#include <sys/types.h>
#include <sys/socket.h>
#include <string.h> 
#include <netinet/in.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h> //use close(),read(),write()...
#define MAXLINE 1023
#define SERV_PORT 1234
char* shift(const char* str)

{
	char* tmp = (char*)malloc((strlen(str)+1)*sizeof(char));
	strcpy(tmp,str);
	char* ret = tmp;
	char* p = tmp+strlen(str)-1;
 while (p > tmp)
	{
 	*p=*p+*tmp;
 	*tmp=*p-*tmp;
 	*p=*p-*tmp;
	--p;
 	++tmp;
	}
 return ret;

}
void do_echo(int sockfd, struct sockaddr *pcliaddr, socklen_t clilen)
{
int n,buflen;
struct sockaddr_in cliaddr;
socklen_t len;
char mesg[MAXLINE];
for(;;)
{
//init cliaddr
buflen  = sizeof(mesg);
       bzero(mesg,buflen);
      bzero(&cliaddr,sizeof(cliaddr));
len = clilen;  // waiting for receive data 
n = recvfrom(sockfd, mesg, MAXLINE, 0, pcliaddr, &len);  //shift the message
strcpy(mesg,shift(mesg));  // sent data back to client 
sendto(sockfd, mesg, n, 0, pcliaddr, len);
}
}

int main(void)
{
int sockfd;
struct sockaddr_in servaddr, cliaddr;
 // create a socket with udp protocol 
sockfd = socket(AF_INET, SOCK_DGRAM, 0); // init servaddr 
bzero(&servaddr, sizeof(servaddr));
servaddr.sin_family = AF_INET;
servaddr.sin_addr.s_addr = htonl(INADDR_ANY);
servaddr.sin_port = htons(SERV_PORT);// bind address and port to socket 
if(bind(sockfd, (struct sockaddr *)&servaddr, sizeof(servaddr)) == -1)
{
perror("bind error");
exit(1);
}
do_echo(sockfd, (struct sockaddr *)&cliaddr, sizeof(cliaddr));
return 0;
close(sockfd);
}