Qt Changes and LGPL

Question

My company uses Qt 4.7.4 under LGPL for a commercial proprietary application.

We had to put two changes into the source code of Qt and re-build its DLLs to resolve some issues. What should we now do under LGPL? Are we to expose our source code? or do we just need to expose the Qt files we've changed? What is the nature of this "exposure"? posting them on a website?

how does that work?

I'm voting to close this question as off-topic because [licensing advice is off-topic on Stack Overflow.](http://stackoverflow.com/help/on-topic) You may be able to get help on [Programmers Stack Exchange, but **read their faq carefully** before proceeding.](http://meta.programmers.stackexchange.com/questions/7265/when-is-a-software-licensing-question-on-topic) — durron597, Jun 03 '15 at 18:50
I'm voting to close this question as off-topic because it is about licensing and legal issues, not programming or software development. [See here](http://meta.stackoverflow.com/a/274964/1402846) for details, and the [help/on-topic] for more. — Pang, Jun 04 '15 at 01:33

score 1 · Accepted Answer · answered Mar 13 '12 at 15:03

1

LGPL is mainly meant for libraries and means that you can use Qt with "closed-source" applications, i.e. ones you do not have to divulge their source code.
Would Qt be GPL, you would have had to open-source your own app, too.

Now, LGPL also means that if you modify the sources of the library, you have to make these modifications open and public.

How this is done is not fixed in stone. If your modifications is of general interest, you should submit a patch to Code Review. see Contribute to Qt for more info.
If it is very specific to your needs, publishing the patch on your website should be sufficient.

answered Mar 13 '12 at 15:03

Chris Browet

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It's really bugs that were fixed in 4.8 - however, because of platform constraints, I cannot upgrade to 4.8 and must remain with 4.7.4 - I guess those small fixes are of no value to the Qt community. – JasonGenX Mar 13 '12 at 15:40
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Well, the patch could be beneficiary to other people in the same situation as you, so I'd still suggest to submit the patch. Even if the patch is not accepted/included, it is so public and findable by others. – Chris Browet Mar 13 '12 at 15:48
Of value or not, if you publicly distribute the application you're required to release those fixes for the Qt library under the LGPL. FYI. – JadziaMD Mar 14 '12 at 13:45
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This is what I'm trying to ascertain, what is the meaning or "release those fixes", put a webpage with download links to it? submit it to somewhere? – JasonGenX Mar 14 '12 at 15:02
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@RM1970 - you have to make the changes (and actually the full source tree you used) 'available' on request to users you distribute your program to. That could be having them downloadable from your site, or offering to send someone a CD if they asked for it (you can even charge for this). – Martin Beckett Mar 14 '12 at 15:51
Just making sure: "and actually the full source tree you used" means = my modified Qt 4.7.4 source tree, NOT the source code for MY commercial application making use of Qt. – JasonGenX Mar 14 '12 at 20:01
Just the Qt sources. That's the point of LGPL. – Chris Browet Mar 14 '12 at 20:23
@RM1970: Yes, the full source code of the LGPL'ed part. A patch only is not enough (but I would it offer as well), but also the fully patched stuff you build the binary from incl. the build scripts. – hakre Mar 18 '12 at 15:49

score 1 · Answer 2 · edited Mar 14 '12 at 12:26

Nothing has really changed, the Qt is still LGPL, your app is still proprietary

You have to make the modified version of the Qt sources available. The strict interpretation by GNU is that you must host your own copy of the Qt source tree that you you modified and make that available.

A more useful one, if these are just small fixes, is that you make patches to the Qt tree available. Ideally you would contribute these fixes back to the Qt source and have them adopted - which saves you from the effort of reapplying them to subsequent versions and makes it a lot easier for your users.

Hopefully with Qt moving to a more open development following the decline of Nokia this will become easier.

Qt Changes and LGPL

2 Answers2