[LinuxPPS] Multiple netlink sockets

[Rodolfo Giometti]

Hello,

I'd like to use several netlink sockets in one process.

Into "man 7 netlink" I read:

       nl_pid is the unicast address of netlink socket.  It's always 0 if  the
       destination  is in the kernel.  For a userspace process, nl_pid is usu-
       ally the PID of the process owning the  destination  socket.   However,
       nl_pid  identifies  a netlink socket, not a process.  If a process owns
       several netlink sockets, then nl_pid can only be equal to  the  process
       ID  for  at  most one socket.  There are two ways to assign nl_pid to a
       netlink socket.  If the application sets nl_pid before calling bind(2),
       then  it  is  up to the application to make sure that nl_pid is unique.
       If the application sets it to 0, the kernel takes care of assigning it.
       The  kernel assigns the process ID to the first netlink socket the pro-
       cess opens and assigns a unique nl_pid to every netlink socket that the
       process subsequently creates.

So to execute the bind() in userland I do:

        /* Bind the socket with the source address */
        memset(&src_addr, 0, sizeof(src_addr));
        src_addr.nl_family = AF_NETLINK;
        src_addr.nl_pid = 0; 
        src_addr.nl_groups = 0;         /* not in mcast groups */ 
        ret = bind(handle->socket, (struct sockaddr *) &src_addr, sizeof(src_addr));
        if (ret < 0) {
                close(handle->socket);
                return ret;
        }

and to replay into kernelland I do:

        /* Send an answer to the userland */
        dbg("start sending replay...");
        NETLINK_CB(skb).pid = 0;        /* from the kernel */
        NETLINK_CB(skb).dst_pid = nlh->nlmsg_pid;
        NETLINK_CB(skb).dst_group = 0;  /* not in mcast groups */

        ret = netlink_unicast(nl_sk, skb, nlh->nlmsg_pid, MSG_DONTWAIT);
        dbg("... replay sent (%d)", ret);
       
When I open the first socket everything works well, but when I use the
second I notice that the request goes into the kernel which in turn
replays using netlink_unicast() but the userland program blocks
forever.

Can you pleae suggest me where I do wrong?

Thanks,

Rodolfo

-- 

GNU/Linux Solutions                  e-mail:    giometti at enneenne.com
Linux Device Driver                             giometti at gnudd.com
Embedded Systems                     		giometti at linux.it
UNIX programming                     phone:     +39 349 2432127
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