GNU Radio 3.9 OOT Module Porting Guide

The major changes in the (in-progress) GNU Radio 3.9 release that will impact OOTs are:


 * C++ modernization (C++11/14?)
 * Replacement of SWIG with Pybind11
 * Cleanup of filter and fft APIs

= Porting Guide =

Porting from 3.8 to 3.9 can be achieved most simply by creating a new OOT module (with the same name as the 3.8 OOT but in a different directory), then performing some manual steps

1. Use the 3.9 gr_modtool to generate a module with the same name (in another directory)

2. Copy the python folder from 3.9 OOT into your 3.8 OOT

3. (in 3.8 OOT) Add the bindings directory to the python directory CMakeLists ./python/CMakeLists.txt → add the line: add_subdirectory(bindings)

4. (in 3.8 OOT) Call gr_modtool bind for each block in your OOT NOTE: If you are doing more than just make function in your public header, e.g. setters/getters or other callback functions, be sure to have pygccxml set up

5. c++ blocks that have python QA will need the import statements updated in that QA.

6. python/bindings/python_bindings.cc needs to be updated for all C++ blocks (in two places!)

7. Replace occurrences of boost::shared_ptr with std::shared_ptr

8. Though not necessary, boost::bind instances for message port handlers can be replaced with lambda functions for performance and simplicity as well as consistency with the in-tree modules

Porting from 3.7 to 3.9 should follow the 3.8 Porting Guide, but basically create a new OOT using 3.9 and add blocks from there, then copy in code.

= Details =

C++ Modernization
Boost shared pointers have been replaced with std:: shared pointers and memory management. At the top level of each block, the instantiation will need to change, e.g.

In include/blockname_xx.h:

typedef std::shared_ptr sptr;

Inbound message ports receive an update too. We move from boost::function to std::function. This affects how message handlers are registered. The preferred style is to use lambdas which is already compatible with GNU Radio 3.8:

set_msg_handler(pmt::mp("message"), [this](pmt::pmt_t msg) { this->handle_msg(msg); });

Pybind11 Python Bindings
As of the GNU Radio 3.9 release, python bindings are handled using pybind11, which is inherently different than they were in previous releases

Dependencies
curl -Lo pybind11.tar.gz https://github.com/pybind/pybind11/archive/v2.4.3.tar.gz mkdir pybind11 && tar xzf pybind11.tar.gz -C pybind11 --strip-components=1 && cd pybind11 mkdir build && cd build cmake .. -DCMAKE_BUILD_TYPE=Release -DPYBIND11_TEST=OFF make && make install
 * pybind11 &gt; 2.4.3 https://pybind11.readthedocs.io/
 * pip does not provide the proper cmake (https://github.com/pybind/pybind11/issues/1379)
 * For some distributions (e.g. Ubuntu <20.04), this will need to be installed from source as 2.4.3 is not the supplied version with package managers


 * pygccxml https://pygccxml.readthedocs.io/en/develop/install.html
 * This is an optional dependency and basic functionality for OOT generation can be performed without pygccxml
 * It is required for automatically generating bindings for most of the GR source tree

Components
Python bindings are contained in the  directory

The bindings for each block exist in blockname_python.cc under the  directory. Additionally, a template header file for each block that is used as a placeholder for the scraped docstrings lives in the  dir

blockname_python.cc
This is the class function enum variable bindings for everything that needs to be exposed through the Python API

python_bindings.cc
The structure of this file is

Comment Block
Each block binding file contains an automatically generated and maintained comment block that informs CMake when the bindings are out of sync with the header file they refer to, and what to do about it

In this case, the flag should be set to 0, and the bindings need to be updated manually. If the flag is set to 1, CMake will override the binding file in the source tree when it detects out of sync bindings. This should only be done in simple cases.

If a block needs pygccxml for the bindings to be properly generated automatically, this should be set to





Out-of-Tree modules
The steps for creating an out of tree module with pybind11 bindings are as follows:


 * 1) Use   to create an out of tree module and add blocks

Update the parameters or functions in the public include file and rebind with 

NOTE: without pygccxml, only the make function is currently accounted for, similar to

If the public API changes, just call  to regenerate the bindings

When the public header file for a block is changed, CMake will fail as it checks the hash of the header file compared to the hash stored in the bindings file until the bindings are updated

Build and install

Docstrings
If Doxygen is enabled in GNU Radio and/or the OOT, Docstrings are scraped from the header files, and placed in auto-generated  files in the build directory on compile. Generated templates (via the binding steps described above) are placed in the  directory and are used as placeholders for the scraped strings

Upon compilation, docstrings are scraped from the module and stored in a dictionary (using ) and then the values are substituted in the template file (using  )

OOT Migration
The easiest way to migrate an OOT to 3.9 is to use  to create a new OOT, use   to create the blocks, and copy code from the previous OOT.

Steps to do this without regenerating a new module are TBD

Caveats
Pybind11 bound methods do not implicitly convert int to enum, so blocks that take enum as input, must have either "raw" or "enum" in the grc yml definition of the block. "Raw" will allow the value to be changed by another variable in the flowgraph.

Block inheritance must be specified completely in the python bindings in order to use the inherited methods. For instance, if a block inherits from sync_block, both block and basic_block must be included in the inheritance specification of the class:

py::class_>(       m, "atsc_interleaver", D(atsc_interleaver))

If your OOT module uses types from or its classes derive from another gr module, it is necessary in  to specify these modules.

For instance, since all OOT modules require the base block types, there is a line.

If you wanted to utilize  objects in your OOT, it would be necessary to add   so that pybind knows to use the bindings already compiled into gnuradio.digital for the constellation objects

If your OOT module uses other classes as parameter, you must either setup a python binding for this class or use an existing one. An example, how to setup a binding for the QWidget class of QT5 used in gr-qtgui, can be found here

If you want to use this binding in your OOT module, you have to import it. This has to be done in the python_bindings.cc inside the PYBIND11_MODULE macro. In case of the QWidget usage this would be

Using default values
For standard types like int,float, etc. you can set default values as usual. will setup the correct bindings in blockname_python.cc.

But if you use something like  gr_modtool generates code like

But at this point pybind does not know the type of parent, so you have to modify the binding code and cast the type.

Another way to come across this issue is to modify your header file from to

pybind generates

which will be handled correctly

Unable to find pydoc.h
fatal error: blockname_pydoc.h: No such file or directory 28 | #include      |          ^ compilation terminated.

blockname_pydoc.h is generated during compilation based on the template in the docstring directory. When the block is first created in blocktool, this template does not exist. Run  to generate the appropriate template used as a placeholder for the scraped docstrings

Also, the scraping of docstrings only takes place at CMake time, so it may be necessary to do a  to re-trigger the scraping

You can also try rm python/bindings/docstring_status which will reset the docstring scraping target in cmake and re-copy the docstring templates

TypeError: 'modulename_python.blockname' object is not subscriptable
This is caused by an incomplete inheritance chain specified in the binding declaration of the block.

Instead of

py::class_(m, "blockname", D(blockname))

Try something like (taking into account your block type)

py::class_(m, "blockname", D(blockname))

Module is Empty
This is usually caused by linker errors that prevent the binding module from being loaded. When you see something like:

>>> import foo >>> dir(foo) ['__builtins__', '__cached__', '__doc__', '__file__', '__loader__', '__name__', '__package__', '__path__', '__spec__']

But you expect foo to have other C++ blocks, try loading the pybind module separately

cd build/python/bindings python3 >>> import foo_python [linker error should be evident here in mangled symbol name]