[Udpcast] UDP Multicast Programming question

[Alain Knaff]

tovis wrote:
> I'm C a programmer, looking for Linux UDP multicast programming
> documentation.
> Early I'm write some UDP multicast program for WIN32, using API, it is
> working on NT4.0 (loop back does not work, you can not switch off own
> packets), W2K and WINXP - where I/O completion port is exist.
> In WIn32 API you can simply setup an UDP socket, bind on apropriate
> interface address and after using IOCTL set ADD_MEMBERSHIP/DROP_MEMBERSHIP
> you can "join" to many groups _ at most I have tested is 4 groups like
> 224.0.0.xxx on the same port number (for example 6000).
> In Linux I had managed to use multicast UDP port but, it seem to be that
> it could not "join" to many groups, to receive packets, only one and you
> should bind on it (for example on 224.0.0.10:6000)!
> Is this right? One UDP socket could receive UDP packets only from one
> multicast group in Linux?
> 
> Sincerelly
>   tovis

You can subscribe to as many multicast groups as you want. However, if 
you want to listen to the same address/port pair from two different 
programs, you need to set the SO_REUSEADDR flag before binding the 
socket. In C, you can do that as follows:

#include <sys/socket.h>
#include <netinet/in.h>
...
int reuse=1
...

if(setsockopt(master,SOL_SOCKET,SO_REUSEADDR,&reuse,sizeof(int)) < 0 ){
     perror("reuse");
     return -1;
}

Regards,

Alain