Experimental Physics and
Industrial Control System

Steven Hartman <[email protected]> · Wed, 19 Dec 2001 12:52:02 -0500 (EST)

John--

You raise some issues which I for one cannot answer. But let me point you
to the GPL FAQ again and give some excerpts which may answer some of your
questions:

http://www.gnu.org/licenses/gpl-faq.html

Q.	If a program released under the GPL uses plug-ins, what are the
	requirements for the licenses of a plug-in.

A.	It depends on how the program invokes its plug-ins. If the program
	uses fork and exec to invoke plug-ins, then the plug-ins are
	separate programs, so the license for the main program makes no
	requirements for them.

	If the program dynamically links plug-ins, and they make function
	calls to each other and share data structures, we believe they
	form a single program, so plug-ins must be treated as extensions
	to the main program. This means they must be released under the
	GPL or a GPL-compatible free software license.

	If the program dynamically links plug-ins, but the communication
	between them is limited to invoking the `main' function of the
	plug-in with some options and waiting for it to return, that is a
	borderline case.

Q.	Can I use the GPL for a plug-in for a non-free program?

A.	If the program uses fork and exec to invoke plug-ins, then the
	plug-ins are separate programs, so the license for the main
	program makes no requirements for them. So you can use the GPL for
	a plug-in, and there are no special requirements.

	If the program dynamically links plug-ins, and they make function
	calls to each other and share data structures, we believe they
	form a single program, so plug-ins must be treated as extensions
	to the main program. This means that linking the GPL-covered
	plug-in with the main program would violate the GPL. However, you
	can resolve that legal problem by adding an exception to your
	program's license which gives permission to link it with the
	non-free main program.

	For more details, see the question above that starts with, "I am
	writing free software that uses a non-free library."

> There are too many complications here to make this issue a trivial
> one.

Agreed. Ultimately, these are issues which won't be resolved on this
listserve and likely will require real legal counsel.

-- 
Steve Hartman
[email protected] | 919-660-2650
Duke Free Electron Laser Laboratory

Experimental Physics and Industrial Control System

Experimental Physics and
Industrial Control System