What do we ever get nowadays from reading to equal the excitement and the revelation in those first fourteen years? -- Graham Greene
The reading of code is likely to be one the most common activities of a computing professional, yet it is seldom taught as a subject, or formally used as a method for learning how to design and program. One reason for this sad situation may have been the lack of high-quality code to read. Companies often protect source code as a trade secret and rarely allow others to read, comment, experiment, and learn from it. In the few cases where important proprietary code was allowed out of a company's closet, it has spurred enormous interest and creative advancements. As an example, a generation of programmers benefited from John Lions' Commentary on the UNIX Operating System that listed and annotated the complete source code of the sixth edition UNIX kernel. Although Lions' book was originally written under a grant from AT&T for use in an operating system course and was not available to the general public, copies of it circulated for years as bootleg nth-generation photocopies.
In the last few years however, the popularity of open-source software has provided us with a large body of code that we can all freely read. Some of the most popular software systems used today, such as the Apache Web server, the Perl language, the gnu/Linux operating system, the BIND domain-name server, and the sendmail mail-transfer agent are in fact available in open-source form. I was thus fortunate to be able to use open-source software, such as the above to write this book as a primer and reader for software code. My goal was to provide background knowledgeand techniques for reading code written by others. By using real-life examples taken out of working, open-source projects I tried to cover most concepts related to code that are likely to appear before a software developer's eyes including programming constructs, data types, data structures, control flow, project organization, coding standards, documentation, and architectures. A companion title to this book will cover interfacing, and application-oriented code including the issues of internationalization and portability, the elements of commonly used libraries and operating systems, low-level code, domain-specific and declarative languages, scripting languages, and mixed language systems.
This book is--as far as I know--the first one to exclusively deal with code-reading as a distinct activity, one worthy on its own. As such I am sure that there will be inevitable shortcomings, better ways some of its contents could have been treated, and important material I have missed. I firmly believe that the reading of code should both be properly taught, and used as a method for improving one's programming abilities. I therefore hope that this book will spur interest to include code reading courses, activities, and exercises into the computing education curriculum so that in a few years our students will learn from existing open-source systems, just as their peers studying a language learn from the great literature.
Supplementary Material
Many of the source code examples provided come from the source distribution of NetBSD. NetBSD is a free, highly portable UNIX-like operating system available for many platforms, from 64-bit AlphaServers to handheld devices. Its clean design and advanced features make it an excellent choice for both production and research environments. I selected NetBSD over other similarly admirable and very popular free UNIX-like systems such as GNU/Linux, FreeBSD, and OpenBSD, because the primary goal of the NetBSD project is to emphasize correct design and well written code thus making it a superb choice for providing example source code. According to its developers, some systems seem to have the philosophy of "if it works, it's right" whereas NetBSD could be described as "it doesn't work unless it's right." In addition, some other NetBSD goals fitted particularly well with the objectives of this book. Specifically, the NetBSD project avoids encumbering licenses, provides a portable system running on many hardware platforms, interoperates well with other systems, and conforms to open systems standards as much as is practical. The code used in this book is a (now historic) export-19980407 snapshot. A few examples refer to errors I found in the code; as the NetBSD code continuously evolves, presenting examples from a more recent version would mean risking that those realistic gems would have been corrected.
I chose the rest of the systems I used in the book's examples for similar reasons: code quality, structure, design, utility, popularity, and a license that would not make my publisher nervous. I strived to balance the selection of languages, actively looking for suitable Java and C++ code. However, where similar concepts could be demonstrated using different languages I chose to use C as the least common denominator.
I sometimes used real code examples to illustrate unsafe, non-portable, unreadable, or otherwise condemnable coding practices. I appreciate that I can be accused of disparaging code that was contributed by its authors in good faith to further the open-source movement and to be improved upon rather than be merely criticized. I sincerely apologize in advance if my comments cause any offence to a source code author. In defense I argue that in most cases the comments do not target the particular code excerpt, but rather use it to illustrate a practice that should be avoided. Often the code I am using as a counter example is a lame duck, as it was written at a time where technological and other restrictions justified the particular coding practice, or the particular practice is criticized out of the context. In any case, I hope that the comments will be received good-humouredly, and openly admit that my own code contains similar, and probably worse, misdeeds.
