Bug#694257: fdk-aac: who knows more?

[Arto Jantunen]

Carlos Alberto Lopez Perez <clopez at igalia.com> writes:

> On 09/05/13 23:27, Adam M. Costello wrote:
>> Fabian Greffrath <fabian at greffrath.com>:
>> Of course, the library would be much more useful if avconv could use it.
>> If libfdk-aac is GPL-incompatible, what does that imply?  That avconv
>> must not require libfdk-aac to be present at runtime?  Could it check
>> for the existence of libfdk-aac and dlopen() it if it's found?  Would
>> that make them independent enough that their licenses wouldn't need to
>> be compatible?
>
> The thing is that libav (ffmpeg) is LGPL (not GPL). So, my understanding
> is that it shouldn't be a problem to use a third-party library (fdk-aac
> or whatever) even if this library is GPL-incompatible (or even proprietary).
>
> I tried to clarify this point with libav developers [1]. But the replies
> I got where not clear to me so I gave up. They seem to be more
> interested in improving the internal AAC encoder of libav.
>
> I still think that it shouldn't be any problem by linking libav with
> fdk-aac or any other library given the LGPL license of libav. But I am
> not a lawyer, maybe I'm wrong.

As far as I understand you are indeed mistaken about the difference
between GPL and LGPL.

The reason why GPL licensed software cannot use a GPL-incompatible
library is that the resulting binary inherits both licenses. If the
licenses are not compatible, the binary cannot be legally distributed at
all (it has no valid license).

The usual reason why certain licenses are not compatible with the GPL is
the clause that says "You may not impose any further restrictions on the
recipients' exercise of the rights granted herein." causing a license
that has a clause that the GPL does not have to immediately become
incompatible (the BSD advertisement clause is the most common one, for
example OpenSSL has this). This problematic clause is the exact same in
both GPL and LGPL.

The difference between the GPL and the LGPL does solve the problem if
the program you are developing wants to link to both LGPL licensed and
GPL incompatible libraries, assuming that the license of the program
itself is not either GPL or LGPL. Parts of libav are GPL and the rest is
LGPL, thus the problem remains.

-- 
Arto Jantunen