简单Echo服务器的实现

1. 要求

    实现Echo服务器的客户端/服务器程序，客户端通过网络连接到服务器，并发送任意一串英文信息，服务器端接收信息后，将每个字符转换为大写并回送给客户端显示。
    这个项目的主要目的：熟悉C语言的网络编程（Socket），为开发更高级的网络应用打好基础。

2. 服务器端代码

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


#define SERVER_PORT 666

int main(void){   
    int server_socket,client_socket;
    struct sockaddr_in server_address;
    struct sockaddr_in client_address;
    socklen_t client_address_length;
    client_address_length = sizeof(client_address);

    /* 1. create server socket
          arguments: AF_INET -> use IPv4
	             SOCK_STREAM -> use tcp
     */
    server_socket = socket(AF_INET,SOCK_STREAM,0);
    // 2. clear server_address and write server address and server port
    bzero(&server_address,sizeof(server_address));
    server_address.sin_family = AF_INET;                 // ipv4
    server_address.sin_addr.s_addr = htonl(INADDR_ANY);  // designate all ip address
    server_address.sin_port = htons(SERVER_PORT);        // designate server port
    // 3. bind sevrer ip and port to server socket
    bind(server_socket,(struct sockaddr *)&server_address,sizeof(server_address));
    // 4. start to listen
    listen(server_socket,1024);

    printf("wait client message .....\n");

    // start process client's request
    int done = 1; 
    while(done){
	    int message_length;
	    char client_ip_address[64];
	    char buffer[256];
	    // start accept client request in order to client socket
        client_socket = accept(server_socket,(struct sockaddr *)&client_address,&client_address_length);
        // print client's ip and port
	    printf("client ip: %s\tport: %d\n",inet_ntop(AF_INET,&client_address.sin_addr.s_addr,client_ip_address,sizeof(client_ip_address)),ntohs(client_address.sin_port));
	    // read client message
	    // -1 to prevent over the border
	    message_length = read(client_socket,buffer,sizeof(buffer)-1);
        // set string end symbol
	    buffer[message_length] = '\0';
        printf("receive[%d]: %s\n",message_length,buffer);
	    // lower letter convert to  upper letter(a-z -> A-Z)
	    for(int i = 0; i < message_length; i ++){
	        buffer[i] = toupper(buffer[i]);
        }
	    // client's message write back to client
	    write(client_socket,buffer,message_length);
	    printf("finished len: %d\n",message_length);
	    // over client's connect
	    close(client_socket);
    }
    // over server's connect
    close(server_socket);
    return 0;
}

3. 客户端代码

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <sys/socket.h>
#include <netinet/in.h>

#define SERVER_PORT 666
#define SERVER_IP "10.0.0.1"

int main(int argc,char *argv[]){
	int client_socket;
	char message[256];
	struct sockaddr_in server_address;
	int n;
	char buffer[256];

	/*if(argc != 2){
		fputs("Error! number of arguments is incorrect!\n",stderr);
		exit(1);
	}
	*/

	memset(&server_address,'\0',sizeof(struct sockaddr_in));
	server_address.sin_family = AF_INET;
	inet_pton(AF_INET,SERVER_IP,&server_address.sin_addr);
	server_address.sin_port = htons(SERVER_PORT);

	// message = argv[1];

	int done = 1;
	while(done){
		memset(buffer,'\0',sizeof(buffer));
		//gets is very dangerous
        gets(message);
		printf("message:%s\n",message);

		client_socket = socket(AF_INET,SOCK_STREAM,0);

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

		write(client_socket,message,strlen(message));

		n = read(client_socket,buffer,sizeof(buffer) - 1);

		if(n > 0){
			buffer[n] = '\0';
			printf("receive: %s\n",buffer);
		} else{
			perror("error!");
		}
		printf("finished!\n");
		close(client_socket);
	}
}

4. 结果(在mininet上进行测试)

    h1为服务器，h2为客户端。二者均可以通信。

5. 参考资料

    https://www.bilibili.com/video/BV14Y411s7yB/?spm_id_from=333.1007.top_right_bar_window_custom_collection.content.click&vd_source=a642bb3ddc5b706845426dc41d73fbda