ZMQ REQ Message Source

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Revision as of 01:34, 12 March 2020 by Duggabe (talk | contribs) (expand Example Flowgraph section)
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The ZMQ REQ Message Source block receives messages from a ZMQ REQ socket and outputs async messages. This block will connect to a ZMQ REP Message Sink.
The website says:
"The REQ-REP socket pair is in lockstep. The client issues zmq_send() and then zmq_recv(), in a loop (or once if that's all it needs). Doing any other sequence (e.g., sending two messages in a row) will result in a return code of -1 from the send or recv call." Likewise, the server "issues zmq_recv() and then zmq_send() in that order, as often as it needs to."


(R): Run-time adjustable

ZMQ socket address specifier. The format of the address is tcp://*:port where * should be for localhost.
Socket timeout in milliseconds, default is 100ms.

Example Flowgraph


Request/Reply pairs can be used on one, or two separate, flowgraphs to exchange messages.

REQ REP msg demo fg.png

External Python client (send only)

An external Python program can send messages to a ZMQ REQ Message Source block. An example flowgraph and Python code follow.

GNU Radio as a server

If the GNU Radio flowgraph(s) is configured as a server, the REQ message is processed by the flowgraph and a message is sent back in a REP message as the response. An example flowgraph and Python code follow.

Server demo fg.png

The Embedded Python block "Server demo" contains the following code:

from gnuradio import gr
import pmt

inputText = ""

class my_sync_block (gr.sync_block):
#  accepts message string from input port
#  capitalizes the string
#  sends message to output port
    def __init__(self):
            name = "Server demo",
            in_sig = None,
            out_sig = None)
        self.set_msg_handler(pmt.intern('msg_in'), self.handle_msg)

    def handle_msg(self, msg):
        global inputText
        inputText = pmt.symbol_to_string (msg)
        # print (inputText)
        if (len (inputText) > 0):
            #  capitalize the string
            outputText = inputText.upper()
            # print (outputText)
            #  Send reply back to client
            self.message_port_pub(pmt.intern('msg_out'), pmt.intern(outputText))

    def work(self, input_items, output_items):
        # with no data ports, there is nothing to do
        return (0)

Source Files

C++ files
Header files
Public header files
Block definition