Freelance, copyright, code, law ...

I am freelancer and working with clients all over the world.

I have some kind of framework done by me
and use it to for my clients project.

The question is:
First I did app for one customer
then another app for other client (using the same library)
then I decide to make a commercial app that I want to sell

Is it ok? Does anybody tell me that you did it for us you can't use it anywhere else.

I really do not understand how it works.
If for one client I wrote a cool function, does it mean I can't use anywhere else.

In effect I think this is fine, but from now on you will want to watch your back too and I think this is what you're feeling now. This is your copyright to resell to anyone your code.

Explicitly in future make it clear that they are paying for a license to use your commercial programming, and maybe supply your code with an liability imdemity(sp?) clause like "software is sold AS IS etc", that they sign and agree that at this moment your code meets their design requirements, and that they are satisfied with the code. Done.

Remember, they are not upholding the license (GPL etc), but buying a product that you are charging for.

Hope this helps

(hit my scales if this makes you feel any better