Insert messages in queues?

GNU Radio
1

If I’m using a block that has a message port output, is there a way to
put
those messages into a message queue? For instance:

self.message_queue = gr.msg_queue()
self.msg_connect((self.gsm_control_channels_decoder, ‘msgs’),
(???message_queue???, 0))

I have used gr.msg_queue() with blocks.message_sink() but that only
accepts
items such as complex, float, etc. I need to take the messages
generated by
any old block and parse them outside the flowgraph.

From what I have read, I thought all blocks that handle messages have
queues, so shouldn’t I be able to pop them directly out of that block?
I
guess I’m confused. Are there two type of message passing; an old one
that
uses queues, and a new one that uses PMT?

Thanks,
Lou

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2

On 11.04.2015 20:38, madengr wrote:

From what I have read, I thought all blocks that handle messages have
queues, so shouldn’t I be able to pop them directly out of that block? I
guess I’m confused. Are there two type of message passing; an old one that
uses queues, and a new one that uses PMT?

Hey Louis,

Yes, there are two ways, and they’re completely different. The old
message queues will be deprecated soon, so anything with gr.msg_queue()
in there will, at one point in the future, stop working.

Look at the Python output of some of the message examples we provide,
also check out this manual page:
http://gnuradio.org/doc/doxygen/page_msg_passing.html

M

3

Thanks Martin. Looking at the gr-blocks/examples/msg_passing, the
tutorial
link, and qa_msg, I only find examples of posting messages. Those make
sense, but is there an equivalent to _post method that pulls messages?

http://gnuradio.org/doc/doxygen/classgr_1_1basic__block.html#a8501714cb14c1a08b4ff55761600f3a5

From the above docs, I see the delete_head_blocking, which is maybe what
I
want. If I have a tagged stream to PDU like below, how do I pull the
message?

sink=blocks.tagged_stream_to_pdu(blocks.float_t, “packet_len”)
sink.to_basic_block.???

Lou

Martin B.-2 wrote

M

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4

Martin,

Yes, pop them outside the block, but also outside of the flow graph.
Exactly reverse of the last example “Posting from External Sources”
here:

http://gnuradio.org/doc/doxygen/page_msg_passing.html

“The last feature of the message passing architecture to discuss here is
how
it can be used to take in messages from an external source. We can call
a
block’s gr::basic_block::_post method directly and pass it a message. So
any
block with an input message port can receive messages from the outside
in
this way.”

I want to take any block with an output message port and _pull that
message
to the outside (of the flow so I can process it in Python).

Presently, with the old queued messaging system, I can use
blocks.message_sink() to take an item into a message queue I can access
outside the flow. Since this will be deprecated, I’d want to use
blocks.tagged_stream_to_pdu() to generate a message, but don’t see a way
to
get those messages stored into a FIFO (queue) and access them outside
the
flow.

As you suggest, maybe the solution is to develop a block, with an input
message port, that performs the callback and stores those messages in a
FIFO. From browsing the documentation, it looks like the message_debug
block will store up messages (I haven’t attempted to read those
messages),
but I’d be worried about overflows. I guess the ideal was to do it
without
developing or modifying any blocks.

Though looking at line 278 of usrp_spectrum_sense.py, when the message
queues are deprecated, what would you do to get the data out of the
flow? I
assume blocks.bin_statistics_f() would have to be re-written to have a
message port output, but then how do you get that message out of the
flow?

https://github.com/gnuradio/gnuradio/blob/master/gr-uhd/examples/python/usrp_spectrum_sense.py#L278

Thanks,
Lou

Martin B.-2 wrote

Cheers,
M

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5

On 12.04.2015 15:22, madengr wrote:

sink=blocks.tagged_stream_to_pdu(blocks.float_t, “packet_len”)
sink.to_basic_block.???

So you want pop messages from outside the block, right? Is the original
block written in C++?

delete_head_block() should work, but this seems unusual. Typically, we
have two mechanisms of handling messages: Dedicated message handler
callbacks, or we pop the queue ourselves in work() using delete_head().

If you’re popping messages outside of the block, the block itself would
never see or use those messages – which is why I’m surprised. You might
be better of writing a mini-Python-block that handles the messages.

Cheers,
M