Chapter 4. Linux Kernel

In this chapter, you will not only learn about the Linux kernel in general, but also specific things about it. The chapter will start with a quick presentation of the history of Linux and its role and will then continue with an explanation of its various features. The steps used to interact with the sources of the Linux kernel will not be omitted. You will only be presented with the steps necessary to obtain a Linux kernel image from a source code, but also information about what porting for an new ARM machine implies, and some of the methods used to debug various problems that could appear when working with the Linux kernel sources in general. In the end, the context will be switched to the Yocto Project to show how the Linux kernel can be built for a given machine, and also how an external module can be integrated and used later from a root filesystem image.

This chapter will give you an idea of the Linux kernel and Linux operating system. This presentation would not have been possible without the historical component. Linux and UNIX are usually placed in the same historical context, but although the Linux kernel appeared in 1991 and the Linux operating system quickly became an alternative to the UNIX operating system, these two operating systems are members of the same family. Taking this into consideration, the history of UNIX operating system could not have started from another place. This means that we need to go back in time to more than 40 years ago, to be more precise, about 45 years ago to 1969 when Dennis Ritchie and Ken Thompson started the development of UNIX.

The predecessor of UNIX was Multiplexed Information and Computing Service (Multics), a multiuser operating system project that was not on its best shape at the time. Since the Multics had become a nonviable solution for Bell Laboratories Computer Sciences Research Center in the summer of 1969, a filesystem design was born and it later became what is known today as UNIX. Over time, it was ported on multiple machines due to its design and the fact that the source code was distributed alongside it. The most prolific contributor to the UNIX was the University of California, Berkeley. They also developed their own UNIX version called Berkeley Software Distribution (BSD), that was first released in 1977. Until the 1990s, multiple companies developed and offered their own distributions of UNIX, their main inspirations being Berkeley or AT&T. All of them helped UNIX become a stable, robust, and powerful operating system. Among the features that made UNIX strong as an operating system, the following can be mentioned:

• UNIX is simple. The number of system calls that it uses are reduced to only a couple of hundred and their design is basic
• Everything is regarded as a file in UNIX, making the manipulation of data and devices simpler, and it minimizes system calls used for interaction.
• Faster process creation time and the fork() system call.
• The UNIX kernel and utilities written in C language as well as a property that makes it easily portable and accessible.
• Simple and robust interprocess communication (IPC) primitives helps in the creation of fast and simple programs that accomplish only one thing in the best available manner.

Nowadays, UNIX is a mature operating system with support for features, such as virtual memory, TCP/IP networking, demand paging preemptive multiprocessing, and multithreading. The features spread is wide and varies from small embedded devices to systems with hundreds of processors. Its development has moved past the idea that UNIX is a research project, and it has become an operating system that is general-purpose and practically fits any needs. All this has happened due to its elegant design and proven simplicity. It was able to evolve without losing its capability to remain simple.

Linux is as an alternative solution to a UNIX variant called Minix, an operating system that was created for teaching purposes, but it lacked easy interaction with the system source code. Any changes made to the source code were not easily integrated and distributed because of Minix's license. Linus Torvalds first started working at a terminal emulator to connect to other UNIX systems from his university. Within the same academic year, emulator evolved in a full-fledged UNIX. He released it to be used by everyone in 1991.

One of the most attractive features of Linux is that it is an open source operating system whose source code is available under the GNU GPL license. When writing the Linux kernel, Linus Torvalds used the best design choices and features from the UNIX available in variations of the operating system kernel as a source of inspiration. Its license is what has propelled it into becoming the powerhouse it is today. It has engaged a large number of developers that helped with code enhancements, bug fixing, and much more.

Today, Linux is an experienced operating system that is able to run on a multitude of architectures. It is able to run on devices that are even smaller than a wristwatch or on clusters of supercomputer. It's the new sensation of our days and is being adopted by companies and developers around the world in an increasingly persified manner. The interest in the Linux operating system is very strong and this implies not only persity, but also offers a great number of benefits, ranging from security, new features, embedded solutions to server solution options, and many more.

Linux has become a truly collaborative project developed by a huge community over the internet. Although a great number of changes were made inside this project, Linus has remained its creator and maintainer. Change is a constant factor in everything around us and this applies to Linux and its maintainer, who is now called Greg Kroah-Hartman, and has already been its kernel maintainer for two years now. It may seem that in the period that Linus was around, the Linux kernel was a loose-knit community of developers. This may be because of Linus' harsh comments that are known worldwide. Since Greg has been appointed the kernel maintainer, this image started fading gradually. I am looking forward to the years to come.