TCP/IP编程指南：C语言实现

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"TCP/IP编程在C语言中的实践指南" 该资源是一本专注于使用C语言进行TCP/IP编程的教程，旨在帮助程序员深入理解网络通信的基本原理和实现方法。书中的内容涵盖了从基础知识到高级特性的全面讲解，适合初学者和有一定经验的开发者。 1. 网络基础介绍部分讲解了网络、数据包和协议的概念，强调了它们在网络通信中的作用。地址的理解是网络通信的关键，书中解释了不同类型的网络地址，包括IPv4和IPv6地址。 2. 基本套接字（Sockets）这部分详细介绍了如何创建和销毁套接字，以及如何指定服务器和客户端的网络地址。TCP客户端和服务器的实现过程被逐步解析，展示了如何建立连接、发送和接收数据。 3. 构建消息在这一章，数据编码、字节顺序（Big-Endian与Little-Endian）、对齐和填充以及数据帧的构建和解析等主题被讨论，这些都是在网络通信中处理数据传输时必不可少的知识。 4. 使用UDP套接字 UDP（用户数据报协议）是一种无连接的服务，提供了比TCP更轻量级的通信方式。书中展示了UDP客户端和服务器的创建，并讲解了如何通过UDP套接字发送和接收数据。 5. 高级套接字编程这章涉及套接字选项、信号处理、非阻塞I/O、多任务编程以及I/O复用（如select和poll），这些内容有助于提升程序的效率和灵活性，处理并发连接。 6. 内部机制深入探讨TCP/IP协议栈的内部工作原理，包括缓冲区管理在TCP中的角色、死锁问题、性能影响、TCP套接字的生命周期，以及分用（Demultiplexing）的原理，帮助读者理解网络通信的底层机制。 7. 域名服务（DNS） DNS是将人类可读的域名转换为网络地址的重要服务。本章阐述了如何在名称与互联网地址之间进行映射，以及如何通过名称查找服务信息，这对于实现网络应用来说至关重要。这本书不仅提供理论知识，还包含了大量的思考问题和实践练习，有助于读者巩固所学，并将理论应用于实际编程中。通过学习此资源，读者将能够熟练地使用C语言编写TCP/IP网络程序，应对各种网络通信挑战。

Chapter 1: Introduction []

pieces of information to identify a particular program: an

Internet address, used by IP, and a

port number, the additional address interpreted by the transport protocol (TCP or UDP).

Internet addresses are 32-bit binary numbers. 1 In writing down Internet addresses for

human consumption (as opposed to using them inside applications), we typically show them

as a string of four decimal numbers separated by periods (e.g., "10.1.2.3"); this is called the

"dotted-quad" notation. The four numbers in a dotted-quad string represent the contents of

the four bytes of the Internet address, thus each is a number between 0 and 255.

Technically, each Internet address refers to the connection between a host and an

underlying communication channel, such as a dial-up modem or Ethernet card. Because each

such network connection belongs to a single host, an Internet address identifies a host as

well as its connection to the network. However, because a host can have multiple physical

connections to the network, one host can have multiple Internet addresses.

Most likely you are accustomed to referring to hosts by

name (e.g., "host.example.com").

However, the Internet protocols deal with numerical addresses, not names. Services exist

that can map names to Internet addresses (see Chapter 7). Since names are just a level of

indirection on top of the Internet protocols, we will stick with using Internet addresses in our

programming examples until later in the book.

The port number in TCP or UDP is always interpreted relative to an Internet address.

Returning to our earlier analogies, a port number corresponds to a room number at a given

street address, say, that of a large building. The postal service uses the street address to get

the letter to a mailbox; whoever empties the mailbox is then responsible for getting the letter

to the proper room within the building. Or consider a company with an internal telephone

system: To speak to an individual in the company, you first dial the company's main number

to connect to the internal telephone system and then dial the extension of the particular

telephone of the individual you wish to speak with. In these analogies, the Internet address is

the street address or the company's main number, whereas the port corresponds to the room

number or telephone extension. Port numbers are 16-bit unsigned binary numbers, so each

one is in the range 1 to 65,535 (0 is reserved).

1.3 Clients and Servers

In our postal and telephone analogies, each communication is initiated by one party, who

sends a letter or dials the telephone call, while the other party responds to the initiator's con-

tact by sending a return letter or picking up the phone and talking. Internet communication

is again similar. The terms

client and server refer to these roles: The client program initiates

1 Throughout this book the term

Internet address refers to the addresses used with the current version of

IP, which is version 4 [11]. Because it is expected that a 32-bit address space will be inadequate for future

needs, a new version of IP has been defined [5]; it provides the same service but has much bigger Internet

addresses (128 bits). "IPv6," as the new version is known, has not been widely deployed; the sockets API

will require some changes to deal with its much larger addresses [6].

[] 1.4 What Is aSocket? 7

communication, while the server program waits passively for and then responds to clients

that contact it. Together, the client and server compose the

application.

The terms "client"

and "server" are descriptive of the typical situation in which the server makes a particular

capability--for example, a database service--available to any client that is able to communi-

cate with it using the TCP/IP protocols.

Whether a program is acting as a client or server determines the general form of its use

of the sockets API to communicate with its

peer.

(The client is the peer of the server and vice

versa.) Beyond that, the client-server distinction is important because

the client needs to know

the server's address and port initially,

but not vice versa. With the sockets API, the server can,

if necessary, learn the client's address information when it receives the initial communication

from the client. This is analogous to a telephone call--the callee in general does not need to

know the telephone number of the caller.

How does a client find out a server's IP address and port number? Usually, the client

knows the name of the server it wants, for example, from a Universal Resource Locator (URL)

such as "http://www.mkp.com," and uses the name resolution service (see Chapter 7) to learn

the corresponding Internet address.

Finding a server's port number is a different story. In principle, servers can use any port,

but the client must be able to learn what it is. In the Internet, there is a convention of assigning

well-known port numbers

to certain applications. The Internet Assigned Number Authority

(IANA) oversees this assignment. For example, port number 21 has been assigned to the File

Transfer Protocol. When you run an FTP client application, it tries to contact the FTP server on

that port by default. A list of all the assigned port numbers is maintained by the numbering

authority of the Internet (see

http://www.iana.org/assignments/port-numbers).

1.4 What Is a Socket?

A socket is an abstraction through which an application may send and receive data, in much

the same way as an open file allows an application to read and write data to stable storage.

A socket allows an application to "plug in" to the network and communicate with other

applications that are also plugged in to the same network. Information written to the socket

by an application on one machine can be read by an application on a different machine, and

vice versa.

Sockets come in different flavors, corresponding to different underlying protocol fami-

lies and different stacks of protocols within a family. In this book we deal only with the TCP/IP

protocol family. The main flavors of sockets in the TCP/IP family are

stream

sockets and

datagram

sockets. Stream sockets use TCP as the end-to-end protocol (with IP underneath)

and thus provide a reliable byte-stream service. Datagram sockets use UDP (again, with IP un-

derneath) and thus provide a best-effort datagram service that applications can use to send

individual messages up to about 65,500 bytes in length. Stream and datagram sockets are

also supported by other protocol suites; however, in this book we deal only with TCP stream

sockets and UDP datagram sockets.

Chapter 2: Basic Sockets []

family and type. Because there is currently only one choice for stream and datagram sockets in

the TCP/IP protocol family, we could specify 0 instead of giving the protocol number explicitly.

Someday, however, there might be other end-to-end protocols in the Internet protocol family

that implement the same semantics. In that case, specifying 0 might result in the use of a

different protocol, which might or might not be desirable. The main thing is to ensure that the

communicating programs are using the same end-to-end protocol.

The return value of socket () is actually an integer: a nonnegative value for success and

-1

for failure. A nonfailure value should be treated as an opaque handle, like a file descriptor.

(In reality, it

a file descriptor, taken from the same space as the numbers returned by open().)

This handle, which we call a

socket descriptor,

is passed to other API functions to identify the

socket abstraction on which the operation is to be carried out.

When an application is finished with a socket, it calls close(), giving the descriptor for

the socket that is no longer needed.

int close(int

socket)

close() tells the underlying protocol stack to initiate any actions required to shut down

communications and deallocate any resources associated with the socket, close() returns 0

on success or -1 on failure. Once close() has been called, it is not possible to send or receive

data through the socket.

2.2 Specifying Addresses

Applications using sockets need to be able to specify Internet addresses and ports to the

kernel. For example, a client must specify the address of the server application with which

it needs to communicate. In addition, the sockets layer sometimes needs to pass addresses

to the application. For example, a feature analogous to "Caller ID" in the telephone network

enables a server to learn the address of each client that communicates with it.

The sockets API defines a generic data type--the sockaddr structure--for specifying

addresses associated with sockets:

struct sockaddr

{

unsigned short sa_family;

char sa_data[14] ;

};

/* Address family (e. g. AF_INET) */

/* Family-specific address information */

The first part of this address structure defines the address family--the space to which the

address belongs. For our purposes, we will always use the constant AF_INET, which specifies

the Internet address family. The second part is a blob of bits whose exact form depends on the

address family. This is a typical way of dealing with heterogeneity in operating systems and

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