How to use clangd C/C++ LSP in any project

clangd.md

How to use clangd C/C++ LSP in any project

tl;dr: If you want to just know the method, skip to How to section

Clangd is a state-of-the-art C/C++ LSP that can be used in every popular text editors like Neovim, Emacs or VS Code. Even CLion uses clangd under the hood. Unfortunately, clangd requires compile_commands.json to work, and the easiest way to painlessly generate it is to use CMake.

For simple projects you can try to use Bear - it will capture compile commands and generate compile_commands.json. Although I could never make it work in big projects with custom or complicated build systems.

But what if I tell you you can quickly hack your way around that, and generate compile_commands.json for any project, no matter how compilcated? I have used that way at work for years, originaly because I used CLion which supported only CMake projects - but now I use that method succesfully with clangd and Neovim.

Method summary

Basically what we need to achieve is to create a CMake file that will generate a compile_commands.json file with information about:

1. All source files
2. All include directories
3. External libraries
4. Precompiler definitions

We can do that easily without really caring about if the CMake-generate result will compile at all - we don't need to rewrite our existing build system, just hack a CMake file that will generate enough information for Clangd to work.

Prerequisities

1. CMake
2. clangd

How to

First, create a CMakeLists.txt file in the root folder of your projects, with content similar to this:

cmake_minimum_required(VERSION 3.8)
project(my_project)

set(CMAKE_EXPORT_COMPILE_COMMANDS ON)

# Change path from /src if needed, or add more directories
file(GLOB_RECURSE sources
        "${CMAKE_SOURCE_DIR}/src/*.c"
        "${CMAKE_SOURCE_DIR}/src/*.cpp"
        )
# Add precompiler definitions like that:
add_definitions(-DSOME_DEFINITION)

add_executable(my_app ${sources})

# Add more include directories if needed
target_include_directories(my_app PUBLIC "${CMAKE_SOURCE_DIR}/include")

# If you have precompiled headers you can add them like this
target_precompiled_headers(my_app PRIVATE "${CMAKE_SOURCE_DIR}/src/pch.h")

(If your project already uses CMake, then you just need to add set(CMAKE_EXPORT_COMPILE_COMMANDS ON) to your main CMakeLists.txt file.)

Modify hacky CMakeLists.txt according to your project structure, and run:

cmake -S . -G "Unix Makefiles" -B cmake

which will generate the CMake output inside cmake directory. Check if compile_commands.json is there.

NOTE: You need to run that command every time you add/remove a source file in your project.

If you need more (ex. include external libraries like Boost), check out CMake documentation

Now you have two options:

1. Symlink compile_commands.json to your root project folder:

ln -s cmake/compile_commands.json .

OR

2. Create .clangd file in your root project folder, with the following contents:

CompileFlags:
  CompilationDatabase: "cmake"

Now open the project in you editor and everything should work (assuming clangd LSP is started).

Hej,

I'm running into another problem.
I have a folder with .c and .h files which is outside of the project folder.
Adding the folder path to "file()" will add all .c files in it correctly to the compile_commands:

file(GLOB_RECURSE sources
        "${CMAKE_SOURCE_DIR}/source/*.c"
        "${CMAKE_SOURCE_DIR}/../Basics/*.c"
        )

However when adding include directories, none of the files or folders are listed in compile_commands:

target_include_directories(DeepSeekLocalTarget
        PUBLIC "${CMAKE_SOURCE_DIR}/../Basics"
        PUBLIC "${CMAKE_SOURCE_DIR}/ThirdParty/libcurl/include"
        PUBLIC "${CMAKE_SOURCE_DIR}/ThirdParty/cJSON"
)

I can include these files directly, without a path, and I don't get any errors in neovim.
Furthermore, I can "go to" the files through the include itself.

BUT I can't find any files from the "Basics" folder when searching for files.
I can find symbols in these files when I serach for workspace symbols.

BUT I can't find any files from the "Basics" folder when searching for files.
I can find symbols in these files when I serach for workspace symbols.

@Takao-7
I don't know how you perform search in NeoVim, but I guess you have your cwd set to your project directory, and whatever you use for searching to search within cwd, which is probably the default behavior for Telescope/fzf-lua/whatever.

So you need to configure whatever you use for search to include that ../Basics directory too.

I thought that the lsp was doing the search, but it makes sense that it doesn’t.

I’m using Kickstart Neovim, so Telescope is searching for files.

Is it possible to automatically add all folders that are found by cmake to the list?
Or would I need to manually add them?

I think that if you will start nvim in the parent directory that should do the trick, assuming you don't have other plugins that affects cwd. So, if your tree looks like this:

my_projects
|- some_utils
|- project_in_c

and you want search to work in both some_utils and project_in_c, then you should:

cd my_projects
nvim            # or nvim . maybe