Apple Public Source License

Apple sometimes distributes the source code to certain components of the Mac OS X under the Apple Public Source License or APSL. The APSL allows these components of Mac OS X to become collaborative projects.

About the license
The current APSL version 2.0 is an "open source" and "free software" license that allows other users to use, examine, share, copy, play with, and improve the software. The principle requirements are that:
 * 1) If you modify a source code file under the APSL and share your changes, then you must license your changes under the APSL (section 2.2).
 * 2) If you distribute a program, then you must make available any APSL source code files that it uses (section 2.3).

The APSL is weaker than the GNU General Public License because the GNU GPL requires the entire program to be free, while the APSL only requires the APSL source code files to be free and allows you use the APSL source code files together with other files in the same program.

The Apple Public Source License is one of the many variants of the Mozilla Public License. All of these variants are similar, but they have subtle differences. For example, the APSL defines "Externally Deploy" in section 1.4 and uses the term in section 2.2(c) such that if you cause the program to provide a service over the internet, then you must make available the APSL source code files. The APSL is here stronger than the GNU GPL.

Older versions
There are three earlier versions of the APSL. The APSL versions 1.0, 1.1 and 1.2 all have problems that the APSL 2.0 does not have.

The APSL originally restricted how anyone (outside of Apple) could use the software. It also permitted Apple to revoke or suspend the license, making the software non-free. So the open source community and the free software community shunned the license and the source code that Apple released under its terms.

Apple wanted more developers to participate in these projects and thus revised the license. The Open Source Initiative found the APSL 1.2 to meet the Open Source Definition and approved that version of the license. However, the Free Software Foundation objected that organizations were not free to modify and use the software internally without sharing the changes. This put the APSL 1.2 into the awkward and normally impossible position of being an "open source" (OSI) license but not a "free software" (FSF) license.

Apple finally negotiated with the Free Software Foundation and drafted the APSL 2.0, removing that last obstacle.

References and further reading

 * Apple Public Source License
 * Open Source Initiative: APSL 1.2
 * FSF's Opinion of the Apple Public Source License (APSL) 2.0
 * FSF's Position on the Older Versions of APSL