Clangd is a language server for C++/C etc. On CentOS 7, I try to use clangd 12 for code auto-completion. Clangd can be easily installed via its binary release.

# My failed attempt to use clang binary release

When I run clangd --version, I see the following error:

clangd_12.0.0/bin/clangd: /lib64/libc.so.6: version GLIBC_2.18' not found (required by clangd_12.0.0/bin/clangd)

The reason is that the shared library libc.so is too old for clangd to run. We need to use a newer version of glibc (at least glibc-2.18).

I tried to clone glibc and build version 2.20 from scratch:

git clone https://mirrors.tuna.tsinghua.edu.cn/git/glibc.git
cd glibc
git checkout glibc-2.20

cd ..
# Note that glibc requires out-source building, i.e., we are forbidden to build glibc inside its source directory.
mkdir glibc-build
../glibc/configure --prefix=$HOME/local make -j 16 make install  I add $HOME/local/lib to LD_LIBRARY_PATH. Then things go badly wrong. Every executable I use segfaults immediately. For example, when I use ls, it will print out:

zsh: segmentation fault (core dumped) ls -F –color=auto

According to this post, this is caused by version mismatch: the glibc version requried to run the executable is different from what is provided right now.

I tried the glibc-6.18 release tar downloaed from here. The same errors happens when I add \$HOME/local/lib to LD_LIBRARY_PATH and run clang.

# Build clangd from source

Maybe we need to build llvm project to use clangd. Here is how to compile llvm from source:

git clone --depth=1 https://github.com/llvm/llvm-project.git

cd llvm-project
mkdir build && cd build
cmake -G "Unix Makefiles" -DLLVM_ENABLE_PROJECTS="clang;clang-tools-extra" -DCMAKE_INSTALL_PREFIX=~/tools/llvm -DCMAKE_BUILD_TYPE=Release ../llvm
make -j 16
make install


Some explanation about the options used:

• -DLLVM_ENABLE_PROJECTS="clang;clang-tools-extra": it specifies which project we want to build. To use clangd and clang-tidy, clang-tools-extra is a must.
• -DCMAKE_INSTALL_PREFIX=~/tools/llvm: it specifies where we want to install llvm.
• -DCMAKE_BUILD_TYPE=Release: it specifies the build type. Release type is smaller than Debug.

For more detail on Cmake options when building llvm, check here.

Since llvm is a huge project, it may take a long time to build. It takes 37m to build on my server with 16 concurrent processes.

This time, clangd works without errors. After installing llvm, do not forget to add llvm binary to your PATH variable.

# How to use clangd

According to clangd documentation, you can either generate a compile_commands.json file for your project or use compile_flags.txt.

For simple project, using compile_flags.txt is sufficient. Your compile flags are written one per line. A sample compile_flags.txt looks like this:

-Wall
-std=c++11
-L/usr/local/lib
-I/usr/local/include
-lfmt
`