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# Clangd
## Introduction
[clangd](https://clang.llvm.org/extra/clangd/) is a clang-based [language server](http://langserver.org/).
It brings IDE features (e.g. diagnostics, code completion, code navigations) to
your editor.
## Getting clangd
See [instructions](https://clang.llvm.org/extra/clangd/Installation.html#installing-clangd).
**Googlers:** clangd has been installed on your glinux by default, just use
`/usr/bin/clangd`.
Alternative: download clangd from the official [Releases](https://github.com/clangd/clangd/releases)
page.
Note: clangd 10.0.0 does not work with Chromium; use one of the pre-release
versions lower down on the Releases page until a newer version is released.
If you prefer to build clangd locally, use the following command to build from
LLVM source, and you will get the binary at
`out/Release/tools/clang/third_party/llvm/build/bin/clangd`.
```
tools/clang/scripts/build_clang_tools_extra.py --fetch out/Release clangd
```
## Setting Up
1. Make sure generated ninja files are up-to-date.
```
gn gen out/Release
```
2. Generate the compilation database, clangd needs it to know how to build a
source file.
```
tools/clang/scripts/generate_compdb.py -p out/Release > compile_commands.json
```
Note: the compilation database is not regenerated automatically. You need to
regenerate it manually whenever build rules change, e.g., when you have new files
checked in or when you sync to head.
If using Windows PowerShell, use the following command instead to set the
output's encoding to UTF-8 (otherwise Clangd will hit "YAML:1:4: error: Got
empty plain scalar" while parsing it).
```
tools/clang/scripts/generate_compdb.py -p out/Release | out-file -encoding utf8 compile_commands.json
```
3. Optional: build chrome normally. This ensures generated headers exist and are
up-to-date. clangd will still work without this step, but it may give errors or
inaccurate results for files which depend on generated headers.
```
ninja -C out/Release chrome
```
4. Use clangd in your favourite editor, see detailed [instructions](
https://clang.llvm.org/extra/clangd/Installation.html#getting-started-with-clangd).
## Index
By default, clangd only knows the files you are currently editing. To provide
project-wide code navigations (e.g. find references), clangd neesds a
project-wide index.
You can pass an **experimental** `--background-index` command line argument to
clangd, clangd will incrementally build an index of Chromium in the background.
Note: the first index time may take hours (for reference, it took 2~3 hours on
a 48-core, 64GB machine).
A full index of Chromium (including v8, blink) takes ~550 MB disk space and ~2.7
GB memory in clangd.
## Questions
If you have any questions, reach out to clangd-dev@lists.llvm.org.
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