Proper license for plugins

[Michael R. Bernstein]

On Sun, 2005-01-16 at 04:43, Benjamin Rossen wrote:
> > 
> > I would guess, but am not sure, that if you created a plugin for Emacs, you 
> > would not be required to release the source code.x
> 
> Yes. Emacs is GPL. You must make the source code available, you may not 
> prevent anyone else from adding to your work and making new derivatives, and 
> you are not permitted to use any other license form.

While I think this is true for Emacs, I don't think this is necessarily
correct generally.

If the plugin is a derivative of GPL code (whether another plugin, or
the host software), the new plugin must clearly be GPL.

The plugin and host software do form a combined work that in-toto should
be covered under the GPL if distributed.

But the plugin itself, if not derived from GPL code, and if distributed
separately, does not necessarily have to be licensed under the GPL. I'm
going to be wishy-washy here, because IANAL, but it strongly depends on
*how* the plugin interfaces with the host software, and also whether the
plugin can be considered an independent work also useful on it's own.

In many cases, the plugin license would at least have to be
GPL-compatible.

This is pretty dodgy though, and 'useful on it's own' is a pretty hard
standard to meet for most plugins.

Your safest bet, if you want to use the GPL for the host software but to
allow proprietary plugins, is to simply grant an exception for plugins,
however you define them.

Another option would be to use the LGPL for your application instead.

This is tricky territory, though, and you *will* want to get the advice
of a lawyer.

I found further discussion of this issue here:
http://www.gnu.org/licenses/gpl-faq.html#GPLAndPlugins
http://www.gnu.org/licenses/gpl-faq.html#LinkingOverControlledInterface
http://www.linuxrising.org/files/licensing.html

-- 
Michael R. Bernstein <webmaven at cox.net>