Constrains of LGPL of Qt

iamcreasy

Hi, I am a new user of Qt & I have some questions regarding the Qt's LGPL license.

Now, so far what I know Qt (LGPL) is allowed to be used in commercial product, as long the Qt libraries are linked dynamically. Is it right?

I have read some wiki articles and google about it, and what I have come up with is that, if you need to provide the dll files with the application then, those dll files are dynamically linked. That means, if I use Qt in any commercial app, where I need to provide dll files of Qt, then I can use the LGPL license, which is free. Is it right?

These licenses details are so hard to get into. :(

Thank you.

edit: fixed QT -> Qt (mariusg)

loladiro

That is also my understanding (both questions).

giesbert

The point here is, that only a Lawyer can answer it correctly. Perhaps the videos from the DevDays help yopu understanding all this: "DevDays about LGPL in Qt":http://developer.qt.nokia.com/elearning/watch/making_the_licensing_decision_lgpl_vs._commercial

jim_kaiser

Thats true that a lawyer is need to understand license terms in general. But, @iamcreasy, i think you have understood it correct. You can create your app using Qt linking dynamically under LGPL terms. Of course, if you make changes to the source code of Qt, you need to also release the code of those changes.

iamcreasy

Thanks.

But, What are the commercial licenses that allows 4 "Freedom", when releasing commercial product using QT(LGPL)?

jim_kaiser

As I understand, you can buy a commercial license from Qt and get commercial support, patches etc.. like we do in our company. You can also make commercial apps using Qt for free with the LGPL license. But if you use some 3rdparty libraries which are not LGPL and instead GPL, then again there are some additional things you might need to do. What do you mean by

bq. allows 4 "Freedom"?

iamcreasy

in the video that @Gerolf mentioned, they(qt) says, the commercial product that you are gonna release using QT(LGPL) must allowed to be modified by the recipients.

(13 Min of the video)
LGPL section 6 requirement , that "Terms permit modification for customer's own use & reverse engineering for debugging such modifications."

So, what are the commercial licenses that allows this terms?

jim_kaiser

I think we need a laywer now :) I haven't seen the video, maybe I should. But my understanding of LGPL is that, you don't have to release your source code (you use Qt with LGPL, your app could just be licensed commercially). But any changes you make to the LGPL code (Qt sources) have to be released publicly, or your customers should receive a copy of the LGPL code on request.

iamcreasy

bq. (you use Qt with LGPL, your app could just be licensed commercially)

Yes, but I think 'that' commercial license has to follow some certain rules. Not any regular commercial license are allowed while using QT's LGPL.

or im wrong! :-s

things are pretty confusing!

loladiro

I think you can use any license as long as you allow the final recipient to switch the LGPL library (or reverse engineer and debug it) to fix any bug that could arise in it (the library). There is also a special LGPL exception granted by Nokia concerning the headers (because they are used directly in your app). What you can't do is modify Qt and not tell anyone (i.e you either have to merge your changes into Qt or provide the sourcecode of your modification).

giesbert

The commercial license allowes you to modify Qt without telling anybody about the changes, write closed code without giving anybody any source of anything :-)

Just the binaries for money. That's what we do in out company. You only have to state some copyrights in your about texts and handbooks.

iamcreasy

bq. LGPL section 6 requirement , that “Terms permit modification for customer’s own use & reverse engineering for debugging such modifications.”

@loladiro that means its about the distributed dll files of QT. Not the application itself.

loladiro

Yes it only applies to modifications you make to the library and provides the end user with the possibility to fix your version of Qt (the binaries you deploy with your app) should they discover a bug.

iamcreasy

That's pretty sweet.

But, why nokia would doing something so open?

Its like using it commercially without any string attached!

loladiro

Using: Yes as long as you dynamically link it (some companies prefer static linking)
Modifing (and enhancing): Not without contributing back to Qt (which is possibly more valuable to Nokia)
Also, Commercial users still like to buy the priority support.
And by getting many people to use Qt, they can establish it as a de facto standard.

Eddy

The official answer is here :
"link":http://developer.qt.nokia.com/faq/answer/why_is_qt_released_under_lgpl

Edit :

@loladiro
You're fast!

iamcreasy

Fantastic. Thank you.

This thread should be sticky. I have seen many people questioning about QT's restriction while choosing LGPL.

Eddy

You could add [sticky] in your title as we do with [solved].
Maybe the trolls will pick it up.

mgran

[quote author="Eddy" date="1307995853"]
You could add [sticky] in your title as we do with [solved].
Maybe the trolls will pick it up.[/quote]

Sorry, a sticky will have to be truly exceptional - we try to avoid them as a general thought. Especially in high traffic forums. I think we have 3 stickies in total now.

</offtopic>

andre

If you want to information to persist, and you think this forum post isn't a good way to do that, I suggest that you either create a wiki page for it, or attach a good docnote at the appropriate licencing page in the documentation.