GRC C++ Generation

Note: This page is a work in progress.

Starting from GNU Radio 3.8, GRC allows you to generate C++ code (not just Python) from your flowgraph. This feature was a SOCIS (ESA Summer of Code in Space) project in 2017. The C++ generation mode is significantly less stable and feature-rich than the default Python mode, since Python is a far more flexible language (and since GRC was created to generate Python code). About 50% of the in-tree blocks support C++ generation. The C++ generation templates have to be written on a per-block basis, which is a considerable amount of work. This also means that it is a considerable amount of work to keep the C++ templates updated, which is why some blocks' templates may be outdated and produce errors. The good news is that these errors often have trivial fixes.

GRC generates C++ code by creating a new directory with three files:

• top_block.cpp
• top_block.hpp
• CMakeLists.txt

All of these files are generated from templates, and changes in your flowgraph will be reflected in them. Additionally, GRC creates a build directory which you can call cmake and make from. Pressing the execute button in GRC will generate, compile and run your flowgraph. However, it is often convenient to compile and run your flowgraph in a separate terminal.

Adding C++ Templates to a Block

This guide builds on the C++ OOT guide. The steps listed here will fall between Modifying the YAML file and Compiling_and_Installing_the_Block.

In order to enable C++ generation for our multDivSelect block, we need to add some additional lines to the YAML file (comments have been removed for brevity):

id: customModule_multDivSelect
label: multDivSelect
category: '[customModule]'
flags: [cpp]

templates:
  imports: from gnuradio import customModule
  make: customModule.multDivSelect(${selector})

cpp_templates:
  includes: ['#include <gnuradio/customModule/multDivSelect.h>']
  declarations: 'customModule::multDivSelect::sptr ${id};'
  link: ['gnuradio-customModule']
  make: 'this->${id} = customModule::multDivSelect::make(${selector});'
  translations:
    'True': 'true'
    'False': 'false'

parameters:
- id: selector
  label: Selector, Multiply (true) or Divide (false)
  dtype: bool
  default: true

inputs:
- label: in0
  domain: stream
  dtype: complex
- label: in1
  domain: stream
  dtype: complex
outputs:
- label: out0
  domain: stream
  dtype: complex

file_format: 1

Troubleshooting

Compilation Issues

error: ‘True’ was not declared in this scope

Consolidate compiler generated dependencies of target cpptest
[ 50%] Building CXX object CMakeFiles/cpptest.dir/cpptest.cpp.o
/home/hkon/oot/gr-customModule/build/cpptest/cpptest.cpp: In constructor ‘cpptest::cpptest()’:
/home/hkon/oot/gr-customModule/build/cpptest/cpptest.cpp:37:80: error: ‘True’ was not declared in this scope
   37 |         this->customModule_multDivSelect_0 = customModule::multDivSelect::make(True);
      |                                                                                ^~~~
make[2]: *** [CMakeFiles/cpptest.dir/build.make:90: CMakeFiles/cpptest.dir/cpptest.cpp.o] Error 1

In C++, the boolean type is either true or false (lowercase), while the corresponding Python type is True or False. Try adding a translations field to the cpp_templates:

cpp_templates:
  (...)
  translations:
    'True': 'true'
    'False': 'false'

GRC will do a search and replace operation on your block's generated code, replacing all instances of True and False with true and false, respectively.