#include <stdio.h>#include <stdlib.h>#include <unistd.h>#include <sys/types.h>#include <sys/socket.h>#include <arpa/inet.h>#include <string.h>#include <errno.h>#define BUFFER_SIZE	1024int main(int argc, char **argv){  int fd;  // 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);  }  char line[BUFFER_SIZE];  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]);  // XXX: step 2, bind(); optional  // XXX: step 3, connect(), optional  // int connect(int sockfd, const struct sockaddr *serv_addr, socklen_t addrlen);  if (connect(fd, (struct sockaddr *) &remote_address, sizeof(remote_address)) < 0)  {    fprintf(stderr, "connect() failed: %s\n", strerror(errno));    exit(1);  }  // char *fgets(char *s, int size, FILE *stream);  while (fgets(line, BUFFER_SIZE, stdin))  {    // XXX: step 4, sendto()/recvfrom  resend:    // ssize_t  sendto(int  s,  const  void  *buf,  size_t len, int flags, const struct sockaddr *to, socklen_t tolen);    //if ((sent = sendto(fd, line, strlen(line), 0, (struct sockaddr *) &remote_address, sizeof(remote_address))) < 0)    //ssize_t send(int s, const void *buf, size_t len, int flags);    //if ((sent = send(fd, line, strlen(line), 0)) < 0)    if ((sent = sendto(fd, line, strlen(line), 0, NULL, 0)) < 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);    }    struct sockaddr_in peer_address;    socklen_t peer_address_length;    ssize_t n;    peer_address_length = sizeof(peer_address);  re_receive:    //ssize_t recvfrom(int s, void *buf, size_t len, int flags, struct sockaddr *from, socklen_t *fromlen);    if ((n = 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", n, inet_ntoa(peer_address.sin_addr), ntohs(peer_address.sin_port));#if 0      int i;      for (i = 0; i < n; i++)      {	fprintf(stdout, "0x%x", buffer[i]);      }#endif      line[n] = '\0';      fprintf(stdout, "DEBUG: %s\n", line);    }  }  // XXX: step 5, close()  return 0;}