can I do that?

Answer 1

I think you need Promela’s if construct (see http://spinroot.com/spin/Man/if.html).

In the process you’re referring to, you probably need the following:

byte var;
if
:: ch1?var -> skip
:: ch2?var -> skip
:: ch3?var -> skip
fi

If none of the channels have anything on them, then “the selection construct as a whole blocks” (quoting the manual), which is exactly the behaviour you want.

To quote the relevant part of the manual more fully:
“An option [each of the :: lines] can be selected for execution only when its guard statement is executable [the guard statement is the part before the ->]. If more than one guard statement is executable, one of them will be selected non-deterministically. If none of the guards are executable, the selection construct as a whole blocks.”

By the way, I haven’t syntax checked or simulated the above in Spin. Hopefully it’s right. I’m quite new to Promela and Spin myself.