IN[59]570 Exercise Session 2

Introduction to Oblig 1.

Joachim Tilsted Kristensen joachkr@ifi.uio.no

University of Oslo, February 02, 2023

Review the revised exercises from last time, and check your solution with the solutions in solutions/session-1 on GitHub.

The Kilroy program.

Here is a more advanced version of the hello world program, that prints to standard out of all connected nodes. Extend the program, to use move. Consult the Emerald language report for the semantics of move.

%                           - -
%                         { 0 0 }
% +===================oOO===(_)===OOo===============+
const WotchIt <-
  object program
    process

      % Greetings.
      const home <- locate self
      stdout.putstring[ "Starting at " || home$LNN.asstring || "\n" ]

      % A counter, and a list of places to go.
      var network : NodeList <- home$activenodes

      % Meet up all nodes in the network.
      for contact in network
        const here <- locate self
        const host <- contact$theNode
        % Remote Method invocation.
        host$stdout.putstring["Comming over. Kind regards, Kilroy\n"]
        % ... call move here ...
      end for

      % Report.
      stdout.putstring["I am done\n"]

    end process
  end program

The object program, should move it self to the other nodes and then back.

Monitors and conditions

The keyword monitor can generally be put in the same places as the word object, and it defines a "monitored" object. (Read Tony Hoare's "Monitors : an operating systems concept" and come back here).

const wait_for_me <-
  monitor class i_am_late

    const invocation <- Condition.create

    export operation stop
      wait invocation
    end stop

    export operation come_on
      signal invocation
    end come_on

  end i_am_late

The object wait_for_me, is a monitored object. It essentially means that you can call a method on it, with guaranteed mutual exclusion.

It starts by creating a condition. You wait for this condition by calling stop, and you can signal it by calling come_on.

1. Make an object that waits for wait_for_me and prints to the screen.
2. Make another object that calls come_on.
3. Run it a couple of times.
4. Does it work?

Challenge.

1. Extend wait_for_me with a method semi_stop that takes an extra argument t of type time. The semantics of semi_stop should be that if invocation has not been signalled within t time. wait_for_me should continue without waiting further.

2. What do you need to extend the cyclic barrier from the Oblig to support a timeout?

3. Extend the cyclic write barrier from the oblig with an API call that waits for the barrier for 10 seconds, and then continues if the barrier has not been signalled.