#include <stdio.h>#include <stdlib.h>#include <unistd.h>#include <sys/types.h>#include <sys/socket.h>#include <sys/select.h>#include <arpa/inet.h>#include <string.h>#include <errno.h>#define BUFFER_SIZE	1024intmain (int argc, char **argv){  int fd;  int rand = 0;  // XXX: step 1, socket();  int socket (int domain, int type, int protocol);  if ((fd = socket (PF_INET, SOCK_DGRAM, 0)) < 0)    {      fprintf (stderr, "Create a new UDP socket failed: %s\n",	       strerror (errno));      exit (1);    }  fd_set rset;  fd_set wset;  fd_set eset;  FD_ZERO (&rset);  FD_ZERO (&wset);  FD_ZERO (&eset);  FD_SET (fd, &eset);  FD_SET (fd, &rset);  FD_SET (fd, &wset);  char line[BUFFER_SIZE] = "hello";  struct sockaddr_in remote_address;  ssize_t sent;  memset (&remote_address, 0, sizeof (remote_address));  remote_address.sin_family = PF_INET;  remote_address.sin_port = htons (atoi (argv[2]));  remote_address.sin_addr.s_addr = inet_addr (argv[1]);  struct sockaddr_in peer_address;  socklen_t peer_address_length;  ssize_t recv;  peer_address_length = sizeof (peer_address);  struct timeval tv;  tv.tv_sec = 1;  tv.tv_usec = 0;  while (1)    {     rand = random () % 3;#if _DEBUG_      printf ("%d\n", rand);#endif      sleep (rand);    //sleep(1);      int sel;      if ((sel = select (fd + 1, &rset, &wset, &eset, &tv)) < 0)	{	  if (EINTR == errno)	    {	      continue;	    }	  else	    {	      fprintf (stderr, "select failed:%s\n", strerror (errno));	    }	}      else if (0 == sel)	{	  fprintf (stdout, "time out...\n");	}      else	{	  if (FD_ISSET (fd, &wset))	    {	    resend:	      if ((sent =		   sendto (fd, line, strlen (line), 0,			   (struct sockaddr *) &remote_address,			   sizeof (remote_address))) < 0)		{		  if (errno == EINTR)		    {		      goto resend;		    }		  else		    {		      fprintf (stderr, "sendto() failed: %s\n",			       strerror (errno));		    }		}	      else		{		  fprintf (stdout, "Sent %d bytes to remote host.\n", sent);		}	    }	}      FD_SET(fd,&rset);      if ((sel = select (fd + 1, &rset, &wset, &eset, &tv)) < 0)	{	  if (EINTR == errno)	    {	      continue;	    }	  else	    {	      fprintf (stderr, "select failed:%s\n", strerror (errno));	    }	}      else if (0 == sel)	{	  fprintf (stdout, "time out...\n");	}      else	{	  if (FD_ISSET (fd, &rset))	    {#if _DEBUG_	      printf ("enter read\n");#endif	    re_receive:	      if ((recv =		   recvfrom (fd, line, BUFFER_SIZE, 0,			     (struct sockaddr *) &peer_address,			     &peer_address_length)) < 0)		{		  if (errno == EINTR)		    {		      goto re_receive;		    }		  else		    {		      fprintf (stderr, "recvfrom() failed: %s\n",			       strerror (errno));		    }		}	      else		{		  fprintf (stdout, "INFO: read %d bytes from %s:%d\n", recv,			   inet_ntoa (peer_address.sin_addr),			   ntohs (peer_address.sin_port));		  fprintf (stdout, "DEBUG: %s\n", line);		}	    }	}    }  close (fd);  return 0;}