服务器代码：
#include
#include
#include
#include
#include
#include
#include
#include
#include
#include
#include
#include
#include
#define PORT 3490
#define MAXLINE 100
#define   MAX(a,b)   (((a)>(b))?(a):(b))  

void sig_chld(int signo)
{
pid_t pid;
int stat;
while ( (pid= waitpid(-1,&stat,WNOHANG)) > 0)
#ifdef DEBUG
printf("Child %d terminated\n",pid);
#endif
return;
}

void str_echo(int sockfd)
{
ssize_t n;
char line[MAXLINE];
fd_set rfds;
int retval;
struct timeval tv;
for (;;) {
memset(line,0,sizeof(line));
FD_ZERO(&rfds);
FD_SET(sockfd, &rfds);
tv.tv_sec = 60;
tv.tv_usec = 0;
retval = select(sockfd+1, &rfds, NULL, NULL, &tv);
  if (retval) {
  #ifdef DEBUG
  printf("Data is available now.\n");
  #endif
  } else {
  #ifdef DEBUG
  printf("No data within five seconds.\n");
  #endif
  break;
  }
}
}
int main(int argc, char **argv)
{
 int listenfd, connfd, udpfd, nready, maxfdp1;
  char mesg[MAXLINE];
  pid_t childpid;
  fd_set rset;
  ssize_t n;
  socklen_t len;
  const int on = 1;
  int i=0;
  struct sockaddr_in cliaddr, servaddr;
  void sig_chld(int);
 
  udpfd = 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(PORT);
 
 
 bind(udpfd, (struct sockaddr *) &servaddr, sizeof(servaddr));
 
 
  printf("The server is waiting datas:\n");
 
 signal(SIGCHLD, sig_chld);
 
 maxfdp1 = MAX(listenfd, udpfd) + 1;
 while(1) {
    
  
    FD_ZERO(&rset);
    listenfd =0;
    FD_SET(listenfd, &rset);
    FD_SET(udpfd, &rset);
     
    
    
     nready = select(maxfdp1, &rset, NULL, NULL, NULL);
    
   if (nready < 0) {
      
   if (errno == EINTR) continue;
   else perror("select error");
      }
   if (FD_ISSET(listenfd, &rset)) {
      len = sizeof(cliaddr);
      connfd = accept(listenfd, (struct sockaddr *) &cliaddr, &len);
      
        if ( (childpid = fork()) == 0) {
        close(listenfd);
        str_echo(connfd);
       
          exit(0);
         }
     
       close(connfd);
       }
  
     if (FD_ISSET(udpfd, &rset)) {
      len = sizeof(cliaddr);
      n = recvfrom(udpfd, mesg, MAXLINE, 0, (struct sockaddr *) &cliaddr, &len);
     
      i++;
       printf("Client %d contains:%s\n",i,mesg);
     }
  }
}
客户端代码
#include
#include
#include
#include
#include
#include
#include
#include
#define PORT 3490
#define MAXDATASIZE 100
int main(int argc, char * argv[])
{
 int sockfd,numbytes,sin_size;
 char buf[MAXDATASIZE] = "hello world!";
 struct hostent *he;
 struct sockaddr_in their_addr;
 if(argc != 2)
 {
  fprintf(stderr,"usage:client hostname\n");
  exit(1);
  }
 if((he = gethostbyname(argv[1])) == NULL)
 {
  herror("gethostbyname");
  exit(1);
  }
 if((sockfd = socket(AF_INET,SOCK_DGRAM,0)) == -1)
 {
  perror("socket");
  exit(1);
  }
  their_addr.sin_family = AF_INET;
  their_addr.sin_port = htons(PORT);
  their_addr.sin_addr = *((struct in_addr *)he -> h_addr);
  bzero(&(their_addr.sin_zero),8);
  
  numbytes = sendto(sockfd,buf,MAXDATASIZE,0,(struct sockaddr *)&their_addr,sizeof(their_addr));
  
  if(numbytes == -1)
  {
   perror("sendto");
   exit(1);
   }
   
  printf("send:%s\n",buf);
   
  close(sockfd);
  return 0;
    
 }