#include <stdio.h>#include <stdlib.h>#include <unistd.h>#include <sys/types.h>#include <sys/socket.h>#include <string.h>#include <errno.h>#include <arpa/inet.h>#include <sys/time.h>////////////////////////////////////////////////////////////////////////////////// Definitions////////////////////////////////////////////////////////////////////////////////#define BUFFER_SIZE	1024////////////////////////////////////////////////////////////////////////////////// Prototypes////////////////////////////////////////////////////////////////////////////////void network_initialize(void);inline void network_register_read(int fd);inline void network_register_write(int fd);inline void network_register_except(int fd);inline void network_unregister_read(int fd);inline void network_unregister_write(int fd);inline void network_unregister_except(int fd);void do_busy(void);int do_read(int fd);int do_write(int fd);int do_except(int fd);////////////////////////////////////////////////////////////////////////////////// Global variables////////////////////////////////////////////////////////////////////////////////fd_set global_read_set;fd_set global_write_set;fd_set global_except_set;int global_max_fd = 0;////////////////////////////////////////////////////////////////////////////////// Function implementition////////////////////////////////////////////////////////////////////////////////int main(int argc, char **argv){  int fd;  if (argc < 3)  {    fprintf(stdout, "Usage: %s <ip> <port>\n", argv[0]);    return 0;  }  network_initialize();  if ((fd = socket(PF_INET, SOCK_DGRAM, 0)) < 0)  {    fprintf(stderr, "Create new UDP socket failed: %s\n", strerror(errno));    exit(1);  }#ifdef _DEBUG_  fprintf(stdout, "Create a new UDP socket %d\n", fd);#endif  struct sockaddr_in server_address;  memset(&server_address, 0, sizeof(server_address));  server_address.sin_family = PF_INET;  server_address.sin_port = htons(atoi(argv[2]));  server_address.sin_addr.s_addr = inet_addr(argv[1]);  //int bind(int sockfd, struct sockaddr *my_addr, socklen_t addrlen);  if (bind(fd, (struct sockaddr *) &server_address, sizeof(server_address)) < 0)  {    fprintf(stderr, "Bind failed: %s\n", strerror(errno));    close(fd);    exit(1);  }#ifdef _DEBUG_  fprintf(stdout, "Bound on %s:%d\n", argv[1], atoi(argv[2]));#endif  network_register_read(fd);  fd_set rset, wset, eset;  struct timeval to;  for (;;)  {    rset = global_read_set;    wset = global_write_set;    eset = global_except_set;    to.tv_sec = 10;    to.tv_usec = 0;    int n;    // int select(int n, fd_set *readfds, fd_set *writefds, fd_set *exceptfds, struct timeval *timeout);    n = select(global_max_fd + 1, &rset, &wset, &eset, &to);#ifdef _DEBUG_    fprintf(stdout, "%d sockets are ready\n", n);#endif    if (n < 0)    {      fprintf(stderr, "select() failed: %s\n", strerror(errno));      switch (errno)      {	case EINTR:	  continue;	default:	  break;      }    }    else if (n == 0)    {      do_busy();    }    else    {      int i;      for (i = 0; i <= global_max_fd; i++)      {	if (FD_ISSET(i, &rset))	{#ifdef _DEBUG_	  fprintf(stdout, "Socket %d is ready for reading.\n", i);#endif	  do_read(i);	}	if (FD_ISSET(i, &wset))	{#ifdef _DEBUG_	  fprintf(stdout, "Socket %d is ready for writting.\n", i);#endif	  do_write(i);	}	if (FD_ISSET(i, &eset))	{#ifdef _DEBUG_	  fprintf(stdout, "Socket %d is ready for excepting.\n", i);#endif	  do_except(i);	}      }    }  }  close(fd);  return 0;}void network_initialize(void){  global_max_fd = 0;  FD_ZERO(&global_read_set);  FD_ZERO(&global_write_set);  FD_ZERO(&global_except_set);}inline void network_register_read(int fd){  FD_SET(fd, &global_read_set);  if (fd > global_max_fd)  {    global_max_fd = fd;  }}inline void network_unregister_read(int fd){  FD_CLR(fd, &global_read_set);}inline void network_register_write(int fd){  FD_SET(fd, &global_write_set);  if (fd > global_max_fd)  {    global_max_fd = fd;  }}inline void network_unregister_write(int fd){  FD_CLR(fd, &global_write_set);}inline void network_register_except(int fd){  FD_SET(fd, &global_except_set);  if (fd > global_max_fd)  {    global_max_fd = fd;  }}inline void network_unregister_except(int fd){  FD_CLR(fd, &global_except_set);}void do_busy(void){  fprintf(stdout, "Do something when network idle.\n");}int do_read(int fd){  char buf[BUFFER_SIZE];  ssize_t n;  memset(buf, 0, sizeof(buf));  struct sockaddr_in peer_address;  socklen_t peer_address_length;  peer_address_length = sizeof(peer_address);  //ssize_t recvfrom(int s, void *buf, size_t len, int flags, struct sockaddr *from, socklen_t *fromlen);  n = recvfrom(fd, buf, sizeof(buf), 0, (struct sockaddr *) &peer_address, &peer_address_length);#ifdef _DEBUG_  fprintf(stdout, "Read %d bytes from %s:%d\n", n, inet_ntoa(peer_address.sin_addr), ntohs(peer_address.sin_port));#endif  fprintf(stdout, "Message from peer: %s\n", buf);  return 0;}int do_write(int fd){#ifdef _DEBUG_  fprintf(stdout, "TODO: write some data to remote host.\n");#endif#if 0  //ssize_t sendto(int s, const void *buf, size_t len, int flags, const struct sockaddr *to, socklen_t tolen);  n = sendto(fd, buf, n, 0, (struct sockaddr *) &peer_address, peer_address_length);#  ifdef _DEBUG_  fprintf(stdout, "Write %d bytes to %s:%d\n", n, inet_ntoa(peer_address.sin_addr), ntohs(peer_address.sin_port));#  endif#endif  return 0;}int do_except(int fd){#ifdef _DEBUG_  fprintf(stdout, "TODO: Handle exception on %d.\n", fd);#endif  return 0;}