> If, however, your software talks to the GPL-ed software through an API > (ergo, libraries compiled or linked into your software), then your > software becomes infected.
I think Thaths has reached the heart of the issue - this is not really a question of law, it's a question of fact depending on the circumstances of each case. As Carey pointed out, there is a lack of case law on this subject because (although it is one of the thorniest areas of the GPL), cases tend to settle before reaching court. As it happens, I had lunch today with a friend who teaches US copyright and Internet law, and I asked him about this. Basically he confirmed what the discussion here has thrown up and said: 1. As long as they are not publicly distributing the product, they can do pretty much what they want with the GPL'ed code. 2. Assuming they are publicly distributing the product, they will almost certainly have to distribute the modified POP3 server under the GPL. 3. The position with the main proprietary program is unclear and requires more detail than AMS provided in his email. It's a question of fact to be decided case by case although rules of thumb like Thaths has outlined may be useful in reaching a conclusion. 4. There seems to be some confusion about "derivative works" here. Based on the facts outlined, it is very unlikely that the *main* proprietary program infringes copyright of the original GPL POP3 server or is a derivative work of it. The modified POP3 server will however, almost certainly be a derivative work of the original POP3 server and infringe copyright in it (these are separate things under US copyright law). However, a "mere aggregation" of the two programs doesn't mean (under *copyright law* - the terms of the GPL may be interpreted differently) that they become one. (This is the point I believe AMS' friend was trying to make - that aggregating the two programs doesn't merge the copyrights in them - the phrasing was a bit unfortunate though - of course software is a work of authorship in which copyright subsists). I hope that clarifies things. In terms of practical legal advice, if it's your GPL program that is being used, a nice lawyer letter (or even a friendly email really) outlining some of the above points in the right way would probably let you open negotiations with them..depends what you want to happen as well.. Badri
