c - C 中的 UDP 套接字编程，调用 recvfrom() 后 printf 不工作？

Question

我正在从事一个有关 c 和 java 套接字编程的项目，使用 UDP 和 TCP(当然是分开的)。我的 UDP 服务器是 Java，我的 UDP 客户端是 C。我遇到了一个问题，在我调用 recvfrom() 之后，printf 和 fprintf 等方法实际上并没有打印所有内容。我也试过fflush。这是那部分代码:

int main(int argc, char **argv) {
   int sockfd, portno, n;
   int serverlen;
   struct sockaddr_in serveraddr;
   struct hostent *server;
   char *hostname;
   char buf[BUFSIZE];
   int GID = 9;

/* check command line arguments and extract port numbers */
   if (argc != 4) {
     fprintf(stderr,"usage: %s <hostname> <port> <myport>\n", argv[0]);
     exit(0);
   }
   hostname = argv[1];
   int serverPort = atoi(argv[2]);                                                 
   int myPort = atoi(argv[3]);
   int portRange = (5 * GID) + 10010;
   if (myPort < portRange || myPort > portRange + 4) {
     fprintf(stderr, "Invalid Port range\n");
     exit(1);
   }

/* socket: create the socket */
   sockfd = socket(AF_INET, SOCK_DGRAM, 0);
   if (sockfd < 0) 
     error("ERROR opening socket\n");

 /* gethostbyname: get the server's DNS entry */
   server = gethostbyname(hostname);
   if (server == NULL) {
     fprintf(stderr,"ERROR, no such host as %s\n", hostname);
     exit(0);
   }

/* build the server's Internet address */
   bzero((char *) &serveraddr, sizeof(serveraddr));
   serveraddr.sin_family = AF_INET;
   bcopy((char *)server->h_addr, 
     (char *)&serveraddr.sin_addr.s_addr, server->h_length);
   serveraddr.sin_port = htons(serverPort);

/* create a message */
   bzero(buf, BUFSIZE);
   buf[0] = 74;
   buf[1] = 111;
   buf[2] = 121;
   buf[3] = 33;
   buf[4] = (myPort >> 8);
   buf[5] = (myPort & 0xFF);
   buf[6] = GID;

/* send the message to the server */
   serverlen = sizeof(serveraddr);
   n = sendto(sockfd, buf, strlen(buf), 0, (struct sockaddr *)&serveraddr, 
     serverlen);
   if (n < 0) 
     error("ERROR in sendto\n");

   bzero(buf, BUFSIZE);

/* print the server's reply if buf[6] != 0*/
   n = recvfrom(sockfd, buf, strlen(buf), 0, (struct sockaddr *)&serveraddr, 
      &serverlen);
  //ERROR OCCURS AFTER RECEIVING

   if (n < 0) 
     error("ERROR in recvfrom\n");
   if (buf[6] == 0 && buf[7] == 1) {
     printf("ERROR: no magic number\n");
     return 0;
   }
   else if (buf[6] == 0 && buf[7] == 2) {
     printf("ERROR: incorrect length\n");
     return 0;
   }
   else if (buf[6] == 0 && buf[7] == 4) {
     printf("ERROR: port number out of range\n");
   }
   printf("%.*s\n", strlen(buf), buf);

return 0;

Answer 1

常见问题。您忽略了 recvfrom() 返回的计数:

printf("%.*s\n", strlen(buf), buf);

应该是

printf("%.*s\n", n, buf);

注意你的recvfrom()调用也不正确。应该是:

n = recvfrom(sockfd, buf, sizeof buf, 0, (struct sockaddr *)&serveraddr, 
  &serverlen);

如果n < 0您不应输入以下 if-else 链。