未定义的端口号作为输出-UDP / C / Linux

Question

I have defined the port number in both client as well as server program. I start the simple udp server program which receives packet from client. The server gets the packet But when i print the client information, Port number is know as Random number (51958) How to get the correct port number . ie the number i have defined.

  #define PORT XYZ
       ...
     if((s = socket(AF_INET, SOCK_DGRAM, IPPROTO_UDP)) == -1)
        diep("socket");

      memset((char *) &si_me, 0, sizeof(si_me));

      si_me.sin_family = AF_INET;
      si_me.sin_port = htons(PORT);
      si_me.sin_addr.s_addr = htonl(INADDR_ANY);

      if(bind(s, (struct sockaddr *) &si_me, sizeof(si_me)) == -1)
        diep("bind");

      for(i = 0; i < NPACK; i += 1) {
        if(recvfrom(s, buf, BUFLEN, 0, (struct sockaddr *) &si_other, &slen) == -1)
          diep("recvfrom()");

        printf("Recieved packet from %s: %d\nData: %s\n\n", inet_ntoa(si_other.sin_addr), ntohs(si_other.sin_port), buf);
      }
      close(s);

/// client

   #define PORT XYZ

   if((s = socket(AF_INET, SOCK_DGRAM, IPPROTO_UDP)) == -1)
                diep("socket");

        memset((char *) &si_other, 0, sizeof(si_other));

        si_other.sin_family = AF_INET;
        si_other.sin_port = htons(PORT);
        if(inet_aton(SRV_IP, &si_other.sin_addr) == 0) {
                fprintf(stderr, "inet_aton() failed\n");
                exit(1);
        }

        {
                for(i = 0; i < NPACK; i += 1) {
                        printf("Sending packet %d\n", index);
                        sprintf(buf, "This is packet%d\n", index);
                     ;
                        if(sendto(s, buf, BUFLEN, 0, (struct sockaddr *) &si_other, slen) == -1)
                                diep("sendto()");
                        index++;
                }

        }
        close(s);

Update If we have data sent on N number of sockets and On the server side we are in a while(1) loop receiving the data , How do we identify the port the client has sent ?

Answer 1

I think it actually is the correct port number, since you're printing the source port of the client (the client host uses a random free one if not else specified) and not the destination port which is the one the server is listening too

If you have multiple sockets you can get the port which is bound to with getsockname

if (getsockname(sock, (struct sockaddr *)&sin, &len) == -1)
    perror("getsockname");
else
    printf("port number of the listening socket %d\n", ntohs(sin.sin_port));

Answer 2

您尚未将客户端套接字绑定到特定端口，因此它获得了系统分配的端口。