Generate JSON Compilation Database using CMake

When working with C or C++ projects, external tools like clangd, Clang-Tidy, and various IDE features rely on accurate knowledge of how each file in the project is compiled. This is where the Compilation Database comes in. Compilation Database is a JSON file (typically named compile_commands.json) that lists all the compile commands used to build each source file in the project. It includes important details such as compiler options, include paths, and source file locations - everything the compiler needs to reproduce the build for each translation unit. This tutorial explains how to generate JSON compilation database using CMake.

Here's a minimal example CMakeLists.txt that enables generation of the compile_commands.json file:

cmake_minimum_required(VERSION 3.27)
project(myapp)

set(CMAKE_CXX_STANDARD 17)

set(CMAKE_EXPORT_COMPILE_COMMANDS ON)

add_executable(${PROJECT_NAME} main.cpp)

The CMAKE_EXPORT_COMPILE_COMMANDS option tells CMake to generate the compile_commands.json file during configuration.

Run the following command to configure the project and generate the compilation database:

cmake -S . -B build -DCMAKE_BUILD_TYPE=Release

After running this, you'll find a compile_commands.json file inside the build directory.

Here's a sample entry from a generated compile_commands.json:

[
{
  "directory": "/home/ubuntu/myproject/build",
  "command": "/usr/bin/c++   -O3 -DNDEBUG -o CMakeFiles/myapp.dir/main.cpp.o -c /home/ubuntu/myproject/main.cpp",
  "file": "/home/ubuntu/myproject/main.cpp",
  "output": "CMakeFiles/myapp.dir/main.cpp.o"
}
]

This entry describes how the file main.cpp is compiled, including the compiler, options, source path, and output path.