There is still some debate -- and this case didn't touch it -- over linking and derivative works. Static linking pretty clearly involves something that requires a license from the copyright holder, but dynamic linking is far from clear. The GPL basically asserts that dynamic linking makes a derivative work, but there's no guarantee that this is how copyright law would actually be interpreted by a court.

There is more to it than just the technical nature of the linking, though - the court would take into account all of the circumstances of the claimed infringement.

For example, if you distributed your commercial software packaged up with a GPL library, with scripts or links or whatever designed to have that library loaded together with the commercial binary as part of a finished product, then the "dynamic" nature of the linking might well be considered to be a technical irrelevancy.

On the other hand, if you distributed your commercial binary on it's own, and expected your customers to find a compatible library to dynamically link against it, then that might be a different matter.

The second case is the more common one, and has come up a few times. See, for example, debates over whether code written to a generic readline-esque API must comply with readline's GPL, even though non-GPL alternatives are available.

So if you code against a GPL library, but don't distribute that GPL library with your code, are you still bound by the GPL? According to the GPL license itself, it seems so, as long as there is not other library that implements with the same interface. But what part of copyright law makes this possible?

Do people distribute their non-free code, which then, somehow bootstraps GPL libraries (from GPL sources)? Would that be compatible with the GPL, or is there a provision against that?

Any project that builds using Maven does that all the time!

(And really it's not simply a question of "non-free" -- there are plenty of open source licenses that are incompatible with the GPL. Is this technically a workaround for the incompatibility between Apache v2 and GPL v2?)

But a non-free villain could bootstrap a build using Maven without even publishing their source at all -- just keep a Maven repository with their binary jars and and a pom.xml that specifies dependencies on a whole bunch of GPL libraries.

Is there a provision against it? I hope so, but I haven't found it yet.

http://wbillingsley.blogspot.com.au/2012/06/does-oracle-v-go...

Presumably, if proprietary code isn't linked with GPL code, it's not a derived work, but two separate works under different licenses. If your situation can tolerate it, you might could write a gplserver running in a separate address space. Or, if it's hardware to hide, video drivers frequently provide a razor-thin kernel driver that does little more than let you mmap the device's I/O space, with all the action sequestered in a proprietary userspace library.

c.f. rlwrap

You're still linking in the free software. If dynamically linking the normal way would be verboten, I can't see why dynamically linking that way would be any different.