FRR development and Visual Studio Code

[rzalamena]

Visual Studio Code is a popular code editor (Stack Overflow Developer Survey) that I have been trying recently and I noticed that it doesn't work out-of-the-box for FRR development. Since I never saw people commenting about it I decided to document the problems I found in case anyone else wants to use the editor at its best capacity.

1. You need the C/C++ extension in order to get code linting/completion. It will be suggested to you when you open a C.

2. For the linter/completion to work in a project with autotools (and without CMake or visual studio project file) we'll need to generate a compile_commands.json file. This step is necessary because otherwise the C/C++ extension won't be able to figure out the include paths and the compilation dependencies (like the flag -DHAVE_CONFIG_H).

   To generate a compile_commands.json file there are several tools available, but I'll focus in the one I use: bear. In an Ubuntu system you can grab it with the command:

   sudo apt install bear

   Before using bear we need to configure our FRR sources with the parameters we want. If you are unfamiliar with the FRR build procedure please read the build manual.

   NOTE to use bear we need to build FRR entirely because the way bear works is by intercepting the build commands and generating the compile_commands.json file.

   This is important to keep in mind with this tool because if you add a new file later on it won't be automatically added to the compile_commands.json file and the linter/completion won't work there.

   Here are the steps I use for generating the compile_commands.json file:

   # Fetch FRR and change into its directory
   git clone [email protected]:FRRouting/frr.git
   cd frr
   
   # Generate configure file:
   ./bootstrap.sh
   
   # Configure the project the way you want
   ./configure --enable-sharpd
   
   # Run the build command with bear.
   bear -- make -j$(nproc)
   # If the above command fails try the following:
   # bear make
   # (newer bear version wants the `--` separator for commands)
   #
   # If you don't have `nproc` just replace it with the number of CPU threads or don't use the `-j` parameter at all.

   Visual Studio code will ask about using it next time you open a C file. You may also open the project configuration and manually specify it in the compileCommands settings. Example:

   cat .vscode/c_cpp_properties.json
   {
       "configurations": [
           {
               "name": "Linux",
               "includePath": [
                   "${workspaceFolder}/**"
               ],
               "defines": [],
               "compilerPath": "/usr/bin/gcc",
               "cStandard": "gnu17",
               "cppStandard": "gnu++17",
               "intelliSenseMode": "linux-gcc-x64",
               "compileCommands": "${workspaceFolder}/compile_commands.json"
           }
       ],
       "version": 4
   }
   

   You can use the Visual Studio Code UI editor to change this. Click on the bottom right button called Linux (for Ubuntu case) then select Edit Configurations (UI).

3. Visual Studio Code will still complain about some errors in files that compile cleanly. This is due some compiler extensions that FRR uses and the editor doesn't know about. To get the linter/completion to work in these files you will need to address these errors either by fixing them (preferably) or, if not possible, by ignoring them.

   I've fixed some of the most annoying errors in this PR: lib: handle visual studio code C/C++ extension #12397.

4. Happy coding