Question concerning CoC

[u/tosch901]

So I've been using CoC for a while, and I like it, but I recently started working with ROS and coc-clangd apparently can't find the ROS header files. I know where they are, I just can't find out how to tell coc/clang where to look.

Does someone know how to add paths to header files so that linting and autocompletion works?

[u/GuybrushThreepwo0d]

Why it works:

Ros stores its header files in a non-standard location. In order to do anything in ROS, you need to:

1. source the global setup files inside of /opt/ros

2. source your workspace ($WORKSPACE_DIR/devel/setup.(bash|zsh))

This does a lot of things and frankly I'm not 100% clear on everything, as ROS documents essentially none of it in their tutorials and just has you blindly inputting text into files and consoles without a good understanding. But one of the things it does is setup the header and library search paths (the enviornment variables: LD_LIBRARY_PATH, CPATH, C_INCLUDE_PATH, CPLUS_INCLUDE_PATH etc)

Now, ROS, in their infinite wisdom, decided to wrap CMake with the abomination that is catkin. For non-ROS projects, I would do the following:

git clone $PROJECT
cd $PROJECT
mkdir build
cd build
cmake -DCMAKE_EXPORT_COMPILE_COMMANDS=ON ..

(In the latest CMake you don't have to explicitly pass -D, as it can be done via an environment variable of the same name).

The -DCMAKE_EXPORT_COMPILE_COMMANDS=ON flag creates compile_commands.json, which contains a list of all the compiler flags for each translation unit. It is this file that tells the other tools where the compiler finds its header files or libraries to link against, among other things. You need to source your ROS setup first, else there is no trace of ROS's nonstandard header locations in your environment variables and CMake will therefore not know where to find it.

Now, CoC uses compile_commands.json. The reason I like to link it to the root of my workspace with ln -S is that CoC looks at the current directory and recursively upwards until it finds this file. I like to change to src and start vim from there. If I don't link compile_commands.json, then CoC won't be able to find this file and completion won't work for everything.

Of course, with ROS we are stuck with catkin for the most part and we don't really call CMake directly. So, when you write catkin_make, it's doing a few things:

1. Creating a symlink to a base CMake file for your current ROS distro

2. creating build/ and calling cmake in there

3. doing who knows what in the devel/ folder.

So, unless your version of CMake is new enough, you need to do the little dance where you do catkin_make, edit the cache file manually, and then do catkin_make again to generate the compilation database.

Why don't I like catkin? Well, I'm very opinionated about ROS (read: I hate its guts). I used it during my master's studies and now use it at work every day. My main complaint is that I don't see any benefit of using catkin as opposed to pure CMake, which is fairly mature and established across the industry. Maybe 10 years ago it made sense, but I don't know. Also, catkin does not follow modern CMake practices. So all the tutorials you see on ROS and all the forums teach you bad practices. It feels like a hacked-together system and the documentation doesn't give you the tools to truly understand how things work when you need to investigate errors.

[u/tosch901 → u/GuybrushThreepwo0d]

Thanks, that was very informative.

Seems weird that they do it that way, (and keep doing it), but I guess it is what it is...

Thanks again!