[Vm-dev] Re: [squeak-dev] Criteria For Plugin Compatibility

Eliot Miranda eliot.miranda at gmail.com
Fri Sep 11 19:00:21 UTC 2015

Hi Chris,

On Fri, Sep 11, 2015 at 9:01 AM, Chris Cunnington <brasspen at gmail.com>

> I had the idea that a plugin from one VM could be dragged into another
> VM's directory and that it could be used just by starting up the image.
> I've done a little experimenting and it seems more of an idea of a reality.
> The functionality is there, but VM developers over the years have not seen
> this as a priority. Typically plugins and their VMs are compiled together.
> Or a person adds one by recompiling a VM compilation rig they already have.

The problems I see with this are

a) a plugin compiled for Spur may not work with V3 or vice verse.  The
issue is the header size of an object.  Some plugins, but not all, use this
define, and the header sizes between Spur and V3 are incompatible.

b) 32-bit plugins won't work with 64-bit VMs, and 64-bit plugins won't work
with 32-bit VMs. Period.

Now there are platform-level packaging technologies, such as fat binaries
on Mac OS X, that allow one to construct plugins that contain more than one
binary.  But this is a lot of work to build and maintain.

So personally I wouldn't put much effort into this level of drag-and-drop
compatibility.  Instead I'd put energy into good error messages so that
when plugins don't work the user can find out why, and that when the wrong
kind of plugin is used the VM doesn't just stumble along, maybe producing
incorrect results, but instead puts the plugin out with a comprehensible

Does this make sense?  I know its a downer, but what you propose is, IMO,
not affordable given our resources.

And I must say, *this is a VM-DEV discussion, not a general purpose Squeak
discussion*, yes?

As I had a rig for the Interpreter VM I decided to compile a plugin and see
> if I could drag it around to other VM directories for use. Most times it
> didn't work.
> I compiled a TheUniversalAnswer plugin which returns 42. I moved it from a
> 4.14.1-3430 VM to a 4.14.1-3414 VM. I could get it to work if I did not use
> the squeak.sh start script. That is, I dragged so.vm-sound-null,
> so.TheUniversalAnswer and so.vm-display-X11 into the same directory as the
> VM binary. That worked.
> The only way to see the external plugins is with #listLoadedModules. But,
> irritatingly, modules are loaded as needed, so once you have proof of using
> the primitive from the plugin, yea, it will appear as a result of that
> Smalltalk listLoadedModules message. So, it's not that useful.
> I dragged so.TheUniversalAnswer around to other VMs such as 4.0.3-2202 and
> 4.13.10-3268 without success. And the unload selectors #forgetModule: and
> unloadModule:, which both use primitive 571 don't appear to work on my
> Ubuntu 15.04.
> So, I don't know if a community of plugins passed around is in the offing.
> What I have learned is that many of the answers are in
> sqUnixExternalPrims.c. The issues are really where does the VM look for
> primitives and what does it do when it finds one. It appears to me, after
> reading both the non-Cog and Cog versions of that file, that this an
> interesting area poorly documented that VM developers alter in haste to get
> on to something else.
> It's pretty important stuff, though. Do you start the VM with the binary
> on the path? Do you use squeak.sh? Where is the VM looking for stuff. Where
> will FFI look for stuff. And so on.
> Chris

best, Eliot
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