[nanomsg] accessing control IDs

  • From: Drew Crawford <drew@xxxxxxxxxxxxxxxxxx>
  • To: nanomsg@xxxxxxxxxxxxx
  • Date: Mon, 5 May 2014 16:27:57 -0500

I have a REP socket.  I’m trying to identify the channel (sender or forwarder) 
on which some message has arrived to the socket.  A transport-layer 
understanding of the sender is not required; any identifying value, such as an 
integer, is sufficient.  Consulting the REQREP spec  suggests that the topmost 
“channel ID”, one of the records in the “backtrace”, is the identifier I’m 
looking for.

Clearly this identifier is not exposed over the nn_recv interface.  I had some 
hopes that it would be accessible in the nn_recvmsg interface, possibly as 
control information, but it seems not to be the case:

>     int client = nn_socket(AF_SP,NN_REQ);
>     int server = nn_socket(AF_SP,NN_REP);
>     nn_connect(client,"inproc://test");
>     nn_bind(server,"inproc://test");
>     nn_send(client,"A",1,0);
>     int rc;
>     void *body;
>     void *control;
>     struct nn_iovec iov;
>     struct nn_msghdr hdr;
>     iov.iov_base = &body;
>     iov.iov_len = NN_MSG;
>     memset (&hdr, 0, sizeof (hdr));
>     hdr.msg_iov = &iov;
>     hdr.msg_iovlen = 1;
>     hdr.msg_control = &control;
>     hdr.msg_controllen = NN_MSG;
>     rc = nn_recvmsg (server, &hdr, 0);
>     print_array(body,rc,"body”); //contains only A
>     printf("msg_iovlen %d\n",hdr.msg_iovlen); //1
>     printf("msg_controllen %d\n",hdr.msg_controllen); //-1

I have consulted a previous mailing thread on this topic which suggests channel 
IDs are manipulated in rep.c.  Indeed, the information I’m looking for seems to 
be moved around between nn_sockbase, nn_msg, nn_rep, and similar structures.  
However I cannot work out a sane way to get those structures from application 

Any suggestions on identifying the sender of a remote message?


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