I could be totally wrong about this. I'm not a lawyer but this is my general understanding of why dual licensing exists.
On Mon, Jan 10 2011, Narendra Sisodiya wrote: [...] > That's exactly I want to understand. Let take a fictitious example I > purchased a book on PyQt. I wrote an excellent close software which is > based on PyQt. To use a third party package, you will need to accept their license - be it the GPL, a BSD type (non-copyleft) license or a more restrictive EULA. If you're using PyQt, you are bound by whatever license the version you're using it released under. If you are using the libre version of PyQt, it is made usable to you under the terms of the GNU GPL (I haven't checked the website myself but that's what this thread suggests). The GNU GPL is a copyleft license which insists that applications derived[1] from software licensed under it also be released using the same licensing terms as itself. This is a hack (one of RMSs best in my opnion) using copyright to make sure that the software "stays" free. It's not possible to take a GPL program and make a non-free version of it (which is possible with non-copyleft free-software licenses like the MIT license). It's generally agreed upon that linking your code with a library constitues a dervied work (I don't know if this has been legally tested). This means that your program if released under a non-free licenses is in violation of the GNU GPL and so you're doing something illegal. Some libraries are released under the LGPL which is the GPL with an extra clause that says that linking against them doesn't constitute a derived work. AFAIK, PyQt is not one of them. > I am start selling this proprietary software in market. Selling has nothing to with software being libre or not. You can sell copies of GPLed code (like RMS himself did with early copies of Emacs). > people started purchasing this software from my website. Fair enough. > They download PyQT. and they run my close software. Your program *requires* PyQt to run so you have to obtain it under a license. If you obtain a GPLed version, you have to make your own application free software to confirm to the GPL. If you want to keep your code closed, you have to obtain a version of PyQt that lets you do this (for which you'll have to pay). This is the point of dual licensing. Another example is the GNU readline library. You cannot make a closed source application that uses it without violating the GPL. If you want to do that, you'll have to contact the copyright owners (the FSF), ask them to give it to you under a different license that allows this for which they might charge you money. [...] Footnotes: [1] The is the term where much of the legalities hide - if I use a GPL'ed library is my code "derived"? If I use code that's generated by a GPL program, is my code "derived"? etc. -- _______________________________________________ BangPypers mailing list BangPypers@python.org http://mail.python.org/mailman/listinfo/bangpypers
