Mac OS X Internals: A Systems Approach

2.3. Darwin

As we saw in Chapter 1, Darwin was released as a fork of a developer release of the Rhapsody operating system, which was an immediate precursor to Mac OS X. An important component of Darwin is the Mac OS X kernel environment, which, together with the Darwin user environment, makes Darwin a stand-alone operating system. Until Apple announced the transition of Mac OS X to the x86 platform in mid-2005, Mac OS X had been a strictly PowerPC-only operating system. In contrast, Darwin has always been supported on both the PowerPC and the x86 platforms.

2.3.1. Darwin Packages

Darwin can be best understood as a collection of open source technologies that have been integrated by Apple to form a fundamental part of Mac OS X. It contains source code both from Apple and from third parties, including the Open Source and Free Software communities. Apple makes Darwin available as a set of packages, where each package is an archive containing source code of some component of Mac OS X. Darwin packages range from trivial ones such as Liby to gigantic ones such as GCC and X11. The exact number of packages in Darwin varies from release to release. For example, Darwin 8.6 (PowerPC)which corresponds to Mac OS X 10.4.6contains about 350 packages. Sources for Darwin components that originated at Apple are usually provided under the Apple Public Source License (APSL), which is a free software license.^[6] The rest of the packages are provided under their respective licenses, such as the GNU General Public License (GPL), the BSD License, the Carnegie Mellon University License, and so on.

^[6] The Free Software Foundation (FSF) classifies versions 1.0, 1.1, and 1.2 of the APSL as nonfree software licenses, whereas version 2.0 of the APSL is classified by the FSF as a free software license.

2.3.2. The Benefits of Darwin

Darwin represents a substantial amount of software that Apple has leveraged from a variety of sources: NEXTSTEP and OPENSTEP, Mach, various BSD flavors (largely FreeBSD), the GNU software suite, the XFree86 project, and so on. More importantly, Apple has integrated such "external" software rather well, making important modifications to optimize and adapt it to Mac OS X. Even though you can configure and control most of this software as you normally would, say, on a traditional Unix system, Mac OS X provides simplified and mostly consistent user interfaces that usually work well by hiding the underlying complexity. Such efficiency in adopting technology from diverse sources, and integrating it to create a synergistic effect, is one of the great strengths of Mac OS X.

2.3.3. Darwin and Mac OS X

It is important to note that Darwin is not Mac OS X. It can be thought of as a subset of Mac OS Xessentially the low-level foundation upon which Mac OS X is built. Darwin does not include many proprietary components that are integral parts of Mac OS X, such as the Aqua look-and-feel, Carbon, Cocoa, OpenGL, Quartz, and QuickTime. Consequently, it also does not support key Apple software such as the iLife suite, iChat AV, Safari, and the Xcode development environment.

Although Darwin lacks the visual technologies of Mac OS X, it is possible to run Darwin with the X Window System providing the graphical user interface.