Q: Combining proprietary code and GPL for in-house use

[Gregor Hoffleit <gregor@mediasupervision.de>]

I thought I had a pretty good understanding of the GPL, but still I'm not
able to decide this (even after browsing
http://www.gnu.org/copyleft/gpl-faq.html):

Let's make up a gedankenexperiment:

I have written a program which I plan to distribute under a proprietary
license. Now I have found a GPL library that I could use to add some
additional, optional features to my program. AFAIK, the GPL doesn't prohibit
linking the GPL library with my program, and it doesn't prohibit me using
this combined work for my own work. It does prohibit, though, distribution
of this combined work as long as I would use that proprietary license for my
own code.

So far, pretty simple.

Now where's the borderline:

My program works well without the GPL library. Now if I sell this program,
and add a module that the customer may link with the GPL library, would I
violate the GPL of the library, and why ? If the customer linked in the
module, would he violate the GPL, and why (he's in the same situation as I
was: linking a GPL module with proprietary code is not forbidden, only the
distribution thereof).

Now what happened if the customer paid somebody else for linking the GPL
library with the module of my program (the consultant would do this on the
machine of the customer, FWTW). Would somebody violate the GPL here. Who and
why ?


If somebody who knows could please comment on this, or point me to a
discussion of this. I just don't see where exactly the violation might be.

Thanks,
    Gregor