Free Software Licensing - Permissive Versus Copyleft Opinions

Permissive Versus Copyleft Opinions

Many users and developers of BSD-based operating systems have a different position on licensing. The main difference is the belief that the copyleft licenses, particularly the GNU General Public License (GPL), are undesirably complicated and/or restrictive. The GPL requires any derivative work to also be released according to the GPL while the BSD license does not. Essentially, the BSD license's only requirement is to acknowledge the original authors, and poses no restrictions on how the source code may be used. As a result, BSD code can be used in proprietary software that only acknowledges the authors. For instance, the IP stack in Microsoft Windows NT 3.1 and Mac OS X are derived from BSD-licensed software.

Supporters of the BSD license argue that it is more free than the GPL because it grants the right to do anything with the source code, second only to software in the public domain. This includes incorporating the BSD-licensed code into proprietary products. The approach has led to BSD code being used in common, widely used proprietary software. In response, GPL supporters claim that the freedom of others to make non-free software from free software is an unjust form of power rather than a necessary freedom. However, some developers might want to include GPLed software in their products and can't do it, simply because the GPL is incompatible with the licenses of other software they include in their product, even if everything is open source.

Code licensed under a permissive free software license, such as the BSD license, can be incorporated into copylefted (e.g. GPL'd) projects. Such code is thus "GPL-compatible". There is no need to secure the consent of the original authors. In contrast, code under the GPL cannot be relicensed under the BSD license without securing the consent of all copyright holders. Thus the two licenses are compatible, but the combination as a whole must be distributed under the terms of the GPL, not the permissive license.

Existing free software BSDs tend to avoid including software licensed under the GPL in the core operating system, or the base system, except as a last resort when alternatives are non-existent or vastly less capable, such as with GCC. (Indeed however, note that as of mid 2010 FreeBSD for example are moving from GCC to the upcoming LLVM compiler, perhaps primarily for this reason.) The OpenBSD project has acted to remove GPL-licensed tools in favor of BSD-licensed alternatives, some newly written and some adapted from older code.