received UDP packet length

sasubillis · 02-12-2010, 05:00 AM

How to get the length of received UDP packet? Using wireshark I could see the correct length of datagram. How can I print this value in my simple udp server program? I am receiving binary data (Non printable ascii characters as a data) So I can not use strlen(buf) which throws incorrect length.

Code:

          if (ret=recvfrom(s, buf, BUFLEN, 0, (struct sockaddr *)&si_other, &slen)!=-1){
                error = ioctl(s, FIONREAD, &value);
                printf(" from ioctl UDP packet length is : %d error is : %d\n", value, error);
          }

udp packet length is always '0' from the above code. Any comments?

I also tried like below

Code:

     if (ret=recvfrom(s, buf, BUFLEN, 0, (struct sockaddr *)&si_other, &slen)!=-1){
                unsigned short iphdrlen;
                struct iphdr* iph = (struct iphdr*)buf;
                iphdrlen =iph->ihl*4;

                printf("IP version :%d\n", ((unsigned int)((iph->version))));
                printf("protocol .. %d\n", ((unsigned int)(iph->protocol)));
                printf("total len .. %d\n", (ntohs(iph->tot_len)));

}

The above code always returns me wrong values from ip header? Any comments?

David1357 · 02-12-2010, 07:57 AM

Quote:

Originally Posted by sasubillis

How to get the length of received UDP packet?

From the man page for "recvfrom":

Code:

RETURN VALUE
       These calls return the number of bytes received, or -1 if an
       error occurred. The return value will be 0 when the peer has
       performed an orderly shutdown.

In other words, the value in "ret" will be "the length of received UDP packet".