#include <sys/types.h>
#include <sys/socket.h>
#include <arpa/inet.h>
#include <stdio.h>
#define BUFLEN 255  int main(int argc, char **argv)
{  struct sockaddr_in peeraddr, localaddr;  int sockfd;  char recmsg[BUFLEN+1];  int socklen, n;  if(argc!=5){  printf("%s <dest IP address> <dest port> <source IP address> <source port>\n", argv[0]);  exit(0);  }  sockfd = socket(AF_INET, SOCK_DGRAM, 0);  if(sockfd<0){  printf("socket creating err in udptalk\n");  exit(1);  }  printf("IP address Checking!\n");  socklen = sizeof(struct sockaddr_in);  memset(&peeraddr, 0, socklen);  peeraddr.sin_family=AF_INET;  peeraddr.sin_port=htons(atoi(argv[2]));  if(inet_pton(AF_INET, argv[1], &peeraddr.sin_addr)<=0){  printf("Wrong dest IP address!\n");  exit(0);  }  memset(&localaddr, 0, socklen);  localaddr.sin_family=AF_INET;  if(inet_pton(AF_INET, argv[3], &localaddr.sin_addr)<=0){  printf("Wrong source IP address!\n");  exit(0);  }  printf("try to bind local address \n");  localaddr.sin_port=htons(atoi(argv[4]));  if(bind(sockfd, &localaddr, socklen)<0){   printf("bind local address err in udptalk!\n");  exit(2);  }  printf("Please Type Some Message To Server\n");    if(fgets(recmsg, BUFLEN, stdin) == NULL) {printf("get nothing exit\n"); exit(0);}  //    printf("end of fgets \n");  if(sendto(sockfd, "HELLO", strlen("HELLO"), 0, &peeraddr, socklen)<0){  printf("sendto err in udptalk!\n");  exit(3);  }  printf("end of sendto \n");  printf("start of recv&send message loop!\n");  for(;;){  /*recv&send message loop*/  printf("Waiting For Message...!\n");  n = recvfrom(sockfd, recmsg, BUFLEN, 0, &peeraddr, &socklen);  if(n<0){  printf("recvfrom err in udptalk!\n");  exit(4);  }else{  recmsg[n]=0;  printf("peer:%s\n", recmsg);    }  printf("Please Type Some Message To Server\n");    if(fgets(recmsg, BUFLEN, stdin) == NULL) exit(0);  if(sendto(sockfd, recmsg, strlen(recmsg), 0, &peeraddr, socklen)<0){  printf("sendto err in udptalk!\n");  exit(3);  }  }
}