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CO538 Anonymous Questions and Answers Keyword Index

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Keyword reference for chan-type


Question 49 (2011):

Submission reference: IN2082

Hi, this is a follow-up to Question 48 (2011).

Maybe I should be a little more specific. Consider the following;

    [snipped code]
    [n]OCCADE.SPRITE! bundle:
      PAR --This could be replicated.
        occade.start.sprite (occade, bundle[0], -1)
        bundle[0][req] ! load.image; "someimage.bmp"
        bundle[0][req] ! move; 20; 10; FALSE
        bundle[0][req] ! visible; TRUE
        occade.start.sprite (occade, bundle[1], -1)
        bundle[1][req] ! load.image; "someimage.bmp"
        bundle[1][req] ! move; 20; 10; FALSE
        bundle[1][req] ! visible; TRUE

    [snipped code]

Currently I have it working in a very horrible way involving the following and a lot of not replicated stuff. (it's currently not very dynamic!);

    OCCADE.SPRITE! t0,t1,t2,t3,t4:                  -- The "Philosophers" sat down
    OCCADE.SPRITE! node0,node1,node2,node3,node4:   -- ... and eating
    OCCADE.SPRITE! f0,f1,f2,f3,f4:                  -- Forks when they are at rest
    OCCADE.SPRITE! l0,l1,l2,l3,l4:                  -- Forks when they are left forks
    OCCADE.SPRITE! r0,r1,r2,r3,r4:                  -- Forks when they are right forks

Answer 49:

The declaration of your bundle array should be:

    MOBILE [n]OCCADE.SPRITE! bundle:

The latter declarations should also be arrays (which enables us to replicate code):

    MOBILE [n.philosophers]OCCADE.SPRITE! t:      -- The "Philosophers" sat down
    MOBILE [n.philosophers]OCCADE.SPRITE! node:   -- ... and eating
    MOBILE [n.philosophers]OCCADE.SPRITE! f:      -- Forks when they are at rest
    MOBILE [n.philosophers]OCCADE.SPRITE! l:      -- Forks when they are left forks
    MOBILE [n.philosophers]OCCADE.SPRITE! r:      -- Forks when they are right forks

But ... you haven't asked a question?

Keywords: occade , chan-type

Referrers: Question 50 (2011)

Question 48 (2011):

Submission reference: IN2081

If you have a CHAN TYPE e.g.

        CHAN TEST.a.one?:
        CHAN TEST.a.two!:
        CHAN TEST.a.three!:

Is it then possible to do the following;

    PROC a ()
      [1000]TEST.a! bundle:

instead of;

    PROC a ()
      TEST.a! b1,b2,b3,...:

Answer 48:

There are syntax errors in your CHAN TYPE declaration. I'm assuming you are not wanting a recursive channel type (i.e. one that can carry its own channel-type-ends in some of its channel fields – although this is possible)?   A syntax-legal declaration is:

    PROTOCOL TEST.a IS INT:    -- say

        CHAN TEST.a one?:
        CHAN TEST.a two!:
        CHAN TEST.a three!:

We can declare an array of channel-type-ends ... but because only MOBILE channel types are currently allowed, such an array must also be declared MOBILE.

However, there is no point declaring an array of just one end of a channel type – we need two arrays: one for each end. Further, declaring channel-type end variables (or arrays) does not construct the channel bundles. As in Java, this is done dynamically:


    PROC a ()

      MOBILE [N.SERVERS]TEST? bundle.server:                    -- elements not initialised
      MOBILE [N.SERVERS]SHARED TEST! bundle.client:             -- elements not initialised


        SEQ i = 0 FOR N.SERVERS
          bundle.client[i], bundle.server[i] := MOBILE TEST     -- construct a channel bundle

	-- all elements of both bundle arrays are now initialised
	-- the number of channel bundles constructed is N.SERVERS
	-- for each i, bundle.client[i] and bundle.server[i] hold
	-- opposite ends of the same channel bundle
	-- for each i, any number of (client) processes may share
	-- use of bundle.client[i] (because it's SHARED)
	-- for each i, only one (server) process may be given and
	-- use bundle.server[i] (because it's not SHARED)

        ...  etc.


Now we can give each bundle.server[i] to its own server process and each bundle.client[i] to as many client processes (for the server at the other end) as we like.

Note: the client end array was declared to hold SHARED ends only because that is a common paradigm. Neither end need be shared ... or both can be shared ... or just the server side.

Note: the material on MOBILE data and channel types is not an examinable part of this course module.

Keywords: chan-type

Referrers: Question 49 (2011)

Question 34 (2011):

Submission reference: IN2064

Hi, is is possible to have channels, both shared and normal in a RECORD? I have this:

      CHAN INT x?:
      CHAN INT y?:

but this doesn't compile when used with SHARED CHANs. Is there a way of doing this, or do I just need to pass the SHARED CHANs individually, as it doesn't seem possible to have an array of them either?

Answer 34:

Fields of a CHAN TYPE RECORD (informally known as channel bundles) can only be CHANs (with field names that specfify the direction of use) – so the direct answer to your question is: no. However, ...

We only declare variables for the ends of such bundles of channels. The type names of these ends are the type names of the bundles, Bundle type ends with the "!" specifier mean that their channel fields must be used in the opposite directions to those declared in the bundle type (they are the opposite ends of the bundle). Note: it's the the type of the bundle ends that carry the "?" or "!" specifiers – not the variables that hold them (whereas, for standard CHANs, direction specifiers go with the channel variables; we apologise for this inconsistency).

Now, bundle type ends (i.e. either end of the whole bundle) may be SHARED, which should give you what you were after (I hope). Many processes may be given the same end of a shared bundle of channels. To use them, just CLAIM the bundle-end variable and use the individual channels as you like (in the correct directions, of course). See the Mobiles slides 39-43 for examples.

Note: the material on these these channel bundles is not an examinable part of this module.

Re. your question about arrays of shared channels (ordinary ones): sorry, these are not supported – see Question 30 (2010). However, arrays of shared ends of channel bundles are supported, :).

Keywords: shared-channel , chan-type

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