前言

不知道大家是否还记得1.0版本程序, 我们现在要对他进行升级了!
所谓回声服务器, 就是客户端发送的数据, 能够像声音一样回弹, 服务器原样返还给客户端.

server.c

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#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <unistd.h>
#include <arpa/inet.h>
#include <sys/socket.h>
#include <netinet/in.h>
#define BUF_SIZE 100

int main() {
    int server_socket = socket(AF_INET, SOCK_STREAM, IPPROTO_TCP);

    if(server_socket < 0){
        printf("socket() error\n");
        return 0;
    }
    struct sockaddr_in server_addr;
    memset(&server_addr, 0, sizeof(server_addr));
    server_addr.sin_family = AF_INET;
    server_addr.sin_port = htons(1234);
    server_addr.sin_addr.s_addr = inet_addr("127.0.0.1");

    //将套接字与IP,端口绑定
    bind(server_socket, (struct sockaddr *) & server_addr, sizeof(server_addr));

    //开始监听
    listen(server_socket, 20);

    struct sockaddr_in client_addr;
    socklen_t client_addr_size = sizeof(client_addr);
    
    int client_socket = accept(server_socket, (struct sockaddr *) & client_addr, &client_addr_size);

    //向客户端发送数据
    char buffer[BUF_SIZE];
    read(client_socket, buffer, sizeof(buffer));
    printf("服务器端接受到的消息：%s\n", buffer);
    write(client_socket, buffer, sizeof(buffer));

    close(client_socket);
    close(server_socket);

}

client.c

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#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <unistd.h>
#include <arpa/inet.h>
#include <sys/socket.h>
#include <netinet/in.h>
#define BUF_SIZE 100

int main(){
    int client_socket = socket(AF_INET, SOCK_STREAM, 0);
    if(client_socket < 0) {
    	printf("socket error! \n");
    	return 0;
    }

    struct sockaddr_in server_addr;
    memset(&server_addr, 0, sizeof(server_addr));
    server_addr.sin_family = AF_INET;
    server_addr.sin_port = htons(1234);
    server_addr.sin_addr.s_addr = inet_addr("127.0.0.1");

    connect(client_socket, (struct sockaddr *) & server_addr, sizeof server_addr);

    //向服务器发送数据
    char buffer_send[BUF_SIZE] = {0};
    printf("输入发送消息：");
    fgets(buffer_send, 100, stdin);
    write(client_socket, buffer_send, sizeof(buffer_send));

    //
    char buffer_recv[BUF_SIZE] = {0};
    read(client_socket, buffer_recv, sizeof(buffer_recv) - 1);

    printf("接收到的消息：%s\n", buffer_recv);
    
    close(client_socket);

}

一点理解:
服务器端创建了两个socket, 一个server_socket, 用来创建连接, 而client_socket用来通信, 客户端创建了一个socket, 没有绑定端口和ip的环节, 我理解它为client_socket, 直接通过connect连接服务器, 之后, 两个client_socket就能互相通信, 客户端写入它, 服务器端就能读取.