Intro to GNURadio and the USRP (Part 4, message blocks)

This week I started playing with the message source and sink blocks within GNURadio and found them quite useful. Especially for accessing (sending and receiving) data outside of the actual GNURadio code. Below is the code, but let me explain what is going on (you can get the code from the source repository, see part 0). I create a top block which has a message_source and a message_sink (and a queue for each). Then I created two functions in the top block, one for sending a packet, and the other for receiving. The code should be fairly simple.

Then in the main portion of the code, I show how this is used. You can interact directly with the queues or by using the two functions.

from gnuradio import gr
from time import sleep

class msg_blocks(gr.top_block):

    def __init__(self):
        gr.top_block.__init__(self, 'Message Blocks Test')

        # initialize the queues
        self.sink_queue = gr.msg_queue()
        self.source_queue = gr.msg_queue()

        # initialize the blocks
        self.msg_source = gr.message_source(gr.sizeof_char, self.source_queue)
        self.msg_sink = gr.message_sink(gr.sizeof_char, self.sink_queue, False)

        self.connect((self.msg_source, 0), (self.msg_sink, 0))

    def send_pkt(self, payload):
        self.source_queue.insert_tail(gr.message_from_string(payload))

    def recv_pkt(self):
        pkt = ""

        if self.sink_queue.count():
            pkt = self.sink_queue.delete_head().to_string()

        return pkt            

if __name__=="__main__":
    tb = msg_blocks()
    tb.start()

    # operate on queues directly
    print 'operating on the queues directly'
    print 'sink queue count:', tb.sink_queue.count()

    print 'inserting:', 'a'
    tb.source_queue.insert_tail(gr.message_from_string('a'))

    sleep(1) # sleep to yield to tb thread

    print 'sink queue count:', tb.sink_queue.count()

    print 'received:', tb.sink_queue.delete_head().to_string()

    # operate using methods
    print 'operating with fuctions supplied by the top block'
    print 'sending:', 'a'*512
    tb.send_pkt('a'*512)
    sleep(1)

    print 'received:', tb.recv_pkt()

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