"Linux Network Development Required Course" 1_Network Programming Core Concepts and Patterns

Question: How to do network programming?

The essence of network programming

Use the interface functions provided by the operating system to enable the application program to have the ability to send and receive network data

The network interface is a function provided by the operating system at the code level

• Network programming is just a fancy way of playing a series of system functions

The application uses the networking capabilities of the operating system through the network interface

• Network programming is domain-specific (network) programming in C

Protocol: Predefined data rules for data communication Address: Integer value used to identify the device in network communication ("xxx.xxx.xxx.xxx" 4-byte integer value can be represented, just to make it easier to remember, convert as a string)
The port number

• The value assigned by the device to send and receive data to indicate a specific connection
• It can be understood as: the data channel used for network communication in the device
• For example, a computer often has only one IP and has multiple applications, at this time, the port is used to distinguish

Role

• Server: Waiting for a connected device
• Client: It is the evil quilt who initiated the connection

ps : 一台设备可同时作为客户端和服务端

Network Knowledge Charging Station

Is a web address an IP address? What is the URL and what is the domain name?

• The URL is not an IP address, but refers to the address of network information resources (such as the address of a specific web page), namely: URL
• A domain name is the name of an IP address, and multiple domain names can point to the same IP address
• Domain Name → DNS → IP Address

Does the protocol have to be incomprehensible binary data?

• A protocol is a convention, ie: pre-defined rules
• Protocols can be based on textual or binary definitions

Little-endian system: a system that adopts little-endian mode, that is, the low byte of data is placed in the low address of memory

 0x12345678  int 4字节
低地址 👉 |78|56|34|12| 👈 高地址

Big-endian system: a system that adopts big-endian mode, that is, the low byte of data is placed in the high address of memory

 0x12345678  int 4字节
低地址 👉 |12|34|56|78| 👈 高地址

Network byte order: The byte order of the URL adopts the big endian mode , so byte conversion needs to be done in the little endian system

network programming mode

 1. 准备连接
2. 连接远程设备
3. 收发数据
4. 关闭丽娜姐

A Preliminary Study of Network Programming Interface

#include <sts/types.h>
#include <sys/socket.h>

function prototype	Function description
int socket(int domain, int type, int protocal);	Create a socket in preparation for a network connection
int connect(int socket, struct sockaddr *addr, socklen_t len);	Connect to a remote device at the specified address
ssize_t send(int fd, const void *buf, size_t n, int flags);	Send data to remote device
ssize_t recv(int fd, void *buf, size_t n, int flags);	Receive data from remote devices
int close(int fd);	close the connection, destroy the socket

Programming experiment: first experience of network programming

 #include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <stdio.h>
#include <unistd.h>
#include <string.h>

int main(void)
{  
   int sock = 0;
   struct sockaddr_in addr = {0};
   char *tosend = "GET /index.html HTTP/1.1\nHOST: www.dt4sw.com\nUser-Agent: TEST\nConnection: close\n\n";
   int len = 0;
   char buf[128] = {0};
   int r = 0;

   sock = socket(PF_INET, SOCK_STREAM, 0);
   if (sock == -1) {
      printf("socket error\n");
      return -1;
   }

   addr.sin_family      = AF_INET;
   addr.sin_addr.s_addr = inet_addr("47.99.217.175");
   addr.sin_port        = htons(80);

   if (connect(sock, (struct sockaddr*)&addr, sizeof(addr)) == -1) {
      printf("connected error\n");
      return -1;
   }

   printf("connect success\n");

   len = send(sock, tosend, strlen(tosend), 0);

   printf("send bytes = %d\n", len);

   len = 0;
   do {
      int i = 0;

      r = recv(sock, buf, sizeof(buf), 0);

      if (r > 0) {
         len += r;
      }

      for (i=0; i<r; ++i) {
         printf("%c", buf[i]);
      }
   } while (r > 0);

   printf("\n");
   printf("recv bytes = %d\n", len);

   return 0;
}

output:

 ./a.out 
connect success
send bytes = 81
HTTP/1.1 200 OK
Content-Type: text/html

......
......

  <script src="./static/js/app.js"></script>
</body>
</html>
recv bytes = 22382

Think: What is the role of the network program written this time?

Answer: Client