GNU Radio ZMQ Blocks REP - REQ - zeromq

Question

I am trying to connect GNU Radio to a python script using the GR ZMQ REP / REQ blocks. GR is running on a Raspberry Pi 4 on router address 192.168.1.25. The python script is on a separate computer, from which I can successfully ping 192.168.1.25. I am setting up the REQ-REP pairs on separate ports, 55555 and 55556.
Flow graph:
import pmt
import zmq
# create a REQ socket
req_address = 'tcp://192.168.1.25:55555'
req_context = zmq.Context()
req_sock = req_context.socket (zmq.REQ)
rc = req_sock.connect (req_address)
# create a REP socket
rep_address = 'tcp://192.168.1.25:55556'
rep_context = zmq.Context()
rep_sock = rep_context.socket (zmq.REP)
rc = rep_sock.connect (rep_address)
while True:
data = req_sock.recv()
print(data)
rep_sock.send (b'1')
Running this code leads to the following error:
ZMQError: Operation cannot be accomplished in current state
The error is flagged at this line:
data = req_sock.recv()
Can you comment on the cause of the error? I know there is a strict REQ-REP, REQ-REP.. relationship, but I cannot find my error.

Answer 1

Your current code has two problems:
You call req_socket.recv(), but then you call rep_sock.send(): that's not how a REQ/REP pair works. You only need to create one socket (the REQ socket); it connects to a remote REP socket.
When you create a REQ socket, you need to send a REQuest before you receive a REPly.
Additionally, you should only create a single ZMQ context, even if you have multiple sockets.
A functional version of your code might look like this:
import zmq
# create a REQ socket
ctx = zmq.Context()
req_sock = ctx.socket (zmq.REQ)
# connect to a remote REP sink
rep_address = 'tcp://192.168.1.25:55555'
rc = req_sock.connect(rep_address)
while True:
req_sock.send (b'1')
data = req_sock.recv()
print(data)
I tested the above code against the following GNU Radio config:
options:
parameters:
author: ''
catch_exceptions: 'True'
category: '[GRC Hier Blocks]'
cmake_opt: ''
comment: ''
copyright: ''
description: ''
gen_cmake: 'On'
gen_linking: dynamic
generate_options: qt_gui
hier_block_src_path: '.:'
id: example
max_nouts: '0'
output_language: python
placement: (0,0)
qt_qss_theme: ''
realtime_scheduling: ''
run: 'True'
run_command: '{python} -u {filename}'
run_options: prompt
sizing_mode: fixed
thread_safe_setters: ''
title: Example
window_size: (1000,1000)
states:
bus_sink: false
bus_source: false
bus_structure: null
coordinate: [8, 8]
rotation: 0
state: enabled
blocks:
- name: samp_rate
id: variable
parameters:
comment: ''
value: '32000'
states:
bus_sink: false
bus_source: false
bus_structure: null
coordinate: [184, 12]
rotation: 0
state: enabled
- name: analog_sig_source_x_0
id: analog_sig_source_x
parameters:
affinity: ''
alias: ''
amp: '1'
comment: ''
freq: '1000'
maxoutbuf: '0'
minoutbuf: '0'
offset: '0'
phase: '0'
samp_rate: samp_rate
type: complex
waveform: analog.GR_COS_WAVE
states:
bus_sink: false
bus_source: false
bus_structure: null
coordinate: [184, 292.0]
rotation: 0
state: true
- name: blocks_throttle_0
id: blocks_throttle
parameters:
affinity: ''
alias: ''
comment: ''
ignoretag: 'True'
maxoutbuf: '0'
minoutbuf: '0'
samples_per_second: samp_rate
type: complex
vlen: '1'
states:
bus_sink: false
bus_source: false
bus_structure: null
coordinate: [344, 140.0]
rotation: 0
state: true
- name: zeromq_rep_sink_0
id: zeromq_rep_sink
parameters:
address: tcp://0.0.0.0:55555
affinity: ''
alias: ''
comment: ''
hwm: '-1'
pass_tags: 'False'
timeout: '100'
type: complex
vlen: '1'
states:
bus_sink: false
bus_source: false
bus_structure: null
coordinate: [504, 216.0]
rotation: 0
state: true
connections:
- [analog_sig_source_x_0, '0', blocks_throttle_0, '0']
- [blocks_throttle_0, '0', zeromq_rep_sink_0, '0']
metadata:
file_format: 1