Re: Writing upgradable data structures

[daveb@geac.UUCP (David Collier-Brown)]

In article <2417@geac.UUCP> Barry Margolin <rutgers!think!barmar>
writes:
> While I was in the Multics group at Honeywell we did some
> investigation of implementing a Multics followon in Ada.  I thought
> the same thing regarding versioned structures, but I was convinced by
> some more experienced Ada people that we could do this using variant
> records.  When you upgrade the structure you add a new variant.  This
> does require recompiling users of the structure, but it doesn't force
> old, stable programs to be recoded unless they want to take advantage
> of the new structure's features.

   The thing that is hard in Ada[tm] was dealing with a structure
which was *later* than the one you expected. Normal Ada
scope/recompilation rules keep versioned structures in a linked
executable from getting out of date with respect to themselves (with
due care, of course), but don't deal well with a structure which is
sent to it from something external.
   Since this technique came from the ARPAnaut world, you might
guess that the data structures can be packets...

   My work-around is to make sure I pass pointers to things, and not
depend on the `size attribute (which is not exactly what I'd call
good practice... I wonder if its even ethical?).

 --dave c-b
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 David Collier-Brown.                 {mnetor yunexus utgpu}!geac!daveb
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