Skip to content

Project Setup

Joel Hock edited this page Apr 9, 2019 · 7 revisions

ccls typically indexes an entire project. In order for this to work properly, ccls needs to be able to obtain the source file list and their compilation command lines.

There are two main ways this happens:

  1. Provide compile_commands.json at the project root
  2. Provide a .ccls file. It is a line-based text file describing compiler flags. Recursively listed source files (headers excluded) will be indexed.

If neither exists, then when ccls starts it will not index anything: instead it will wait for LSP clients to open files and index only those files.


Guillaume Papin(@Sarcasm) has a thorough article about compilation databases.

Generally this file is not checked into source control, but rather is generated by the build system. This means it's best to generate it in a new project before starting the ccls server in that project.

Because this file is auto-generated it's not easy to customize. As a result it's possible to provide both compile_commands.json and .ccls in the same project and have the .ccls configuration enhance the options from compile_commands.json.

If your compile_commands.json is not kept in the project root, set the initialization option compilationDatabaseDirectory to an alternative directory containing compile_commands.json.


% ln -s Debug/compile_commands.json

Caveat on Windows: CMake dumps Windows shell command line directly into command, depends on how ccls was built, it may confuse this field as command line from a POSIX shell, in which Windows path separator '\' is a escape character. You can use jq to convert such entries to use arguments which does not have this issue:

jq '[.[] | {directory: .directory, file: .file, arguments: .command | split(" ") | map(select(length > 0)) | map(sub("\\\\\""; "\""; "g"))}]' < compile_commands.json

Build EAR

Bear is a tool that generates a compilation database for clang tooling. It can be used for any project based on Makefile.

bear make
# generates compile_commands.json



scan-build is a python package that can generate a compilation database for clang tooling (uses Bear as a backend). This too can be used for any project based on a Makefile.

intercept-build make all # generates compile_commands.json from the `make all` ruleset


# Format: ninja -t compdb rule_names... > compile_commands.json
ninja -C out/Release -t compdb cxx cc > compile_commands.json


Load the clang_compilation_database tool in your wscript:

def configure(conf):
./waf configure build
ln -s build/compile_commands.json


buck build :helloworld#compilation-database
ln -s $(buck targets --show-output :helloworld#compilation-database | cut -d ' ' -f 2)

stdout of an external command

If the initialization option "compilationDatabaseCommand" is set, the command will be executed by ccls to provide the JSON compilation database. ccls will read its stdout rather than read compile_commands.json. This may be useful when ccls cannot parse the compile_commands.json correctly (e.g. MSVC cl.exe, Intel C++ Compiler options)

ccls shell script wrapper:

/path/to/Release/ccls --init='{"compilationDatabaseCommand":"/tmp/c/x"}' "$@"


# cat >> /tmp/initialization-options # stdin is initialization options
print '[{"arguments":["c++","-c",""],"directory":"/tmp/c","file":""}]'

Suppose the project is at /tmp/c, /tmp/c/x /tmp/c will be executed with stdin=initializationOptions and the stdout should be a JSON compilation database.

An example to scrub Intel C++ Compiler options (or, even easier, check out clang.excludeArgs in the Initialization options):

#!/usr/bin/env python3
import json
import os
import sys
with open(os.path.join(sys.argv[1], 'compile_commands.json')) as f:
    db = json.load(f)
    for entry in db:
        args = entry['arguments']
            # Intel C++ Compiler option that is unknown to clang
        except ValueError:
    json.dump(db, sys.stdout)

.ccls File

.ccls is a line-based text file at the project root. Its main function is to specify compiler flags needed to properly index your code: -I -D etc. Each line consists of one argument to be added to the compiler command line. No whitespace splitting is performed on the argument, thus -I foo cannot be used (use -Ifoo or -I\nfoo for example).

Subdirectories of the project can also contain .ccls files, if needed, to specify compiler flags specific to those directories.

A line may optionally start with one or more % directives, which specialize the argument on that line.

Available directives include:


By default .ccls compiler flags are applied only to files not listed in compile_commands.json. If this directive appears first in .ccls then after compile_commands.json is parsed, the rest of the .ccls arguments will be appended to the compiler flags for files found in compile_commands.json.

%c / %cpp / %objective-c / %objective-cpp

This argument should be added only when parsing C (%c), C++ (%cpp), Objective-C (%objective-c), or Objective-C++ (%objective-c++) files.

%h / %hpp

This argument should be added only when indexing C header files (%h: *.h) or C++ header files (%hpp: *.hh *.hpp). Note, *.h files are considered as C, not C++.

You may add these lines to make every *.h parsed as C++:

%h -x
%h c++-header

Note, if your project has both C and C++ files, a.h's flags may be inferred from a C file and thus parsed as C. You may run into parsing errors like unknown type name 'class'.

Compiler driver

The compiler driver (the first line unless %compile_commands.json is used) can usually just be clang. clang++ is usually unnecessary, and incorrect if some files are C.

Note that clang and clang++ are different, but the difference is only related to linking (what default runtime libraries are passed) and is not relevant for the frontend actions ccls performs.

.ccls examples

Example A

%c -std=c11
%cpp -std=c++2a
%h %hpp --include=Global.h

*.h *.hh *.hpp files will be parsed with extra --include=Global.h

Example B

%c -std=c11
%cpp -std=c++14
%c %cpp -pthread
%h %hpp --include=Global.h

It appends flags so clang should not be used.

Example: -march=armv7a

See If the compiler driver is a GCC cross-compiler, --target= may be required. Suppose arm-linux-gnueabi-gcc -march=armv7a is used, add a --target=:


Otherwise clang will error: unknown target CPU 'armv7a'.

compile_commands.json examples

Linux kernel

wget '' -O .config
yes '' | make config
bear make -j bzImage modules
ccls -index ~/Dev/Linux -init='{"clang":{"excludeArgs":[
], "extraArgs":["--gcc-toolchain=/usr"]}}'


mkdir Debug; cd Debug
../configure --enable-optimize=no --enable-debug --prefix=~/.local/stow/musl
bear make -j
cd ..; ln -s Debug/compile_commands.json
You can’t perform that action at this time.