What is Copyleft?

[Ryan S. Dancey]

Here's a question I thought I'd never have to ask.

What is a Copyleft?

The reason I ask this question relates to RMS's recent pronouncements about
Apple's psuedo-open license terms.  He says, in part, that one of the flaws
of the license is that:

"It is not a true copyleft, because it allows linking with other files which
may be entirely proprietary."

I the working definition of "copyleft" I have been using is:

"A way of using contract law (through a copyright license) to ensure that
everyone has the freedom to copy, modify and distribute a given work.  It
takes the copyright law and turns it inside-out.   Instead of being used to
limit what you can do with a copyright work, a copyleft ensures that your
freedom can't be abridged."

Now, let me say that for the purposes to which RMS developed the GPL in the
first place, his indication of a "flaw" with the Apple license is completely
consistent.  However, I would say that the ability to link with non-free
code, while an incompatibility with the GPL, isn't a copyleft problem.

If the license allowed a user to link to non-free code, and distribute the
combination in object-form only, then I would say that it was a copyleft
problem, because free code would be rendered non-free (the gestalt work
would have two copyright interests; the Free part, and the non-Free part,
and thus the work as a whole couldn't be distributed without additional
permissions).

If I write a copyleft free program for Windows, I should be able to load and
link at runtime to any DLL in the system, regardless of whether or not that
DLL is free code or not, shouldn't I?  How else could a Windows program ever
be written using the GPL? (I don't know enough about Linux to have an
opinion about Linux code).

The copyleft concept is supposed to ensure that any material I use or modify
which is based on copylefted content has to obey the same terms as the
original copyleft license, correct?

The concept of "copyleft" itself shouldn't be so specific as to include
material related to the linking model of computer software, should it?

Ryan