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In case I would dynamically link to a LGPL-licensed library (in this case Qt) in a commercial application: Which way do I have to provide the library's source code?

  • Is a link to the project homepage sufficient? Or would I need to host the source code on our servers? For which period of time after distribution of the software would this website need to be kept up? (or would "on request" be also compliant with the LGPL?)
  • Is it still necessary to offer the source code in a physical way (e.g. on CD via snail mail) for 3 years (any difference between LGPL 2.1 and LGPL 3 here?)?

I've found this in the GNU FAQ:

Can I put the binaries on my Internet server and put the source on a different Internet site?

Yes. Section 6(d) allows this. However, you must provide clear instructions people can follow to obtain the source, and you must take care to make sure that the source remains available for as long as you distribute the object code.

I've highlighted the relevant part as bold text. However, I cannot believe, that this is the complete truth. If I distribute an application only once, then I could put the source code online and take it down a day later. So this still doesn't answer my question...

There is another question with a related topic here, but it has a different attempt than mine. It only asks for alternatives to shipping the source code directly with the binaries, while my question aims to clarify which circumstances and conditions I would have to consider / respect when sharing the LGPL library source code.

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    The "possible duplicate" covers only a very small part of my question. So I would not consider it as a real duplicate. – mozzbozz Nov 6 '14 at 10:44
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    Thanks for the link, @gnat. I've clarified my question in respect to what I want to know - now it should also be more obvious that this is not a duplicate. – mozzbozz Nov 6 '14 at 13:34
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    @mozzbozz: If you want a more authoritative source on the GPL than the GPL FAQ, you probably need to talk to a lawyer, not random strangers on the internet. – Bart van Ingen Schenau Nov 7 '14 at 9:48
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    @BartvanIngenSchenau: I know; I don't want "real legal advice", but I can't believe that noone has thought about this question before. Maybe there is something in the GPL license text what I skipped accidentally or whatever. This has to be something what others have evaluated already. If this should really be unclear (because not further specified), then a lawyer won't "help" anyway: 3 lawyers, 4 opionions ;) (Please also consider voting for reopening the question after my edit yesterday. Thanks!) – mozzbozz Nov 7 '14 at 10:00
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As a rule of thumb, I'd say you should make the source available for as long as you're providing support for the software that uses it, or (preferably) as long as you can tell it's being used. It's difficult to give any solid guidelines without knowing more about the software you're producing, and what area you're in. To be on the safe side, I'd include a link somewhere in your documentation to the project home page and a note regarding what version of the software your application uses.

  • Qt specific update/correction: the Qt licensing FAQ section 3.7. What are my obligations when using Qt under the LGPL? states "...this has to be under your control, so a link to the source code provided by the Qt Project or Qt Company is not sufficient." – SensorSmith Jun 1 '18 at 22:58
  • It sounds like they want you to provide an archive of the source for the release you're using, so you should build the version of Qt you're using from source, then zip that up into a tarball and include it as part of (or adjacent to) the source download for your project. – TMN Jun 4 '18 at 11:37
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  1. If you distribute it only once, just distribute it together the binaries and you're done.
  2. If you are not sure, upload it to a public free repository (bitbucket, github) and it's no longer your responsability (as long as you actually stopped distributing the binaries anyway).
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In your particular case (you are mentioning Qt) I believe that in practice giving a link to Qt - including to the download page of the particular version of Qt your are basing your product on - is probably enough.

However, be sure that the link is valid (you have no control on how Qt people are managing their website). This means that you should at least watch it every week (or setup a script doing that).

However, as a courtesy, you'll better add a local copy on your web site of the tar source archive of the relevant version of Qt, and keep that copy as long as you offer for sale your product using Qt or some maintenance or services on it.

IMHO (and I am not a lawyer) it is not much a matter of legal obligations, but much more a matter of being nice with your customers and the Qt community at large. And a local copy of Qt source code on your company's web site don't cost that much to it!

At last, your company might buy a different license of Qt.

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