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I modified a GPL opensource project and distributed the binary free of charge on Google Play. I'm planning to release source code per request via email only.

My question is if someone ask for the source code, can I charge them? If they refuse, what will happen, do I still have to send the source code to them?

Alternatively, can I put a restriction on my derived code that it cannot be used for commercial product? Even if the original project allows it.

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    "I'm planning to release source code per request via email only" -- Please read the GPL regarding the Written Offer, valid for 3 years. To comply with the license you must actually supply an offer to whomever you give the binary. Also, depending on which license (GPLv2 and GPLv3), you may not be able to only supply the source via email. Depending on how you read the GPLv2, you may have to offer to ship it to them (and you can charge for that shipping). See: Requiring personal info for source code – Brandin Oct 19 '18 at 8:01
  • "If they refuse, what will happen" - Why would they refuse to pay? Maybe you are charging more than the cost of your distribution (this is not allowed by the GPL), or maybe you are insisting on a method of payment that cannot be used in some areas (not explicitly stated by the GPL). You should elaborate on the situation to get a better answer to that part of your question. – Brandin Oct 19 '18 at 8:08
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  • Thanks for comments. Just want to know if there is any way to avoid people sell clones of the app. If not, then i will comply to GPL terms. – Quyen Le Oct 19 '18 at 11:13
  • You are allowed to sell copies, but in return, everyone else is also allowed to. If you want to separate your own code, to keep your own code proprietary, you need to restrict yourself to using software whose license allows that, such as LGPL. – Brandin Oct 19 '18 at 11:18
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My question is if someone ask for the source code, can I charge them?

You may charge them for the binary, but once anyone has the binary, you may not impose any charge greater than cost-recovery should they also want the source code. Specifically, GPLv2 allows you to make "a charge no more than your cost of physically performing source distribution", which unless you post actual media to the requester, is likely to be nearly zero. GPLv3 has similar language, but it makes it clearer that network access to source cannot be charged for, and that postal access only satisfies your licence obligations in the case of certain kinds of binary shipment, which do not include Google Play.

If they refuse, what will happen, do I still have to send the source code to them?

If they refuse to pay, it is unclear whether you are still obliged to send them source but may then proceed against them for the cost, or whether you may withhold the source until the cost is defrayed. In any case, if you're planning on relying on this part of the GPL to save you from source distribution obligations, I would strongly advise against that.

Alternatively, can I put a restriction on my derived code that it cannot be used for commercial product? Even if the original project allows it.

Both GPLs require that you licence the derivative product under the same version of the GPL, and neither of them allows such restrictions. This requirement applies to the whole of the derivative product, both the original parts, and the parts that you have added.

  • Thanks, im just trying to know if there is anyway to avoid people download the source code and sell a clone of this app. Anw, seems like it permitted. If so, then i will comply. – Quyen Le Oct 19 '18 at 11:12
  • @QuyenLe it is indeed permitted, though they, too, have to give out source to anyone who gets the binaries and requests it. If you're happy with this answer, could you accept it by clicking the tick outline next to it? This puts the question to bed and stops it floating around forever. Please accept my apologies if you already know this. – MadHatter Oct 19 '18 at 12:51
  • Additional question,can i put another term to my derived work's license, for example, author attribution? – Quyen Le Oct 19 '18 at 13:48
  • Terms 1,2(c) of GPLv2 covers attribution.... (You can't distribute GPL-derived code under a different license. Adding terms means that it is a different license) (You can add a copyright notice on startup, which they won't be allowed to remove (in terms of 2(c)), just change from "Copyright 2018 Original authors, MadHatter" to "Copyright 2018 Original authors, MadHatter, modifier") – Gert van den Berg Oct 19 '18 at 14:12
  • What Gert said. And in general additional requirements are disallowed by GPL, which is implicit in GPLv2 and explicit in GPLv3 s7. – MadHatter Oct 19 '18 at 14:20
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see also @madHatter's answer

There you must supply the source, in a manner similar to the binary, so artificially increasing your cost, would be a violation.

There is no way to stop someone taking your source code, studying it, using it, modifying it, distributing it, and/or charging for distribution. However they must continue to comply with the GPL. Therefore you can ask them for a copy of their source code. They must continue to pass-on all of the freedoms that the GPL insists on.

  • Actually, the source and binary don't have to be distributed in the same way. It's legal to post the binary on-line with a written offer to get the source on optical media, although I can't see why I'd want to do that. Moreover, redistribution is optional, so if I take the code and modify it for my own use I don't have to show anyone else a darn thing. If I do distribute it, it has to be under the GPL, of course. – David Thornley Oct 25 '18 at 18:31
  • For GPLv3 it makes clear that one option is to offer the source code online, but it says that in that case, the online access must be free of charge. For GPLv2 the license actually says that if you make a Written Offer you must offer the source "on a medium" which to me implies something like a CD, a USB key, etc. and you may charge only for that distribution (which to me implies the cost of the CD, the cost of the shipping, etc.). For GPLv2 there's nothing prohibiting you from offering the source online, of course. – Brandin Oct 26 '18 at 5:12

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