先写服务端：

创建连接
接收客户端的数据
等待客户端发包，若客户端没有建立连接，则阻塞
接收数据

再写客户端

建立连接
传输数据
关闭

代码

udpserver

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

//#define PORT 8900
void print_usage(char * cmd)
{
        fprintf(stderr," %s usage:\n",cmd);
        fprintf(stderr,"%s IP_Addr [port]\n",cmd);
}

int main(int argc,char** argv)
{
	struct sockaddr_in server;
	struct sockaddr_in client;
	int len;
	int port;
	int sockfd;
	int sendnum;
	int opt;
	int recvnum;
	char send_buf[2048];
	char recv_buf[2048];
        int addr_len = sizeof(struct sockaddr_in);  //add

        if ((1>argc)||(argc>2))
        {
                print_usage(argv[0]);
                exit(1);
        }
      
  	if (2==argc)
 	{
		port = atoi(argv[1]);
	}

	//port= PORT;
	memset(send_buf,0,2048);
	memset(recv_buf,0,2048);
	
      opt = SO_REUSEADDR;

      if (-1==(sockfd=socket(AF_INET,SOCK_DGRAM,0)))  //change
      {
	 perror("Create socket error!\n");
	 exit(1);
      }
     
      /*
      setsockopt(listend,SOL_SOCKET,SO_REUSEADDR,&opt,sizeof(opt));

      #ifdef DEBUG
	printf("the listen id is %d\n",listend);
      #endif
      */

     memset(&server,0,sizeof(struct sockaddr_in));
     server.sin_family = AF_INET;
     server.sin_addr.s_addr = htonl(INADDR_ANY);
     server.sin_port = htons(port);

     if (-1==bind(sockfd,(struct sockaddr *)&server,sizeof(struct sockaddr)))
     {
	perror("bind error\n");
	exit(1);
     }

     /*
     int addr_len = sizeof(struct sockaddr_in);
     recvnum = recvfrom(listend, recv_buf, sizeof(recv_buf), 0, (struct sockaddr *)&client, &addr_len);
     recv_buf[recvnum]="\0";
     printf("Client %s : %s\n", inet_ntoa(client.sin_addr), recv_buf);

    
    if (-1==listen(listend,5))
    {
	perror("listen error\n");
	exit(1);
    }

    while (1)
    {
    

        if (-1==(connectd=accept(listend,(struct sockaddr*)&client,&len)))

    	{
		perror("create connect socket error\n");
		continue;
    	}

   	#ifdef DEBUG
		printf("the connect id is %d",connect);
		printf("the client ip addr is %s",inet_ntoa(client.sin_addr));
   	#endif
	
	sendnum = sprintf(send_buf,"hello,the guest from %s\n",inet_ntoa(client.sin_addr));
       if ( 0 >send(connectd,send_buf,sendnum,0))
	{
		perror("send error\n");
		close(connectd);
		continue;
	}
	
   	#ifdef DEBUG
		printf("the send num is %d",sendnum);
		printf("the client ip addr is %s",inet_ntoa(client.sin_addr));
   	#endif
        */

	while(1) //Many messages
	{
        	if (0>(recvnum = recvfrom(sockfd,recv_buf,sizeof(recv_buf),0,(struct sockaddr *)&client,&addr_len)))
		{
			perror("recv error\n");
			//close(connectd);
			continue;
		}
		recv_buf[recvnum]='\0';

		printf ("The message from the client is: %s\n",recv_buf);

		if (0==strcmp(recv_buf,"quit"))
        	{
			perror("The client break the server process.\n");
			//close(connectd);
			break;
		}

		sendnum = sprintf(send_buf,"The message from client is %s\n",recv_buf);
        	//send(connectd,send_buf,sendnum,0);
 		sendto(sockfd, send_buf, sizeof(send_buf), 0, (struct sockaddr *)&client, sizeof(client));

		//close(connectd);
		continue;

   	}

    	close(sockfd);
    	return 0;
}

udpclient

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

#define PORT 8900

void print_usage(char * cmd)
{
	fprintf(stderr," %s usage:\n",cmd);
	fprintf(stderr,"%s IP_Addr [port]\n",cmd);
}


int main(int argc,char** argv)
{
	struct sockaddr_in server;
	int ret;
	int len;
	int port;
	int sockfd;
	int sendnum;
	int recvnum;
	char send_buf[2048];
	char recv_buf[2048];
	int addr_len = sizeof(struct sockaddr_in);

	if ((2>argc)|| (argc >3))
	{
		print_usage(argv[0]);
		exit(1);
	}

       if (3==argc) 
       {
		port = atoi(argv[2]);
       }

    	if (-1==(sockfd=socket(AF_INET,SOCK_DGRAM,0))) //change
	{
		perror("can not create socket\n");
		exit(1);
	}

	memset(&server,0,sizeof(struct sockaddr_in));
	server.sin_family = AF_INET;
	server.sin_addr.s_addr = inet_addr(argv[1]);
	server.sin_port = htons(port);

	if (0>(ret=connect(sockfd,(struct sockaddr*)&server,sizeof(struct sockaddr))))
	{
		perror("connect error");
		close(sockfd);
		exit(1);
	}

	//memset(send_buf,0,2048);
	//memset(recv_buf,0,2048);

   	while(1)
	{
		printf("What words do you want to tell to server:\n");
		//gets(send_buf);
 		fgets(send_buf, 2048, stdin);
        	//#ifdef DEBUG
		//printf("%s\n",send_buf);
  		//#endif 
		//sprintf(send_buf,"i am lg,thank for your servering\n");

		if (0>(len=sendto(sockfd,send_buf,sizeof(send_buf),0, (struct sockaddr *)&server, sizeof(server))))
		{
			perror("send data error\n");
			close(sockfd);
			exit(1);
		}

		if (0>(len=recvfrom(sockfd,recv_buf,sizeof(recv_buf),0, (struct sockaddr *)&server, &addr_len)))
		{
			perror("recv data error\n");
			close(sockfd);
			exit(1);
		}
	
		recv_buf[len]='\0';
		printf("The message from the server is:%s\n",recv_buf);
	}
	close(sockfd);
}