Beej's Guide: Internet Socket编程入门与系统调用详解

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《网络编程指南：使用Internet套接字》（Beej's Guide to Network Programming Using Internet Sockets）是由Brian "Beej" Hall编写的权威教程，适用于网络编程初学者和有一定经验的开发者。该指南于2005年11月发布，版权属于Brian Jorgensen。本书主要介绍如何利用互联网套接字进行网络编程，涵盖了从基础概念到系统调用的深入讲解。第1章是简介，介绍了目标读者（适合各种级别的程序员，包括初学者），平台和编译器的选择，以及官方网站、注意事项（如Solaris/SunOS程序员和Windows程序员可能遇到的问题）、电子邮件政策、镜像站点设置、翻译者注意事项，以及版权和分发政策。章节2探讨了什么是套接字，它是网络通信的基本组件。这里有两种类型的Internet套接字：流套接字（TCP）和数据报套接字（UDP）。接下来，作者解释了底层网络原理和如何处理套接字的数据结构。在第3章，作者详细讨论了数据处理和结构，强调了转换本地数据格式的必要性，如IP地址的处理。这部分着重于如何有效地与网络协议交互。第4章深入到系统调用层面，讲解了关键函数如socket()（获取文件描述符）、bind()（绑定端口）、connect()（连接到远程主机）、listen()（监听连接请求）、accept()（接受连接）、send()和recv()（双向通信）、sendto()和recvfrom()（发送和接收多播或广播消息）、close()和shutdown()（关闭套接字连接）的功能和用法。此外，还介绍了getpeername()（获取连接对方的信息）和gethostbyname()（获取主机名对应的IP地址）等函数。《Beej's Guide to Network Programming Using Internet Sockets》提供了一套全面且实用的网络编程指南，帮助读者掌握套接字编程的基础知识和实践技巧，无论是构建简单的服务器客户端应用还是更复杂的网络服务，都能从中获益匪浅。

Beej’s Guide to Network Programming Using Internet Sockets 10

a1 = inet_ntoa(ina1.sin_addr); // this is 192.168.4.14

a2 = inet_ntoa(ina2.sin_addr); // this is 10.12.110.57

printf("address 1: %s\n",a1);

printf("address 2: %s\n",a2);

will print:

address 1: 10.12.110.57

address 2: 10.12.110.57

If you need to save the address, strcpy() it to your own character array.

That’s all on this topic for now. Later, you’ll learn to convert a string like “whitehouse.gov”

into its corresponding IP address (see DNS, below.)

3.2.1: Private (Or Disconnected) Networks

Lots of places have a ﬁrewall that hides the network from the rest of the world for their

own protection. And often times, the ﬁrewall translates “internal” IP addresses to “external” (that

everyone else in the world knows) IP addresses using a process called Network Address Translation,

or NAT.

Are you getting nervous yet? “Where’s he going with all this weird stuff?”

Well, relax and buy yourself a drink, because as a beginner, you don’t even have to worry

about NAT, since it’s done for you transparently. But I wanted to talk about the network behind the

ﬁrewall in case you started getting confused by the network numbers you were seeing.

For instance, I have a ﬁrewall at home. I have two static IP addresses allocated to me by

the DSL company, and yet I have seven computers on the network. How is this possible? Two

computers can’t share the same IP address, or else the data wouldn’t know which one to go to!

The answer is: they don’t share the same IP addresses. They are on a private network with 24

million IP addresses allocated to it. They are all just for me. Well, all for me as far as anyone else

is concerned. Here’s what’s happening:

If I log into a remote computer, it tells me I’m logged in from 64.81.52.10 (not my real IP).

But if I ask my local computer what it’s IP address is, it says 10.0.0.5. Who is translating the IP

address from one to the other? That’s right, the ﬁrewall! It’s doing NAT!

10.x.x.x is one of a few reserved networks that are only to be used either on fully disconnected

networks, or on networks that are behind ﬁrewalls. The details of which private network numbers

are available for you to use are outlined in RFC 1918

, but some common ones you’ll see are

10.x.x.x and 192.168.x.x, where x is 0-255, generally. Less common is 172.y.x.x, where y goes

between 16 and 31.

Networks behind a NATing ﬁrewall don’t need to be on one of these reserved networks, but

they commonly are.

ftp://ftp.rfc-editor.org/in-notes/rfc1918.txt

Beej’s Guide to Network Programming Using Internet Sockets 11

4. System Calls or Bust

This is the section where we get into the system calls that allow you to access the network

functionality of a Unix box. When you call one of these functions, the kernel takes over and does

all the work for you automagically.

The place most people get stuck around here is what order to call these things in. In that, the

man pages are no use, as you’ve probably discovered. Well, to help with that dreadful situation,

I’ve tried to lay out the system calls in the following sections in exactly (approximately) the same

order that you’ll need to call them in your programs.

That, coupled with a few pieces of sample code here and there, some milk and cookies (which

I fear you will have to supply yourself), and some raw guts and courage, and you’ll be beaming

data around the Internet like the Son of Jon Postel!

4.1. socket()–Get the File Descriptor!

I guess I can put it off no longer–I have to talk about the socket() system call. Here’s the

breakdown:

#include <sys/types.h>

#include <sys/socket.h>

int socket(int domain, int type, int protocol);

But what are these arguments? First, domain should be set to “PF_INET”. Next, the type

argument tells the kernel what kind of socket this is: SOCK_STREAM or SOCK_DGRAM. Finally, just

set protocol to “0” to have socket() choose the correct protocol based on the type. (Notes:

there are many more domains than I’ve listed. There are many more types than I’ve listed. See

the socket() man page. Also, there’s a “better” way to get the protocol, but specifying 0 works

in 99.9% of all cases. See the getprotobyname() man page if you’re curious.)

socket() simply returns to you a socket descriptor that you can use in later system calls, or

-1 on error. The global variable errno is set to the error’s value (see the perror() man page.)

(This PF_INET thing is a close relative of the AF_INET that you used when initializing the

sin_family ﬁeld in your struct sockaddr_in. In fact, they’re so closely related that they

actually have the same value, and many programmers will call socket() and pass AF_INET as

the ﬁrst argument instead of PF_INET. Now, get some milk and cookies, because it’s times for

a story. Once upon a time, a long time ago, it was thought that maybe a address family (what

the “AF” in “AF_INET” stands for) might support several protocols that were referred to by their

protocol family (what the “PF” in “PF_INET” stands for). That didn’t happen. And they all lived

happily ever after, The End. So the most correct thing to do is to use AF_INET in your struct

sockaddr_in and PF_INET in your call to socket().)

Fine, ﬁne, ﬁne, but what good is this socket? The answer is that it’s really no good by itself,

and you need to read on and make more system calls for it to make any sense.

4.2. bind()–What port am I on?

Once you have a socket, you might have to associate that socket with a port on your local

machine. (This is commonly done if you’re going to listen() for incoming connections on a

speciﬁc port–MUDs do this when they tell you to “telnet to x.y.z port 6969”.) The port number is

Beej’s Guide to Network Programming Using Internet Sockets 12

used by the kernel to match an incoming packet to a certain process’s socket descriptor. If you’re

going to only be doing a connect(), this may be unnecessary. Read it anyway, just for kicks.

Here is the synopsis for the bind() system call:

#include <sys/types.h>

#include <sys/socket.h>

int bind(int sockfd, struct sockaddr *my_addr, int addrlen);

sockfd is the socket ﬁle descriptor returned by socket(). my_addr is a pointer to a struct

sockaddr that contains information about your address, namely, port and IP address. addrlen

can be set to sizeof(struct sockaddr).

Whew. That’s a bit to absorb in one chunk. Let’s have an example:

#include <string.h>

#include <sys/types.h>

#include <sys/socket.h>

#include <netinet/in.h>

#include <arpa/inet.h>

#define MYPORT 3490

main()

{

int sockfd;

struct sockaddr_in my_addr;

sockfd = socket(PF_INET, SOCK_STREAM, 0); // do some error checking!

my_addr.sin_family = AF_INET; // host byte order

my_addr.sin_port = htons(MYPORT); // short, network byte order

my_addr.sin_addr.s_addr = inet_addr("10.12.110.57");

memset(&(my_addr.sin_zero), ’\0’, 8); // zero the rest of the struct

// don’t forget your error checking for bind():

bind(sockfd, (struct sockaddr *)&my_addr, sizeof(struct sockaddr));

There are a few things to notice here: my_addr.sin_port is in Network Byte Order. So is

my_addr.sin_addr.s_addr. Another thing to watch out for is that the header ﬁles might differ

from system to system. To be sure, you should check your local man pages.

Lastly, on the topic of bind(), I should mention that some of the process of getting your own

IP address and/or port can be automated:

my_addr.sin_port = 0; // choose an unused port at random

my_addr.sin_addr.s_addr = INADDR_ANY; // use my IP address

See, by setting my_addr.sin_port to zero, you are telling bind() to choose the port for

you. Likewise, by setting my_addr.sin_addr.s_addr to INADDR_ANY, you are telling it to

automatically ﬁll in the IP address of the machine the process is running on.

If you are into noticing little things, you might have seen that I didn’t put INADDR_ANY into

Network Byte Order! Naughty me. However, I have inside info: INADDR_ANY is really zero! Zero

still has zero on bits even if you rearrange the bytes. However, purists will point out that there

could be a parallel dimension where INADDR_ANY is, say, 12 and that my code won’t work there.

That’s ok with me:

my_addr.sin_port = htons(0); // choose an unused port at random

Beej’s Guide to Network Programming Using Internet Sockets 13

my_addr.sin_addr.s_addr = htonl(INADDR_ANY); // use my IP address

Now we’re so portable you probably wouldn’t believe it. I just wanted to point that out, since

most of the code you come across won’t bother running INADDR_ANY through htonl().

bind() also returns -1 on error and sets errno to the error’s value.

Another thing to watch out for when calling bind(): don’t go underboard with your port

numbers. All ports below 1024 are RESERVED (unless you’re the superuser)! You can have any

port number above that, right up to 65535 (provided they aren’t already being used by another

program.)

Sometimes, you might notice, you try to rerun a server and bind() fails, claiming “Address

already in use.” What does that mean? Well, a little bit of a socket that was connected is still

hanging around in the kernel, and it’s hogging the port. You can either wait for it to clear (a minute

or so), or add code to your program allowing it to reuse the port, like this:

int yes=1;

//char yes=’1’; // Solaris people use this

// lose the pesky "Address already in use" error message

if (setsockopt(listener,SOL_SOCKET,SO_REUSEADDR,&yes,sizeof(int)) == -1) {

perror("setsockopt");

exit(1);

}

One small extra ﬁnal note about bind(): there are times when you won’t absolutely have to

call it. If you are connect()ing to a remote machine and you don’t care what your local port is (as

is the case with telnet where you only care about the remote port), you can simply call connect(),

it’ll check to see if the socket is unbound, and will bind() it to an unused local port if necessary.

4.3. connect()–Hey, you!

Let’s just pretend for a few minutes that you’re a telnet application. Your user commands you

(just like in the movie TRON) to get a socket ﬁle descriptor. You comply and call socket(). Next,

the user tells you to connect to “10.12.110.57” on port “23” (the standard telnet port.) Yow!

What do you do now?

Lucky for you, program, you’re now perusing the section on connect()–how to connect to a

remote host. So read furiously onward! No time to lose!

The connect() call is as follows:

#include <sys/types.h>

#include <sys/socket.h>

int connect(int sockfd, struct sockaddr *serv_addr, int addrlen);

sockfd is our friendly neighborhood socket ﬁle descriptor, as returned by the socket() call,

serv_addr is a struct sockaddr containing the destination port and IP address, and addrlen

can be set to sizeof(struct sockaddr).

Isn’t this starting to make more sense? Let’s have an example:

#include <string.h>

#include <sys/types.h>

#include <sys/socket.h>

#include <netinet/in.h>

剩余79页未读，继续阅读

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