Qt moving from LGPL to commercial halfway through

Question

I've never understood this bit about licensing on the Qt website.

Qt Commercial Developer License The Qt Commercial Developer License is the correct license to use for the development of proprietary and/or commercial software with Qt where you do not want to share any source code.

You must purchase a Qt Commercial Developer License from us or from one of our authorized resellers before you start developing commercial software. The Qt Commercial Developer License does not allow the incorporation of code developed with the Qt GNU LGPL v. 2.1 or GNU GPL v. 3.0 license versions into a commercial product.

If you are starting to develop an app while you're not sure if you'll ever want to sell it (using LGPL), how would they prevent you from moving to the commercial license at some point? As long as the API is the same you simply recompile / link, no?

What am I missing?

Answer 1

Formerly Qt was available on GPL and the Commercial License. This used to mean that a Commercial License was need to do closed source. After the Nokia acquisition Qt was also licensed under the LGPL - after this fact, the only difference between Qt Commercial and LGPL are about shipping modified versions of Qt or Qt Solutions and the support contract.

Answer 2

The license text is probably legally not binding. It effectively adds an extra restriction to the GPL, namely "you cannot use your GPL code later in combination with the Qt commercial license". From clause 6 of the GPL: "You may not impose any further restrictions on the recipients' exercise of the rights granted herein."

Answer 3

I believe that the text only refers to code that has already been distributed under LGPL, and therefore cannot be closed-sourced by switching Qt license.

I think you have nothing to worry about: nobody know/cares where the undistributed code you wrote came from (Commercial Qt or LGPL Qt). As long as it hasn't been released under a LGPL license, nothing can happpen.

Answer 4

If you didn't do any modifications to the Qt Sources, yes you can go ahead and sell it.

From Qt Licensing documentation, Can create proprietary applications in accordance with the LGPL v. 2.1 terms ..

Also from the Documentation,

Qt GNU LGPL v. 2.1 Version

This version is available for development of proprietary and commercial applications in accordance with the terms and conditions of the GNU Lesser General Public License version 2.1.

Support services are available separately for purchase.

Answer 5

It is the Commercial licence that adds the restriction. When they sell their software under their own commercial licence, they are free to add restrictions within the limits of the law. They are allowed to stipulate that you cannot licence software under their commercial licence that was developed using the LGPL version of the software. It is difficult for them to police, certainly, but they may just be relying on most people to do the right thing (and the fact that their Commercial licence isn't too expensive anyway).

There is nothing stopping you from selling software developed with the LGPL version, and you don't have to publish your source code. The only real consequence of using the LGPL is that you cannot technically link to the QT libraries statically. That isn't common with Qt so it's not much to worry about. And of course you don't get the same support.

To release using the commercial version, you need to purchase the commercial version from Qt (which gets you a licence code). The source is the same, but you are not using the commercial 'form' of Qt unless you buy it or steal it.

Answer 6

As you can sell your software using the Qt LGPL version (it's not only GPL anymore), this point should be moot for most users.