1.11 Free and Open-Source Operating Systems

 The study of operating systems has been made easier by the availability of a vast number of free software and open-source releases. Both free operating systems and open-source operating systems are available in source-code format rather than as compiled binary code. Note, though, that free software and open-source software are two different ideas championed by different groups of people (see http://gnu.rog/phliosophy/open-source-misses-the-point.html/ for a discussion on the topic). Free software (sometimes referred to as freellibresoftware) not only makes source code available but also is licensed to allow no-cost use, redistribution, and modification. Open-source software does not necessarily offer such licensing. Thus, although all free sotrware is open source, some open-source software is not "free." GNU/Linux is the most famous open-source operating system, with some distributions free and others open source only (http://www.gnu.org/distros/). Microsoft Windows is a well-known example of the opposite closed-cource approach. Wiundows is proprietary software-Microsoft owns it, restricts its use, and carefully protects its source code. Apple's macOS opoerating system comprises a hybrid approach. It contains an open-source kernel named Darwin but includes proprietary, closed-source components as well.

Starting with the source code allows the programmer to produce binary code that can be executed on a system. Doing the opposite-reverse engineering the source code from the binaries-is quite a lot of work, and useful items such as comments are never recovered. Learning operating systems by examining the source code has other benefits as well. With the source code in hand, a student can modify the operating system and then compile and run the code to try out these changes, which is an excellent learning tool. This text includes projects that involve modifying operating-system source code, while also describing algorithms at a high level to be sure all important operating-system topics are covered. Throughout the text, we provide pointers to examples of open-source code for deeper study.

There are many benefits to open-source operating systems, including a community of interested(and usually unpaid)programmers who contribute to the code by helping to write it, debug it, analyze it, provide support, and suggest changes. Arguably, open-source code is more secure than closed-source code because many more eyes are viewing the code. Certainly, open-source code has bugs, but open-source advocates argue that bugs tend to be found and fixed faster owing to the nbumber of people using and viewing the code. Companies that earn revenue from selling their programs ofter hesitate to open-source their code, but RED Hat and a myriad of other companies are doing just that and showing that commercial companies benefit, rather than suffer, when they open-source their code. Revenue can be generated though support contracts and the sale of hardware on which the software runs, for example.