From the license point of view:
Is it possible to use a plugin written under "LGPLv3+" in a program protected by "GPLv2"?
The plugin is built as a ".so" library and at runtime linked to the application.


This answer is predicated on the assumption that dynamic linking does make a derivative work for copyright purposes, which is not a settled issue (pro, contra). That said, in my opinion, you may not do this. Any additional code used in a GPLv2 program falls under GPLv2 s2b, which says that

You must cause any work that you distribute or publish, that in whole or in part contains or is derived from the Program or any part thereof, to be licensed as a whole at no charge to all third parties under the terms of this License

LGPLv3 permits on-the-fly relicensing under GPLv3 (more accurately, it is GPLv3, with a set of optional extra permissions), but that's no help, because the FSF say that "GPLv2 is, by itself, not compatible with GPLv3".

The FSF also directly address your point, and say that "LGPLv3 is not compatible with GPLv2 by itself".

If the original work had been GPLv2+, you could have taken it under GPLv3, and (as the FSF agree) there would have been no problem. But it's not, so you can't. Sorry.

  • Isn't the question about using a plugin and not about "copying, distribution and modification"? The GPL V2 only applies to the latter: "Activities other than copying, distribution and modification are not covered by this License; they are outside its scope"). Or is "using" a plugin already a "modification"?
    – riQQ
    Apr 13 at 17:08
  • 1
    Since when is merely using a software covered by the GPL?
    – Polygnome
    Apr 13 at 17:28
  • @riQQ if dynamic linking creates a derivative work then distributing the plugin is distributing a modified version of the original work, which as you point out is very much an activity restricted by the GPL.
    – MadHatter
    Apr 13 at 19:10
  • @MadHatter yes, but they are talking about using not distributing.
    – riQQ
    Apr 13 at 19:10
  • 2
    @apsillers I think I agree with you, but we don't have any reason to believe such is the case from the OP's question. I'm increasingly finding the question ambiguous, I must confess. For the record: I think it's perfectly lawful to download from one place GPLv2 code that accepts plugins, and a completely-unrelated plugin from another place under LGPLv3+, and run the two together. I just find it incredibly unlikely that it would work at all, much less do anything useful, and so I don't think the question's about that.
    – MadHatter
    Apr 13 at 19:47

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