#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	1024char *banner = "Hello world!";int main(int argc, char **argv){  int fd;  if (argc < 3)  {    fprintf(stdout, "Usage: %s <ip> <port>\n", argv[0]);    return 0;  }  if ((fd = socket(PF_INET, SOCK_DGRAM, 0)) < 0)  {    fprintf(stderr, "Create UDP socket failed: %s\n", strerror(errno));    exit(1);  }#ifdef _DEBUG_  fprintf(stdout, "Created a new UDP socket %d\n", fd);#endif  char buf[BUFFER_SIZE];  memset(buf, 0, sizeof(buf));  struct sockaddr_in remote_address;  struct sockaddr_in peer_address;  socklen_t peer_address_length;  int n;  memset(&remote_address, 0, sizeof(remote_address));  remote_address.sin_family = PF_INET;  remote_address.sin_addr.s_addr = inet_addr(argv[1]);  remote_address.sin_port = htons(atoi(argv[2]));  //memcpy(buf, banner, strlen(banner));  //char *fgets(char *s, int size, FILE *stream);  fgets(buf, sizeof(buf), stdin);  //ssize_t sendto(int s, const void *buf, size_t len, int flags, const struct sockaddr *to, socklen_t tolen);  n = sendto(fd, buf, strlen(buf), 0, (struct sockaddr *) &remote_address, sizeof(remote_address));#ifdef _DEBUG_  fprintf(stdout, "Sent %d bytes to remote host %s:%d\n", n, argv[1], atoi(argv[2]));#endif  memset(buf, 0, sizeof(buf));  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, "Received %d from remote host %s:%d\n", n, inet_ntoa(peer_address.sin_addr), ntohs(peer_address.sin_port));#endif  fprintf(stdout, "Message from remote: %s\n", buf);  close(fd);  return 0;}