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It's not supposed to be comparable to the GPL. It allows people to tinker and distribute but forces people to pay if they want to build a commercial product around RISC OS.

Ah ok, it is a 'shared source' commercial license, not 'open source'. So they are free to define how they want their product to be used. Few companies give away their products to be used, sold and distributed for free.

http://osgeek.blogspot.com

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Clause 1.2.5 looks obnoxious and grants Castle a special privilege, but it doesn't seem to contradict Open Source Definition.
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2.4 This Licence is not an OEM licence. Consequently, You may not incorporate
or embed RISC OS or any Derivative Work or any part of them in any hardware
product which is intended for commercial sale, nor may You distribute, sell,
supply or otherwise dispose of any such hardware product unless You have
obtained the prior written consent of Castle.

1.2.3 You shall ensure that each Derivative Work distributed by You is distributed and licensed in its entirety at no charge on the terms of this Licence to any third party who is prepared to accept the terms of this Licence. If any part of any Derivative Work distributed by You can reasonably be considered an independent and separate work then this Licence shall not apply to any such part where You distribute such part as an independent and separate work;

The license prohibits distribution of derivative works in non-gratis products. This prevents some common practices such as charging a small fee to ship the software on physical media. The intent might be that nobody but Castle can profit from RISC OS, but their license can prevent distributors from merely breaking even.

Further, the question of what could "reasonably be considered an independent and separate work" negatively impacts the enforceability of this license. If they want to allow certain kinds of modifications to be redistributable on their own under an arbitrary license, then they need to define in explicit terms what kinds of modifications are considered independent and/or separate. It could be as simple as declaring that original source files or original class definitions qualify (on their own) for exemption from these license terms.

Finally, the definition of Derivative Work is almost laughable:

The phrase "Derivative Work" means any derivative work which is based on, or derived from, RISC OS or any part of RISC OS (whether by modification or translation) and any work subsequently derived from such Derivative Work or any part of it PROVIDED THAT such work is only intended to be used in conjunction with an embodiment (whether physical or emulated) of one or more versions of the ARM processor architecture.

What does this mean? A derivative work is a work that is derived from the original or another derivative? Nice recursive definition. I don't know whether this is strong or weak copyleft because apparently they think that everybody agrees on what constitutes derivation in the context of software.

I would highly discourage even the most experienced software vendors from drafting an open source or "shared source" license behind closed doors with no public review process. Especially when the license is intended to propagate to derivative works (i.e. copyleft), is is crucially important to get the license right. Whatever they intend this license to entail, Castle has clearly failed, and if they go through with distributing code under this license, they are going to feel very vulnerable in court when somebody inevitably interprets this license in a creative way.

Edited 2007-05-19 05:22

"but it doesn't seem to contradict Open Source Definition."

A cursory look says it breaks the Open Source definition in 3 separate ways.

1: Free redistribution - the license prevents you from shipping hardware with this code without paying a royalty.

6: No Discrimination against field of endeavour - The license prevents you from using the code on anything other the ARM based platforms.

8: License must not be specific to a product - This license is written specifically for the RISC OS product, and could not be used on other projects.