Thread: Issue receiving data from socket (multicast)

Hi I'm having some kind of problem here that I can't identify ...

I want to get UDP data that is sent to a multicast address so my guess is to create a socket and bind it to the multicast address and port where the data is sent to . I'm using openSUSE and coding in c++

I configure the socket

Code:

    struct sockaddr_in mysocket;
    struct ip_mreq group;
    int sd;
    int dblength;
    char dbuffer[1024];
    int reuse = 1;

try{
	sd = socket(AF_INET, SOCK_DGRAM, 0);
        setsockopt(sd, SOL_SOCKET, SO_REUSEADDR, (char *)&reuse, sizeof(reuse));
           
        memset((char *) &mysocket, 0, sizeof(mysocket));
        misocket.sin_family = AF_INET;
        misocket.sin_port = htons((unsigned int)multicastport.c_str());
        misocket.sin_addr.s_addr = htonl(INADDR_ANY); 
    
    bind(sd, (struct sockaddr*)&misocket, sizeof(misocket));
          
    group.imr_multiaddr.s_addr = inet_addr(multicastaddress.c_str());
    group.imr_interface.s_addr = inet_addr("192.168.0.8");
                setsockopt(sd, IPPROTO_IP, IP_ADD_MEMBERSHIP, (char *)&group, sizeof(group));
          }

And here I try to read from the socket :

Code:

int readLength = -1;
        printf("\nReading from socket");

        try{
        while((readLength=read(sd, dbuffer, dblength)) >=0){
            //some data processing here when it works
            cout << readLength <<endl;
            printf(dbuffer);
        }
        if(readLength < 0){
            printf("\nError buffer empty\n");
        }

        close (sd);
        printf("\nReading .... OK\n");}
        catch(unsigned int a){
            perror("Error reading");
            close(sd);
        }

I get this output

Code:

Reading from socket
Error buffer empty
Reading .... OK

It compiles and executes "ok" and I know for sure (wireshark captures and server's log) that the UDP packets are being sent so I don't know why I'm not receiving... socket not correctly configurated?

Any idea where am I failing?

JessH

JessH,

I concur that you are giving the port a value, but taking the value returned by 'c_str()' from multicastport (which I believe is a pointer to a c style string) and casting it to an unsigned integer won't likely be the same as using 'atoi' to get the integer value of the string. Thus, recvfrom blocks waiting for input from this arbitrary port number rather than the number contained in the c style string. So the program would seem to hang....

The member sin_port of mysocket here:

Code:

misocket.sin_port = htons((unsigned int)multicastport.c_str());

Is a short representing the port number. But the value which has been assigned to it is that of the pointer to the string returned by multicastport.c_str(). Is that not true?

Best Regards,

New Ink -- Henry

I LOVE YOU ! That was it, I owe you a big one !!! I finished my project !!!! It's working great ! wow I'm so happy !!! A huge thank you !!!!

Originally Posted by JessH

I LOVE YOU ! That was it, I owe you a big one !!! I finished my project !!!! It's working great ! wow I'm so happy !!! A huge thank you !!!!

Heh, I wish my professors would give projects like this.

Originally Posted by new_ink2001

JessH,

I checked with W. Richard Stevens UNIX Network Programming: Networking APIs: Sockets and XTI Volume 1 2nd Ed. New Jersey: Prentice Hall 1998. ISBN 0-13-490012-X. The socket options you've set are what concern me. For example, SO_REUSEADDR was mentioned back in the section about TCP sockets about page 194. The Echo Server example on page 214 (the UDP specific material) looks like:

Code:

int main(int argc, char **argv)
{
   int sockfd;
   struct sockaddr_in servaddr, cliaddr;

   sockfd = socket( AF_INET, SOCK_DGRAM, 0 );

   bzero( &servaddr, sizeof( servaddr ) );

   servaddr.sin_family = AF_INET;
   servaddr.sin_addr.s_addr = htonl( INADDR_ANY );
   servaddr.sin_port = htons( SERV_PORT );

   bind( sockfd, (SA*) &servaddr, sizeof( servaddr ) );

   // uses the socket to do the echo service...

   return 0;
}

I just copied it from the book, and he used some fancy wrapper functions for handling errno and such. But there really wasn't that much to a UDP socket. I didn't notice you setting the port in your code block, perhaps there's something there. The constant in Stevens' code SERV_PORT was defined in his library as some well known port number (a short integer in fact). You are welcome to tinker with his example.

Also, we didn't usually use 'read' either, and Stevens actually used recvfrom and sendto in his implementations...maybe something there too. I checked with Paul S. Wang An Introduction to ANSI on UNIX Boston: PWS Publishing 1993. ISBN 0-534-14232-X. Read doesn't pad with the string terminator, it reads up to the number passed as the length parameter. I don't think that's your problem unless the length parameter didn't get set properly (it wasn't in your post).

Best Regards,

New Ink -- Henry

I have this book and I for some reason, I can't stand those wrapper functions sometimes, I found myself flipping through pages to verify something. I have the first and second editions.

Syscal,

When I was in networking class, it took a minute to get the API down too, so it was a little bit of a pain. It turned into a lesson on the compiler interface when the pthread libraries showed up too. I haven't been able to invest in the second volume of Stevens book yet, I imagine it is a good one.... May I ask to what curriculum it belongs (e.g. MSCS, BSCS,...)?

But the level of cohesion using those wrappers is much higher (from what I recall in software engineering it's cohesion and the coupling in his library is nearly identical). That is to say, one doesn't have to re-write all of the error handling and debugging information every time one uses the system call function. It's analogous to marrying the error-handling to the system call in one package.

I've found that a second pair of eyes and some thoughtful reading can be priceless sometimes. I'm starting to think that's the purpose of this board.... You (all) are welcome...it keeps me sharp keeping up with such a great crowd.

Best Regards,

New Ink -- Henry