This book follows the earlier authoritative and full-length descriptions of the design and implementation of the 4.3BSD and 4.4BSD versions of the UNIX system developed at the University of California at Berkeley. Since the final Berkeley release in 1994, several groups have continued development of BSD. This book details FreeBSD, the system with the largest set of developers and the most widely distributed releases. Although the FreeBSD distribution includes nearly 1000 utility programs in its base system and nearly 10,000 optional utilities in its ports collection, this book concentrates almost exclusively on the kernel.
UNIX-like systems include the traditional vendor systems such as Solaris and HP-UX; the Linux-based distributions such as Red Hat, Debian, Suse, and Slackware; and the BSD-based distributions such as FreeBSD, NetBSD, OpenBSD, and Darwin. They run on computers ranging from laptops to the largest supercomputers. They are the operating system of choice for most multiprocessor, graphics, and vector-processing systems, and are widely used for the original purpose of timesharing. The most common platform for providing network services (from FTP to WWW) on the Internet, they are collectively the most portable operating system ever dev eloped. This portability is due partly to their implementation language, C Kernighan & Ritchie, 1989 (which is itself a widely ported language), and partly to the elegant design of the system.
Since its inception in 1969 Ritchie & Thompson, 1978, the UNIX system has developed in several divergent and rejoining streams. The original developers continued to advance the state of the art with their Ninth and Tenth Edition UNIX inside AT&T Bell Laboratories, and then their Plan 9 successor to UNIX. Meanwhile, AT&T licensed UNIX System V as a product before selling it to Novell. Novell passed the UNIX trademark to X/OPEN and sold the source code and distribution rights to Santa Cruz Operation (SCO). Both System V and Ninth Edition UNIX were strongly influenced by the Berkeley Software Distributions produced by the Computer Systems Research Group (CSRG) of the University of California at Berkeley. The Linux operating system, although developed independently of the other UNIX variants, implements the UNIX interface. Thus, applications developed to run on other UNIX-based platforms can be easily ported to run on Linux.
Berkeley Software Distributions
The distributions from Berkeley were the first UNIX-based systems to introduce many important features including the following:
- Demand-paged virtual-memory support
- Automatic configuration of the hardware and I/O system
- A fast and recoverable filesystem
- The socket-based interprocess-communication (IPC) primitives
- The reference implementation of TCP/IP
The Berkeley releases found their way into the UNIX systems of many vendors and were used internally by the development groups of many other vendors. The implementation of the TCP/IP networking protocol suite in 4.2BSD and 4.3BSD, and the availability of those systems, played a key role in making the TCP/IP networking protocol suite a world standard. Even the non-UNIX vendors such as Microsoft have adopted the Berkeley socket design in their Winsock IPC interface.
The BSD releases have also been a strong influence on the POSIX (IEEE Std 1003.1) operating-system interface standard, and on related standards. Several features—such as reliable signals, job control, multiple access groups per process, and the routines for directory operations—have been adapted from BSD for POSIX.
Early BSD releases contained licensed UNIX code, thus requiring recipients to have an AT&T source license to be able to obtain and use BSD. In 1988, Berkeley separated its distribution into AT&T licensed and freely redistributable code. The freely redistributable code was licensed separately and could be obtained, used, and redistributed by anyone. The final freely redistributable 4.4BSD-Lite2 release from Berkeley in 1994 contained nearly the entire kernel and all the important libraries and utilities.
Two groups, NetBSD and FreeBSD, sprang up in 1993 to begin supporting and distributing systems built from the freely redistributable releases being done by Berkeley. The NetBSD group emphasized portability and the minimalist approach, porting the systems to nearly forty platforms and pushing to keep the system lean to aid embedded applications. The FreeBSD group emphasized maximal support for the PC architecture and pushed to ease installation for, and market their system to, as wide an audience as possible. In 1995, the OpenBSD group split from the NetBSD group to develop a distribution that emphasized security. Over the years there has been a healthy competition among the BSD distributions, with many ideas and much code flowing between them.
Material Covered in this Book
This book is about the internal structure of the FreeBSD 5.2 kernel and about the concepts, data structures, and algorithms used in implementing FreeBSD’s system facilities. Its level of detail is similar to that of Bach’s book about UNIX System V Bach, 1986; however, this text focuses on the facilities, data structures, and algorithms used in the FreeBSD variant of the UNIX operating system. The book covers FreeBSD from the system-call level down—from the interface to the kernel to the hardware itself. The kernel includes system facilities, such as process management, virtual memory, the I/O system, filesystems, the socket IPC mechanism, and network protocol implementations. Material above the system-call level—such as libraries, shells, commands, programming languages, and other user interfaces—is excluded, except for some material related to the terminal interface and to system startup. Following the organization first established by Organick’s book about Multics Organick, 1975, this book is an in-depth study of a contemporary operating system.
Where particular hardware is relevant, the book refers to the Intel Personal Computer (PC) architecture. Because FreeBSD has emphasized development on the PC, that is the architecture with the most complete support, so it provides a convenient point of reference.
Use by Computer Professionals
FreeBSD is widely used to support the core infrastructure of many companies worldwide. Because it can be built with a small footprint, it is also seeing increased use in embedded applications. The licensing terms of FreeBSD do not require the distribution of changes and enhancements to the system. The licensing terms of Linux require that all changes and enhancements to the kernel be made available in source form at minimal cost. Thus, companies that need to control the distribution of their intellectual property build their products using FreeBSD.
This book is of direct use to the professionals who work with FreeBSD systems. Individuals involved in technical and sales support can learn the capabilities and limitations of the system; applications developers can learn how to effectively and efficiently interface to the system; system administrators without direct experience with the FreeBSD kernel can learn how to maintain, tune, and configure the system; and systems programmers can learn how to extend, enhance, and interface to the system.
Readers who will benefit from this book include operating-system implementors, system programmers, UNIX application developers, administrators, and curious users. The book can be read as a companion to the source code of the system, falling as it does between the manual pages and the code in detail of treatment. But this book is neither exclusively a UNIX programming manual nor a user tutorial (for a tutorial, see Libes & Ressler 1988). Familiarity with the use of some version of the UNIX system (see, for example, Stevens 1992) and with the C programming language (see, for example, Kernighan & Ritchie 1989) would be extremely useful.
Use in Courses on Operating Systems
This book is suitable for use as a reference text to provide background for a primary textbook in a first-level course on operating systems. It is not intended for use as an introductory operating-system textbook; the reader should have already encountered terminology such as memory management, process scheduling, and I/O systems Silberschatz et al., 2002. Familiarity with the concepts of network protocols Comer, 2000; Stallings, 2000; Tanenbaum, 2003 will be useful for understanding some of the later chapters.
This book can be used in combination with a copy of the FreeBSD system for more advanced operating systems courses. Students’ assignments can include changes to, or replacements of, key system components such as the scheduler, the paging daemon, the filesystems, thread signalling, various networking layers, and I/O management. The ability to load, replace, and unload modules from a running kernel allows students to experiment without the need to compile and reboot the system. By working with a real operating system, students can directly measure and experience the effects of their changes. Because of the intense peer review and insistence on well-defined coding standards throughout its 25-year lifetime, the FreeBSD kernel is considerably cleaner, more modular, and thus easier to understand and modify than most software projects of its size and age.
Exercises are provided at the end of each chapter. The exercises are graded into three categories indicated by zero, one, or two asterisks. The answers to exercises that carry no asterisks can be found in the text. Exercises with a single asterisk require a step of reasoning or intuition beyond a concept presented in the text. Exercises with two asterisks present major design projects or open research questions.
This text discusses both philosophical and design issues, as well as details of the a...