Re: [Discuss-gnuradio] Writing own gnuradio Block, optional output port

How many ports GRC enforces depends on the .xml file in your module's grc/ folder, so you will have to add a
<optional>1/<optional>
to the respective <soruce> definition in your .xml.

Regarding operating on out1 iff there's a second output port:
you can either overwrite the check_topology()[1] method and memorize the number of output streams, or you can just use the fact that work()[2] is called with a

gr_vector_void_star &

output_items

which is but a typedef for a std::vector<void*>, which means it's a vector of pointers to your output buffers. And hence, the number of available output buffers is just
output_items.size().

You must NEVER write to out1=output_items[1] if there is only one stream. That would be a bug, and in most cases actually lead to a segmentation fault and cancellation of your program.

Best regards,
Marcus

[1] https://gnuradio.org/doc/doxygen/classgr_1_1basic__block.html#aca13d6f083397aada5b44d0f349ce151
[2] https://gnuradio.org/doc/doxygen/classgr_1_1sync__block.html#a38942c5d8bee5232404ab71ada9a3956

On 16.12.2015 23:40, Charles Bridger wrote:

I've created a signal source and want to create a second port which outputs  the same signal, except one sample behind. The problem I'm having is that I  don't know how to make it "optional". If I change the code in the private  constructor from:      [Constructor Code}  ...  gr::io_signature::make(1,1,sizeof(float)),  ....     to    gr::io_signature::make(1,2,sizeof(float)),    It still makes me  plug something into the port when I use the block in  GRC, even when I don't want to use the second output port.    I don't know if my code in the work method is relevant but I address the tw  output ports like this:    float *out0 = (float *) output_items[0];  float *out1 = (float *) output_items[1];    for(int i = 0; i < noutput_items; i++)  {      out0[i] = stuff;      out1[i] = stuff - 1; // <- I want that to be optional when I put the  block in GRC.  }    I've also tried setting the io_signature::make to 1,-1,sizeof(float) but  that's less than ideal and doesn't work if in GRC i set number of outputs  to 1.    Again thank you all for your time.    Cheers,  Charles    

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