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Re: [Discuss-gnuradio] Problem TCP blocks

From: Marcus Müller
Subject: Re: [Discuss-gnuradio] Problem TCP blocks
Date: Tue, 12 May 2015 13:06:57 +0200
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:31.0) Gecko/20100101 Thunderbird/31.6.0

Hi Andy,

oh yes, I don't like that block ;) but I'm happy it's there.

So the problem lies in the way this block is structured, and how GNU Radio runs:

tcp_sink is just a hier block -- it's nothing but a small wrapper around the file_descriptor_sink[1]. The cool thing about sockets is that you can, as soon as you have a connection, get a file descriptor for that connection and read and write from/to it, just like a file.
So tcp_sink just sets up a listening socket, and as soon as that connects, it uses the resulting file descriptor [2] to set up a file_descriptor_sink.
That is all nice -- but: this means that the constructor of tcp_sink blocks for as long as it takes till someone connects to this socket.
Which means that the script setting up your flow graph just pauses for as long as it waits for the first socket to get its connection set up; the second socket is not in listening mode, yet, at this point. So you're doing everything right, it's just so that your second tcp server isn't even created before the first one connects!

In principle, this is something you'd avoid; waiting for sockets to connect is typically the job of a gr::block::start() method rather than a gr::hier_block2's constructor, for exactly this reason. However, tcp_sink is there for "historical reasons", and it's mightily handy in many cases.

If your application doesn't scream for performance:
You might just want to use gr_modtool to create a python source, let that block's constructor set up a socket where you call sock.listen(), and give that block a start(self) method, which calls sock.accept, and stores the returned connected socket in a property of the block; work() would then write to that socket. Something like [3]

    def start(self):
        Block start method, called when user calls tb.run/tb.start
        self.connection, _ = self.listener.accept()

    def stop(self):

    def work(self, input_items, output_items):
        in0 = input_items[0]
        nbytes = len(in0)*self.itemsize
        nnext = nbytes // self.itemsize * self.itemsize
        last_pos = 0
        sent = 0
        remaining = nbytes
        rawbuffer = in0.data

        while nnext:
            sent = self.connection.send(rawbuffer[last_pos:last_pos+nnext]) #um, yeah.
            if not sent: ## we couldn't send a single byte --> connection is dead
                return -1 ## We're done.
            remaining -= sent
            last_pos += sent
            nnext = remaining // self.itemsize * self.itemsize # to make sure we only send whole items
        return sent // self.itemsize

[1] http://gnuradio.org/doc/doxygen/classgr_1_1blocks_1_1file__descriptor__sink.html
[2] https://github.com/gnuradio/gnuradio/blob/master/grc/grc_gnuradio/blks2/tcp.py#L70
[3] https://github.com/marcusmueller/gr-tcp/blob/master/python/tcp_sink.py really, just wrote this in a hurry. didn't even care to syntax check. for illustration purposes only.
On 05/12/2015 10:38 AM, Andreas Ladanyi wrote:
iam trying to setup a grc project with 2 TCP sinks server blocks each with own TCP port but both TCP blocks with the same IP.  So i want to open 2 different TCP Ports at the same host and same IP.  At the other grc project iam trying to setup 2 TCP source client blocks which connect to the sockets from the first grc project.
nmap tells me that only one port on the server side is open and the other server port is closed. iptables is down. So iam wondering that it isnt possible to get both ports open. Iam testing this scenario on the same host system. Do i understand something wrong in GRC / Python socket concept ?

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