GPL and closed source

Henningsen alg at
Thu Jul 12 16:34:08 UTC 2001

While I'm even less of a lawyer than most folks here, I disagree. My opinion
is that you *can* write a closed source plugin to work with a published
interface, and distribute it for a fee, and nobody can stop you from doing
that. I also remember having read here that RMS is of the opinion that if
the interface only has been implemented in a GPL-program, then my stance is
wrong, and you cannot distribute your closed source plugin. So let us consider:

It is pretty clear from the text of the GPL that you cannot distribute your
closed source plugin *linked into* the GPL-program. But if you distribute
the GPL-program and the plugin separately, I don't see any problem with
users throwing the two together. I don't think such users would contravene
the GPL, since they're doing no developing or distributing of code, just
using several programs as they see fit on their own computers.

The argument that shows why RMS' opinion on this matter (assuming he was
quoted correctly) is wrong and you *can* do this is that you could at any
time develop a closed source program that implements the published interface
to which your plugin links. If the FSF were to sue you, you'd have to take
the stance that you are distributing the plugin first, before writing the
closed source replacement/improvement of the core GPL-program. So IMO, the
fact that you *could* implement the interface yourself in a closed source
program makes it impossible to fault you for selling a closed source plugin
to that future program.

Problems might arise if the interface itself were proprietary. If yours is a
serious commercial venture, you should talk to a lawyer about that possibility.

Again, IANAL, I'm just going by my understanding of the GPL and what I
consider just and fair.

As a final thought, I think the FSF would be crazy to test the GPL in court
on this issue, where the claim they're making is much weaker than in other
cases of potential license violation (like all the GPL code that was linked
to Qt before that was GPL).

Peter Henningsen

At 07:39 AM 7/12/01 -0700, you wrote:
>> Let's say that I've developped a programm under GPL.
>> Is it possible to create Closed-source plugins for the GPL programm and sell
>> them ?
>If you own the copyright to the larger framework, this can be
>done legally.  You'll want to make sure that you either
>completely own, or have disclaimers or assignment of rights
>documents from all developers.  If the framework is yours, you
>can take all or part of it closed source at will - you own it.
>However, if the framework is not clearly under your copyright,
>then you can't do this, since you cannot link closed source to an
>open source framework - and pluggable modules are considered to
>be linking.
>This is my non-legal opinion, of course.

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