Criteria for Plugin Compatibility

Posted on: September 11, 2015

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.

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.


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