Creating C++ OOT with gr-modtool

This tutorial describes how to create a custom c++ block and use it in a flowgraph:
 * Create a new c++ block using gr_modtool
 * Modify the c++ .h and .cc code so the block will function
 * Modify the YAML file so it can be read in GRC
 * Install and run the block in a flowgraph

The previous tutorial, Creating Python OOT with gr-modtool, describes how to create a Python block in an OOT module. This c++ OOT tutorial builds upon the previous Python one, so it is is suggested to at least complete the Creating an OOT Module portion of that tutorial before completing this one.

Installation Note
This tutorial was written using GNU Radio v3.10.1.1 on Ubuntu 21.10, installed using the Ubuntu PPA from the Installation Wiki Page. The basic GNU Radio install using:

$ sudo apt-get install gnuradio

does not come with the proper libraries needed to compile and install OOT modules. Consider installing the following packages before continuing:

$ sudo apt-get install gnuradio-dev cmake libspdlog-dev clang-format

Adding a New Block
Move to the gr-customModule directory created in the Creating Python OOT with gr-modtool tutorial: cd your-path/gr-customModule

Add a new block named multDivSelector: $ gr_modtool add multDivSelector

The types of blocks will be displayed:

GNU Radio module name identified: customModule ('sink', 'source', 'sync', 'decimator', 'interpolator', 'general', 'tagged_stream', 'hier', 'noblock')

Enter sync as the block type: Enter block type: sync

Enter cpp as the language: Language (python/cpp): cpp Language: C++ Block/code identifier: multDivSelector

Enter the name or organization of the copyright holder: Please specify the copyright holder: YourName

Create a boolean variable selector with a default value of true: Enter valid argument list, including default arguments: bool selector=true

Select whether or not QA code is desired: Add Python QA code? [Y/n] n Add C++ QA code? [Y/n] n

Multiple files will then be created or modified: Adding file 'lib/multDivSelector_impl.h'... Adding file 'lib/multDivSelector_impl.cc'... Adding file 'include/gnuradio/customModule/multDivSelector.h'... Adding file 'python/customModule/bindings/docstrings/multDivSelector_pydoc_template.h'... Adding file 'python/customModule/bindings/multDivSelector_python.cc'... Adding file 'grc/customModule_multDivSelector.block.yml'... Editing grc/CMakeLists.txt...

Editing the c++ Header
Many of the files are automatically generated wrapper code that do not need to be modified. However, the multDivSelector_impl.h and multDivSelector_impl.cc files defines the operation of the block and must be modified. Open the file with a text editor:

$ gedit lib/multDivSelector_impl.h &

The following code will be displayed:

Create a boolean private member _selector which will hold the value of the selector parameter:

Press CTRL + S to save the file.

Editing the c++ impl
The .cc file needs to be modified to define the desired operation of the block. Open the file with a text editor:

$ gedit lib/multDivSelector_impl.cc &

The code will be as displayed:

Remove the pragma messages and define the input and output type to be gr_complex:

Update to two inputs and store the value of the selector parameter using the private member _selector as defined in multDivSelector_impl.h:

Modify the work function by removing the pragma message, defining the variables in0 and in1 corresponding to the two input ports, and multiply the two inputs if _selector is true and divide them if _selector is false:

Press CTRL + S to save the file.

Editing the YAML File
Open the YAML file using a text editor:

$ gedit gedit grc/customModule_multDivSelector.block.yml &

The code will be displayed:

Update the parameter definition with the information for selector:

Update the input port and output port definitions:

Press CTRL + S to save the file.

Compile and Install
The block needs to be compiled and installed. Ensure you are in the gr-customModule directory:

$ cd your-path/gr-customModule

If the build/ directory already exists, remove it:

rm -rf build/

Create the build directory:

$ mkdir build

Move into the build directory:

cd build

Run cmake to build the makefiles:

cmake ..

Compile the module:

make

Install the module:

sudo make install

The new files will then be installed:

-- Install configuration: "Release" -- Up-to-date: /usr/local/lib/cmake/gnuradio-customModule/gnuradio-customModuleConfig.cmake -- Up-to-date: /usr/local/include/gnuradio/customModule/api.h -- Installing: /usr/local/include/gnuradio/customModule/multDivSelector.h -- Installing: /usr/local/lib/x86_64-linux-gnu/libgnuradio-customModule.so.1.0.0.0 -- Installing: /usr/local/lib/x86_64-linux-gnu/libgnuradio-customModule.so.1.0.0 -- Installing: /usr/local/lib/x86_64-linux-gnu/libgnuradio-customModule.so -- Installing: /usr/local/lib/cmake/gnuradio-customModule/gnuradio-customModuleTargets.cmake -- Installing: /usr/local/lib/cmake/gnuradio-customModule/gnuradio-customModuleTargets-release.cmake -- Installing: /usr/local/lib/cmake/gnuradio-customModule/gnuradio-customModuleConfig.cmake -- Up-to-date: /usr/local/lib/python3.9/dist-packages/gnuradio/customModule/__init__.py -- Up-to-date: /usr/local/lib/python3.9/dist-packages/gnuradio/customModule/addSubSelect.py -- Installing: /usr/local/lib/python3.9/dist-packages/gnuradio/customModule/__init__.pyc -- Installing: /usr/local/lib/python3.9/dist-packages/gnuradio/customModule/addSubSelect.pyc -- Installing: /usr/local/lib/python3.9/dist-packages/gnuradio/customModule/__init__.pyo -- Installing: /usr/local/lib/python3.9/dist-packages/gnuradio/customModule/addSubSelect.pyo -- Installing: /usr/local/lib/python3.9/dist-packages/gnuradio/customModule/customModule_python.cpython-39-x86_64-linux-gnu.so -- Set runtime path of "/usr/local/lib/python3.9/dist-packages/gnuradio/customModule/customModule_python.cpython-39-x86_64-linux-gnu.so" to "" -- Up-to-date: /usr/local/share/gnuradio/grc/blocks/customModule_addSubSelect.block.yml -- Installing: /usr/local/share/gnuradio/grc/blocks/customModule_multDivSelector.block.yml

Some of the files are listed as Up-to-date because they correspond to the addSubSelect block created in the Creating Python OOT with gr-modtool tutorial. All of the files corresponding to multDivSelect are now instlaled to /usr/local/.