# Using Clangd with Libvirt

clangd is an implementation of the language server protocol for C and C++.

When paired with an LSP-compatible editor or IDE (e.g. emacs, vim, vscode), clangd can helpful when working with libvirt's C sources e.g. navigating the code base.

Whilst other C LSPs are available, clangd should work correctly with the libvirt because clang is a supported compiler for libvirt, and clangd is part of the same code base as clang.

If clang is the default compiler on your system, then clangd can be used as soon as meson setup has been run.

If gcc is your build environment's default compiler, then additional steps are required to use clangd:

clangd looks for a compile_commands.json file in the top level directory of the project and also in the build/ subdirectory to discover which include paths, compiler flags etc. should be used when it parses each source file.

Meson creates a compile_commands.json in the build directory. Meson defaults to the system's default C compiler. When the default compiler is gcc, its compile_commands.json output cannot be used with clangd due to differences in compiler invocation flags when building libvirt.

Create a separate build directory with a clangd compatible compile_commands.json as follows:

    CC=clang CXX=clang++ meson setup build-clang

There are a small number of source files that are generated as part of the build process. In order to navigate this generated source code, you should also execute a build in this directory:

    ninja -C build-clang

Point clangd (v12 or later) at the correct compile_commands.json by placing the following into a .clangd file in the root of the project:

    ---
    CompileFlags:
      CompilationDatabase: "build-clang"

Please note that if you are not using clang for your daily development, the build-clang directory can get out of sync with the current state of the project over time. When you update your git checkout, new files may be added or configuration options changed and clangd may start to behave unpredictably. In this case, you will need to update your compilation database by doing a new build in the build-clang directory. This should allow clangd to work reliably again.