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Re: [Discuss-gnuradio] Message passing as input and output in block (C++

From: Jose Torres Diaz
Subject: Re: [Discuss-gnuradio] Message passing as input and output in block (C++ coding)
Date: Tue, 9 Oct 2012 11:06:36 +1030

Hi Josh,

Thanks for the info. So, for example if I want to modify the blob_to_stream example as blob_to_blob (ie. input a message and then output a message). The .cc files is as shown below, but I got errors when I compile because of the type of variables:

 In member function ‘virtual int blob_to_blob_impl::work(const InputItems&, const OutputItems&)’:
/lib/blob_to_blob.cc:78:43: error: no matching function for call to ‘blob_to_blob_impl::post_msg(int, const pmt_t&, const char*&, pmt::pmt_t&)’
/lib/blob_to_blob.cc:78:43: note: candidates are:
/grextras/include/gnuradio/block.h:204:10: note: void gnuradio::block::post_msg(size_t, const gr_tag_t&)
/grextras/include/gnuradio/block.h:204:10: note:   candidate expects 2 arguments, 4 provided
/grextras/include/gnuradio/block.h:215:10: note: void gnuradio::block::post_msg(size_t, const pmt_t&, const pmt_t&, const pmt_t&)
/grextras/include/gnuradio/block.h:215:10: note:   no known conversion for argument 3 from ‘const char*’ to ‘const pmt_t& {aka const boost::intrusive_ptr<pmt::pmt_base>&}’

The file blob_to_blob.cc is:

#include <gnuradio/extras/blob_to_blob.h>
#include <gr_io_signature.h>
#include <cstring> //std::memcpy

using namespace gnuradio::extras;

static const pmt::pmt_t BLOB_KEY = pmt::pmt_string_to_symbol("blob_stream"); //From stream to blob

class blob_to_blob_impl : public blob_to_blob{
   blob_to_blob_impl(const size_t item_size):
            gr_make_io_signature(0, 0, 0),
            gr_make_io_signature(1, 1, item_size),
            msg_signature(true, 1)
        _offset = 0;

    //From stream to blob example
    std::stringstream str;
        str << name() << unique_id();
        _id = pmt::pmt_string_to_symbol(str.str());


    int work(
        const InputItems &input_items,
        const OutputItems &output_items
        //Here the blob is received
        //loop until we get a blob or interrupted
        while (_offset == 0){
      _msg = this->pop_msg_queue(); //Reading the incoming blob
            if (pmt::pmt_is_blob(_msg.value)) break;
        if (pmt::pmt_blob_length(_msg.value) == 0) return -1; //empty blob, we are done here

        //calculate the number of bytes to copy
        const size_t nblob_items = (pmt::pmt_blob_length(_msg.value) - _offset)/_item_size;
        const size_t noutput_bytes = _item_size*std::min<size_t>(output_items[0].size(), nblob_items);

        //perform memcpy from blob to output items
        const char *blob_mem = reinterpret_cast<const char *>(pmt::pmt_blob_data(_msg.value)) + _offset;
        //std::memcpy(output_items[0].get(), blob_mem, noutput_bytes); //Putting in the output blob_mem

    //post the message to downstream subscribers
        //this->post_msg(0, BLOB_KEY, blob, _id);
    this->post_msg(0, BLOB_KEY, blob_mem, _id); //Using blob mem

        //adjust the offset into the blob memory
        _offset += noutput_bytes;
        if (pmt::pmt_blob_length(_msg.value) == _offset) _offset = 0;

        return noutput_bytes/_item_size;

    const size_t _item_size;
    gr_tag_t _msg;
    size_t _offset;
  pmt::pmt_t _id; //From strem to blob example

blob_to_blob::sptr blob_to_blob::make(const size_t item_size){
  return gnuradio::get_initial_sptr(new blob_to_blob_impl(item_size));

Many thanks again for your kind help,


On Tue, Oct 9, 2012 at 4:25 AM, Josh Blum <address@hidden> wrote:

On 10/07/2012 09:56 PM, Jose Torres Diaz wrote:
> Hi All,
> I have another question about the message passing technique. In this
> opportunity, I would like to create a block that takes message as input,
> but also that post message downstream as output. In my initial attempt, I'm
> using the original blob_to_stream example (see below), the incoming message
> is taken using:
> _msg = this->pop_msg_queue();
> and then, in order to output the incoming message, it is used:
> std::memcpy(output_items[0].get(), blob_mem, noutput_bytes);
> However,
> *(1) Is it possible to use an alternative way like this:
> post_msg(0,_msg.key, _msg.value,_id);   ?

Yes, absolutely. This is the correct way to pass the message downstream.


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