C语言与Unix系统编程入门

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"C语言与Unix系统编程是Clemson University教授Adam Hoover撰写的一本教材，专注于使用C语言进行系统编程，并结合Unix操作系统进行深入学习。本书由ADDISON-WESLEY出版社出版，覆盖了从基础的C编程到复杂的Unix系统接口的广泛主题，适合初学者和有一定经验的开发者作为学习资料。" 在《System Programming with C and Unix》这本书中，作者通过C语言这一强大的工具，引导读者进入系统编程的世界。C语言因其高效、灵活和接近硬件的特性，成为系统编程领域中的首选语言。书中可能涵盖了以下核心知识点： 1. **C语言基础**：介绍C语言的基本语法，包括变量、数据类型、运算符、控制结构（如if语句、循环）、函数、指针等。这些基础知识是理解和编写任何C程序的基础。 2. **内存管理**：讨论C语言中的动态内存分配和释放，以及如何有效地使用堆和栈，避免内存泄漏。 3. **文件操作**：讲解如何在C语言中读写文件，包括文件打开、读取、写入、关闭等操作，以及文件定位和错误处理。 4. **Unix系统调用**：介绍Unix系统提供的丰富API，如进程控制（创建、终止、通信）、信号处理、I/O操作、文件系统操作等，这些都是进行系统编程的关键。 5. **进程间通信**：涵盖管道、套接字、共享内存、信号量等Unix下实现进程间通信的方式，这对于构建多进程或多线程应用至关重要。 6. **系统接口和库函数**：讲解如何使用系统头文件，如`<unistd.h>`、`<sys/types.h>`、`<sys/socket.h>`等，以及它们提供的各种功能。 7. **编译和调试**：介绍如何在Unix环境下编译、链接C程序，以及使用调试工具如`gdb`来查找和修复程序错误。 8. **Unix shell脚本**：简单介绍如何编写和使用shell脚本，以提高日常的命令行工作效率。 9. **错误处理和调试技巧**：强调在系统编程中正确处理错误的重要性，以及调试策略和技巧。 10. **实践项目**：可能包含一系列的实践项目，帮助读者将理论知识应用于实际问题，提升系统编程能力。这本书通过实例和练习，帮助读者掌握C语言和Unix系统编程技能，对于想要深入理解操作系统底层机制和开发系统级软件的程序员来说，是一本极具价值的参考资料。

1.1

●

What is System Programming? 3

This text was written with three goals. First, it supports teaching about the

tools and concepts of system programming. Second, it should help the reader ele-

vate his or her programming skill beyond an introductory level. Third, it provides

a rigorous regimen of programming exercises and examples that allow the reader

to practice and develop the skills and concepts of system programming. To help

achieve these goals, example code pieces and programs are provided throughout

the text. Each chapter ends with numerous questions and exercises that can be

undertaken to strengthen understanding of the material.

Besides the concepts of system programming, this text explores the lower-

level data types: bits and bytes, bit operations, arrays, strings, structures, and

pointers. This material is covered with an emphasis on memory and understand-

ing how and why these different data types are used. It is common for a student

to be less comfortable with these topics than with other basic programming con-

cepts, such as loops and conditionals. The coverage of the lower-level data types

is intended to reinforce an earlier exposure to these topics. The goal is to ad-

vance the programming skill of the reader to the point where these lower-level

data types are well and comfortably used.

1.1.1 Required Background

This text assumes that the reader has a basic understanding of programming,

such as variables, loops, conditionals, and control ﬂow. For example, the reader

may have completed a single semester of study covering an introduction to C

programming. If the reader studied a different language, such as Java or C++,

then Section 1.5 can be studied to provide background on the equivalent C

syntax. The text also assumes that the reader has a working computer system,

withaCcompiler, text editor, shell, and debugger already installed. The reader is

assumed to be familiar with the basic operation of the computer system, such as

navigating a directory or folder hierarchy and executing programs. Lastly, some

shell knowledge is also assumed, such as that obtained from an introductory

programming course.

1.1.2 Why Unix?

The majority of the material presented in this text can be studied on any com-

puter system, using any operating system. However, it would be naive not to

recognize the two most prevalent operating systems at the time of this writing:

4 Chapter 1

●

Introduction

Microsoft Windows

™

and Unix.

For reasons about to be explained, this text ad-

vocates the study of system programming concepts on a Unix system. Note that

this discussion centers on which is to be preferred for study. There are other on-

going debates as to which operating system has the better business model, better

development, and other issues. The interested reader is directed to seek other

sources for discussion on these debates.

The Windows operating system is designed to simplify computer usage. A

Windows computer is essentially a closed system. The system is designed to be

turnkey, in line with the business model of providing the typical user with a sys-

tem that is as easy to use as possible. This design strategy necessarily obstructs

getting “under the hood,” to keep the typical user from doing something harm-

ful to the system. In addition, Microsoft publishes limited information on the

internal workings and design of Windows. This is also a business policy to pro-

tect their design from competitors. Finally, Windows is monolithic, not allowing

for various parts of the system to be disconnected or swapped for alternatives.

Once again, this is a business decision; Microsoft wants to sell its products and

only its products, so it makes its system fully integrated. It is straightforward to

understand the business motivations for the closed design, scarcity of published

details, and non-modularity of Windows. However, the very properties that aid

the typical computer user can be frustrating for the student studying system op-

eration.

Unix, on the other hand, and speciﬁcally Linux, has different design princi-

ples. It is open source, so that all details of its inner workings can be studied. It

is completely modular, so that any system component can be swapped for an al-

ternative. For example, the Linux kernel is developed completely independently

of the desktop environment. Within the kernel, a Linux computer operator has

many choices as to how to conﬁgure the operation of the system. The kernel itself

can be swapped or modiﬁed. One could argue that these properties prevent the

more widespread adoption of Unix by typical computer users, who are not inter-

ested in this ﬂexibility and openness. However, it is these design properties that

make Unix an attractive choice for a student studying system operation.

Throughout this text, the examples shown are taken from a computer run-

ning the Linux operating system. For the most part, these examples can be run

just as easily on a Windows or other Unix system. There are some important

design issues that differentiate Unix/Linux and Windows, such as the multiuser

versus single-user nature of the systems. These differences will be discussed in de-

tail at the appropriate places. Beyond these considerations, deep down under the

hood, the system concepts are largely similar.

1. In this book, Unix refers to any Unix-like system, including Linus, Mac OS X, BSD variants, etc.

1.2

●

The Three Tools 5

1.1.3 Why C?

Selection of a particular programming language is an old debate in computing.

For application development, the debate still rages. For system programming,

however, very few experts argue for a language other than C. The reason is simple:

C is closest to the hardware. All programming languages provide various lev-

els of abstraction to assist in program development. For example, the concept

of a named variable, as opposed to a numeric memory address, tremendously

simpliﬁes program development. Out of all the commonly used programming

languages, C provides for the least abstraction and hence is closest to the hard-

ware. Most single C statements translate simply to machine code. The available

data types in C tend to reﬂect what the hardware directly supports. Accessing

memory via indirection (pointers) provides the programmer with the ability to

access all parts of the system.

Historically, the development of the Linux kernel, as well as the development

of the original Unix operating system, was done in C. Most system software

is developed in C. Device drivers are almost always written in C. An indirect

beneﬁt of being close to the hardware is speed. Code written in C tends to execute

faster than code written in other languages. For a programmer who intends to

work on system software, or who intends to develop code that closely interacts

with hardware (peripherals or the main system), studying concepts using the C

language provides opportunities to develop the most practical skills.

This choice does not preclude the study of other languages or advocate learn-

ing only C. Other programming concepts outside the scope of this text may be

more readily studied and implemented using another programming language.

However, it is the opinion of this author that a ﬁrm understanding of the pro-

gramming language closest to the hardware better supports an understanding

and proper use of a more abstract programming language.

1.2

●

The Three Tools

The three main tools of a system programmer are a shell, a text editor, and a de-

bugger. Familiarity with these tools increases programming skill and decreases

the time it takes to get programs working properly. The following serves as an in-

troduction to these tools and their interdependency. The real trick is in knowing

how to use all three together. Noticeably absent from this list is a compiler. A com-

piler is a powerful tool that can certainly aid in program development. However,

the compiler as a tool is addressed more fully in Section 6.1 during a discussion

of program building.

6 Chapter 1

●

Introduction

1.2.1 Shell

It is assumed that the reader is familiar with basic operations in a shell, such as

generating a ﬁle listing. There are a number of good books and online references

that provide the necessary details to operate a speciﬁc shell. Appendix B offers a

list of common commands. This section is concerned with why a shell should be

used, and how a shell assists with program development.

A shell is a program that allows the user to run other programs. A shell

is usually executed in a terminal. Historically, a terminal was a simple display

and keyboard that was connected to a computer, providing a text-only interface

to the computer. Today, most operating systems provide a graphical interface

in which multiple virtual terminals can be run simultaenously, each running a

separate shell. If a speciﬁc terminal is reserved for system administration or error

messages on behalf of the entire computer system, then it is sometimes referred

to as a console. In practice, the words shell, terminal, and console are often used

interchangeably.

Most Unix systems provide several methods to start a shell. Some systems

provide a shell after login, without the beneﬁts of a graphical desktop. On other

systems a shell must be started manually through a menu or mouse click in-

terface. Once started, a typical shell looks like the ones shown in Figure 1.1.

Commands are entered into the shell through a text-only interface. The shell

informs the user that it is waiting for its next command via a prompt. In the ﬁg-

ure, the prompt is

ahoover@video>, which is the user name and machine name.

Some shell conﬁgurations show the current directory or other information in

the prompt. Throughout this text, the shell prompt will hereafter be shown as

ahoover@video> to promote clarity.

A typical command is the name of a program, which starts execution of

that program. For example, both the shells in Figure 1.1 show the execution of

the

ls program, which provides a listing of ﬁles in the current directory. Many

of the programs run in a shell have text-only input and output, similar to

ls.

However, it is also possible to run graphics- (GUI-) based programs from a shell.

For example, Figure 1.2 shows the result of typing

xclock at the prompt; it starts

the xclock program.

This method for starting programs may seem strange to those familiar with

today’s desktop approach to running programs. Clicking on an icon and perus-

ing through a pull-down menu are typical operations used to run a program.

Why then start a program, the shell, just to run other programs? The answer is

ﬂexibility. The desktop mouse and menu operations provide limited options in

how a program is run. Typically, the desktop and menu shortcuts run a program

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