UDP client connecting process correct, failure to transmit in Unix environment

Question

I am trying to create my first UDP client server program in C language slightly modified from the man page of linux gai_strerror(3) - Linux man page. Although that I am using several error functions to check the correct process of the code when I reach the transmission part it fails. Based on what I see the connection is correct there is no error so in theory the message should be transmitted, but in practice it fails. I even open wireshark on local mode to see if there are any data transmitted, but nothing is transmitted. Since this is my first client Server program, and I am new into programing I can not think any other way to debug my code. What am I missing?

Addition:

On the terminal two things are printed:

connected: Success

and the error that is printing is Error sending data!

Any advice would be much appreciated.

#include <sys/types.h>
#include<errno.h>
#include <sys/socket.h>
#include <netdb.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <string.h>
#include <inttypes.h>
#include <sys/time.h> /* The header includes gettimeofday() */  
#include <time.h>  /* The header includes time types which we will use for day time */
#include <math.h>  /* The header includes mathematical declarations */

int main(int argc, char *argv[]) {

 struct timeval start; /* struct timeval for tv_sec and tv_usec */
 struct addrinfo bind_info;
 struct addrinfo *servInfo, *rp; /* addrinfo is filled with destination host information */
 int socket_fd, retrieve_info; /* socket_fd represents the file descriptor of socket, rtt round time trip */
 ssize_t send;

 uint32_t tran = 0;
 char *ip_serv = argv[1]; /* Set argv[1] as the server IP */
 char *port_number = argv[2];

 memset(&bind_info, 0, sizeof(struct addrinfo));
 bind_info.ai_family = AF_INET;
 bind_info.ai_socktype = SOCK_DGRAM;
 bind_info.ai_protocol = IPPROTO_UDP;
 bind_info.ai_flags = AI_PASSIVE;

 if ((retrieve_info = getaddrinfo(ip_serv, port_number,&bind_info, &servInfo)) !=0 ) {
   fprintf(stderr, "getaddrinfo: %s\n", gai_strerror(retrieve_info));
   exit(EXIT_FAILURE);
 }

 for (rp = servInfo; rp != NULL; rp = rp->ai_next) {

   if ((socket_fd = socket(rp->ai_family, rp->ai_socktype, rp->ai_protocol)) == -1) {
     perror("socket");
     continue;
   }

   if (connect(socket_fd, rp->ai_addr, rp->ai_addrlen) != -1) {
     perror("connected");
     break; /* We made it enough stop */
   }
   close(socket_fd); /* Release socket after binding */
  }

  if (rp == NULL) {
    fprintf(stderr, "failed to connect\n");
    exit(2);
  }

  freeaddrinfo(servInfo);

  gettimeofday(&start, NULL); /* timezone should be NULL */

  long int start_time = (start.tv_sec * 1000000 + start.tv_usec);

  tran = htonl(start_time);

  send = sendto(socket_fd,&tran,sizeof(tran),0,(struct sockaddr *) rp->ai_addr, rp->ai_addrlen);

  if (send == 0) {
    printf("Transmission to server was correct!\n");
  }
  else {
    fprintf(stderr, "Error sending data!\n");
  }
  exit(0);
 } /* end of int main (argv[]) */

Answer 1

You should read the manual page for sendto, as you seem to not understand its return value.

It will return the number of bytes it sent, or -1 on error. If it fails the call will set errno just like any other system call, and you can use perror like you do for the other functions when they fail.

So in your case you will say that the call was successful when the call have sent zero bytes, and say that the call failed in all other cases (which includes actual successful sending).