How does Qt accomplish both GPL and commercial licensing? In Qt there are some modules which are licensed with GPL or commercial license: https://doc.qt.io/qt-5/qtmodules.html#gpl-licensed-addons How I read this: I can use those modules in SW that I sell and I don't have to publish the source code, if I buy the commercial license (because there's the word &quot;or&quot; -&gt; I can pick either GPL or commercial license). If I'm right so far, my next question is: How do they accomplish this? In case they use the GPL licensed code in their libraries, GPL says that they should publish the source code and so should I, right? So I guess they must have coded their own versions of those libraries and sell those under commercial license? Please explain this like to a 5-year old.
Their code, their rules A copyright holder is free to offer their work under none, one or many licences. They can, at the same time, use their own work however they see fit without regard to the licences they have given/sold to others (except, they can only give one person an exclusive licence). As an analogy, let’s say I own a fleet of cars. I can drive my cars anytime I want. I can let Jim drive my cars anytime he wants for free. I can let Mary drive a specific car on Thursdays and only within 10km of the depot. I can let Joe drive my cars providing he pays me $50 a day. And I’m not going to let Fred drive them at all because Fred’s a jerk. Each of those is a different licence.
It depends on what you are selling to the enterprises. You could be selling just a licence to use the program, and retain all the rights on the source code. You could be selling the rights to the source code. For #1, the source code is yours and you can do what it pleases with it1. In fact many programs offer both an open-source licence (which usually forces the user to make its changes to the code publicly available) and a commercial one (which allows the user to keep the modifications of the source code for themselves). For #2, the source code is no longer yours so it is not up to you to decide what to do with it. 1I am assuming no other agreements imposing limits on those; for example that none of your contracts to your customers have a clause forbidding you from making the code available to the public.
If your own software includes software covered by the GPLv2 (for example by copying source code, or by linking dynamically) then your own software is also covered by the GPLv2, and you will have to provide the source code. This is called a "work based on the Program" on the GPLv2. In this case, however, it seems that your own software does not include software covered by the GPLv2, but you want to put it onto an SD card together with software covered by the GPLv2. That would most likely fall under "mere aggregation of another work", as long as your software and the other software do not interact very closely (such as dynamic linking). To quote the GPLv2: In addition, mere aggregation of another work not based on the Program with the Program (or with a work based on the Program) on a volume of a storage or distribution medium does not bring the other work under the scope of this License. So in that case the GPLv2 does not cover your program. You will still have to supply the source code for the GPLv2 software on the SD card. This is covered by section 3 of the GPLv2. Basically, you have two options: send along the complete source code for all the software (would be quite bothersome for a complete distro) or provide a written offer to provide the source code on demand to anyone who asks (you may charge for this, but only to cover your cost) GPLv2 contains a third option, but that only applies to non-commercial distribution. Practically speaking, it should be enough to include a README.txt or similar explaining that the SD card contains software covered by GPLv2, and that you will provide the source code on demand for a certain, reasonable fee (say $5 or $10 per CD). In practice, it is unlikely that anyone would ask for this, as the source code can usually be downloaded for free elsewhere, but if someone does ask, you just charge them $5 and send a CD. Of course, to reduce legal risks it may be prudent to contact a lawyer for your jurisdiction, as this is only general advice.
It seems that you don’t understand what parody is. If you do understand, please explain how it’s even possible to parody computer code. What you can do with “open source” code depends on the licence the copyright holder(s) release it under. For some very permissive licences you can do what you suggest, for most, you can’t.
You can licence your copyright under as many licences as you like to as many people as you like It's your copyright - you can do what you want with it. What you can't do is give someone an exclusive licence and then give licences to others - that would be a breach of contract with the exclusive licensee. How you let people know about the available licences is also up to you - your bio on Stack Exchange is fine.
Recall the basic principle of copyright law, as detailed in 17 USC 106: The copyright holder has the exclusive right to make copies; prepare derivative works; or distribute copies by sale, rental, lease, or lending. Other people can legally do these things only if they are given permission by the copyright holder, typically via a license. (Remember, the literal meaning of the word license is permission.) Often, the copyright holder will require a prospective licensee to accept various terms and conditions before the license will be granted. If there is &quot;no EULA&quot;, or if there is one but the purchaser has not agreed to its terms, then the purchaser has not been granted any such license, hence does not have permission to do any of the things listed above. If they do so anyway, it is illegal copyright infringement and they will be liable for damages. To use a firewall analogy, copyright law is &quot;default deny&quot;. So let's take your questions one by one: Do they own the software? US law has no concept of literally owning software. The closest thing is owning the copyright, which the purchaser certainly does not. It still belongs to the vendor that wrote the software (or whoever they may have later transferred it to). Can they legally alter the code of the program they purchased? No, that would be preparing a derivative work. The copyright holder has not granted them a license to do that. (There are some exceptions for purposes such as reverse engineering and interoperability, see 17 USC 1201(f)). Can they legally redistribute it No; again, that is the exclusive right of the copyright holder, and the purchaser has not received their permission. or transfer ownership? Maybe, if the first sale doctrine applies. Its application to software is complicated. The user has a better case for being able to sell the software if it exists as some tangible object which is transferred (physical media, pre-installed on hardware, etc). Can they legally modify the code of the program for others who have also purchased the same package? No, that would be preparing a derivative work. Can they take that software and install it on a secondary machine? No, that would be making a copy.
To use an API over a network connection (as opposed to, e.g., the Windows API), a user communicates a request to the API host, or server, and awaits a response. The host of the remote API can refuse to serve requests from users for probably any reason. Such services often require users to accept a license as a condition of using the service, and they may charge a fee as a condition of the license. A license to use a service is obviously not necessarily bound to a license for the use of its source code, just as the ability to reach the service is not dependent on the ability to see (let alone use) the code in any format, whether it be the source code or some compiled form of the code. In essence, source licenses and service licenses have different primary goals, at least inasmuch as the source license seeks to restrict someone who has actual physical access to compiled code, and possibly source code. Service licenses do not have that concern, though I have seen service licenses that also prohibit decompiling. This is probably the result of a CYA attitude among lawyers: the language is already in the standard software license text, and it doesn't hurt anything to leave it in, and it could help if a service user somehow managed to download the program code.
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