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Two of the most common free software licenses are the BSD license and the GNU General Public License (GPL). Whereas software released with either of these licenses is considered "free", the licenses differ substantially in the way the source code can be used. The relative merits and shortcomings of either is a common cause of flame wars. The main differences between the two licenses is that revised BSD licenses are permissive while the GPL is copyleft. The GPL requires the software to always be free, including derivative works, by requiring the software to always be licensed under the GPL. The BSD license only requires acknowledging the original authors, and imposes few restrictions on how the source code may be used. As a result, BSD code can become proprietary software. For instance, parts of Mac OS X and the IP stack in Microsoft Windows are derived from BSD-licensed software. Code licensed under the BSD license can be relicensed under the GPL (is "GPL-compatible") without securing the consent of all original authors. Code under the GPL cannot be relicensed under the BSD license without securing the consent of all original authors, since the BSD license does not require the source code to be always freely available. The Free Software Foundation provides the GNU Lesser General Public License that differs by having a weaker copyleft clause concerning linking of libraries between non-copyleft (proprietary or permissive) licensed code. Supporters of the BSD license argue against the copyleft nature of the GPL. They argue that the BSD license is more free than the GPL, because it grants the right to do nearly anything with the source code, second only to software in the public domain, and that the nature of the BSD license has encouraged the inclusion of well-developed standard code into proprietary software. BSD supporters feel that the GPL takes away fundamental rights from the users, forcing them to write their own software for tasks that are covered by GPL software if they wish to redistribute it with a non-GPL-compatible license. Existing BSD-licensed software distributions tend to avoid including software licensed under the GPL in the core operating system, except as a last resort when alternatives are non-existent or vastly less capable, such as with GCC. The OpenBSD project, for example, has acted to remove GPL-licensed tools in favour of BSD-licensed alternatives, some newly written and some adapted from older code. GPL supporters claim that mandating that derivative works remain GPL-licensed fosters the growth of free software, as developers who use GPL code have to share their improvements with the community. GPL supporters claim the derivative work license requirement is more a form of power than a freedom,* and that the BSD license allows people to "hoard" the work of others without having to give anything back.
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