compile_commands.json with CMake

Kent - April 20, 2024

compile_commands.json is a file capturing the exact commands files have been compiled with, regardless of the build system. It captures the compiler, source file, include path and definitions used to build a certain source file. With this information, external tools can “rebuild” the source file as it is. It is very useful for source code tools like and clang-tidy.

With CMake, only the generators Ninja and Makefiles will generate compile_commands.json. Any other generator will silently ignore it.

Since the feature is controlled by an environment flag, there are 3 ways of telling CMake that is should create that file.

Using the environment block in CMakePresets.json.
Permanently alter your environment with .bashrc or Windows User Environment.
Invoke CMake with -D and or export/set.

Number 1. is by far the easiest and most portable solution. The other will not be transferred to other systems automatically.

A minimal CMakePresets.json with support for vcpkg with automatic bootstrapping. The important bit to enable compile_commands.json is CMAKE_EXPORT_COMPILE_COMMANDS.

If vcpkg is not needed, remove the linje with toolChainFile.

{
  "version": 6,
  "cmakeMinimumRequired": {
    "major": 3,
    "minor": 25,
    "patch": 0
  },
  "configurePresets": [
    {
      "name": "default",
      "hidden": true,
      "displayName": "Default Parent Configuration",
      "binaryDir": "${sourceDir}/build-cmake/${presetName}",
      "toolchainFile": "${sourceDir}/vcpkg/scripts/buildsystems/vcpkg.cmake",
      "installDir": "${sourceDir}/build-cmake/${presetName}-install",
      "environment": {
        "CMAKE_EXPORT_COMPILE_COMMANDS": "1"
      }
    },
    {
      "name": "linux",
      "inherits": "default",
      "cacheVariables": {
        "VCPKG_TARGET_TRIPLET": "x64-linux"
      },
      "generator": "Unix Makefiles",
      "condition": {
        "type": "equals",
        "lhs": "${hostSystemName}",
        "rhs": "Linux"
      }
    },
    {
      "name": "darwin",
      "inherits": "default",
      "cacheVariables": {
        "VCPKG_TARGET_TRIPLET": "x64-linux"
      },
      "generator": "Unix Makefiles",
      "condition": {
        "type": "equals",
        "lhs": "${hostSystemName}",
        "rhs": "Darwin"
      }
    }
  ]
}

Minimal CMakeLists.txt:

cmake_minimum_required (VERSION 3.25.0)
project (compile_commands_example)
set(CMAKE_CXX_STANDARD 23)
set(CMAKE_CXX_STANDARD_REQUIRED ON)

add_executable(compiler-cmd compiler-cmd.cpp)

Minimal C++23 source file:

#include <print>

auto main() -> int
{
    std::print("foo {:d}\n\n", 42);
    return 0;
}

Configure with cmake --preset linux or cmake --preset darwin.

Program output:

foo 42

Example output from compile_commands.json (truncated) built on a Mac:

[
{
  "directory": "/Users/kent/src/build-cmake/darwin",
  "command": "/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/c++   -std=gnu++2b -arch arm64 -isysroot /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX14.5.sdk -o CMakeFiles/compiler-cmd.dir/compiler-cmd.cpp.o -c /Users/kent/src/compiler-cmd.cpp",
  "file": "/Users/kent/src/compiler-cmd.cpp",
  "output": "CMakeFiles/compiler-cmd.dir/compiler-cmd.cpp.o"
}
]

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compile_commands.json with CMake

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