[License-discuss] Strong and weak copyleft

[Ben Tilly]

Hopefully you find this response more useful than my first attempt.

My understanding of both the GPL and LGPL licenses is described by the
following phrase which appears in section 2 in both the LGPL v2 and
GPL v2:

    These requirements apply to the modified work as a whole. If
identifiable sections of that
    work are not derived from the Program, and can be reasonably
considered independent
    and separate works in themselves, then this License, and its
terms, do not apply to those
    sections when you distribute them as separate works. But when you
distribute the same
    sections as part of a whole which is a work based on the Program,
the distribution of the
    whole must be on the terms of this License, whose permissions for
other licensees extend
    to the entire whole, and thus to each and every part regardless of
who wrote it.

This clarifying language unfortunately disappeared from v3.  But the
text of both v3 licenses still fits with the interpretation suggested
by this section.  Namely that if you distribute a work that contains
an (L)GPLed component, the only way to satisfy the license on that
component is to apply rules from that license on other code in that
work that is not legally derived from said component.

In the LGPL and GPL, both versions 2 and 3, there is clarifying
language making it clear that there is no restriction placed upon code
that merely happens to be distributed in a form that is aggregated
with the licensed component.  The license only restricts a unified
work.

With that said, the difference between "weak" and "strong" is what
requirements are put on code which has been combined with, but is not
legally derived from, the component in question.

The LGPL imposes fairly mild restrictions, such as not preventing
reverse engineering.  (A lot of code is distributed under licenses
that forbid reverse engineering.)

The GPL imposes very strong restrictions.  Namely that the entire
application must be available under the GPL.

On Thu, Apr 9, 2015 at 1:35 PM, Lawrence Rosen <lrosen at rosenlaw.com> wrote:
> Maybe we can summarize so far:
>
> ULTRA-STRONG            (AGPL)
> STRONG          (GPL)
> MORE THAN WEAK  (LGPL)
> ALMOST WEAK             (EPL)
> WEAK                    (MPL)
> VERY WEAK               (APACHE)
> ULTRA-WEAK              (CC0)
>
> This rather simple scale is not reflected in copyright law or any relevant
> cases. It is not part of the Free Software Guidelines or the Open Source
> Definition. It bears no resemblance whatsoever to the definition of
> "derivative work." It is based here in this thread on obscure quotes from
> various websites or opinions about "license author's intent" without quoting
> the actual provisions of the licenses that enable these vague distinctions.
>
> This is one of the issues raised by the VMware complaint in Germany, and
> we're expecting a court to make a decision about how strong the GPL is. This
> email thread is still not very helpful.
>
> /Larry
>
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