3. 基于UDP协议的网络程序

下图是典型的UDP客户端/服务器通讯过程(该图出自[UNPv13e])。

图 37.3. UDP通讯流程

UDP通讯流程

以下是简单的UDP服务器和客户端程序。

/* server.c */

#include <stdio.h>

#include <string.h>

#include <netinet/in.h>

#include "wrap.h"



#define MAXLINE 80

#define SERV_PORT 8000



int main(void)

{

	struct sockaddr_in servaddr, cliaddr;

	socklen_t cliaddr_len;

	int sockfd;

	char buf[MAXLINE];

	char str[INET_ADDRSTRLEN];

	int i, n;



	sockfd = Socket(AF_INET, SOCK_DGRAM, 0);



	bzero(&servaddr, sizeof(servaddr));

	servaddr.sin_family = AF_INET;

	servaddr.sin_addr.s_addr = htonl(INADDR_ANY);

	servaddr.sin_port = htons(SERV_PORT);

    

	Bind(sockfd, (struct sockaddr *)&servaddr, sizeof(servaddr));



	printf("Accepting connections ...\n");

	while (1) {

		cliaddr_len = sizeof(cliaddr);

		n = recvfrom(sockfd, buf, MAXLINE, 0, (struct sockaddr *)&cliaddr, &cliaddr_len);

		if (n == -1)

			perr_exit("recvfrom error");

		printf("received from %s at PORT %d\n",

		       inet_ntop(AF_INET, &cliaddr.sin_addr, str, sizeof(str)),

		       ntohs(cliaddr.sin_port));

    

		for (i = 0; i < n; i++)

			buf[i] = toupper(buf[i]);

		n = sendto(sockfd, buf, n, 0, (struct sockaddr *)&cliaddr, sizeof(cliaddr));

		if (n == -1)

			perr_exit("sendto error");

	}

}
/* client.c */

#include <stdio.h>

#include <string.h>

#include <unistd.h>

#include <netinet/in.h>

#include "wrap.h"



#define MAXLINE 80

#define SERV_PORT 8000



int main(int argc, char *argv[])

{

	struct sockaddr_in servaddr;

	int sockfd, n;

	char buf[MAXLINE];

	char str[INET_ADDRSTRLEN];

	socklen_t servaddr_len;

    

	sockfd = Socket(AF_INET, SOCK_DGRAM, 0);



	bzero(&servaddr, sizeof(servaddr));

	servaddr.sin_family = AF_INET;

	inet_pton(AF_INET, "127.0.0.1", &servaddr.sin_addr);

	servaddr.sin_port = htons(SERV_PORT);

    

	while (fgets(buf, MAXLINE, stdin) != NULL) {

		n = sendto(sockfd, buf, strlen(buf), 0, (struct sockaddr *)&servaddr, sizeof(servaddr));

		if (n == -1)

			perr_exit("sendto error");



		n = recvfrom(sockfd, buf, MAXLINE, 0, NULL, 0);

		if (n == -1)

			perr_exit("recvfrom error");

	  

		Write(STDOUT_FILENO, buf, n);

	}



	Close(sockfd);

	return 0;

}

由于UDP不需要维护连接,程序逻辑简单了很多,但是UDP协议是不可靠的,实际上有很多保证通讯可靠性的机制需要在应用层实现。

编译运行server,在两个终端里各开一个client与server交互,看看server是否具有并发服务的能力。用Ctrl+C关闭server,然后再运行server,看此时client还能否和server联系上。和前面TCP程序的运行结果相比较,体会无连接的含义。


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