How do I tell CMake to use Clang on Windows?

Question

I have a C++ project that builds using CMake. I usually build on OSX but now I am trying to get a Windows version working too. I would like to use Clang on Windows for compatibility reasons.

I installed the pre-compiled Clang 3.8 binary from LLVM:

C:\Program Files\LLVM\bin\clang.exe
C:\Program Files\LLVM\bin\clang++.exe

It is also installed on my PATH:

>clang++
clang++.exe: error: no input files

I have two questions:

1. How do I tell CMake to use clang++ when I call cmake --build?
2. How can I check before building which compiler CMake is configured with?

Answer 1

You also need - in addition to the Clang compilers itself - an build/link environment for Windows.

The latest CMake 3.6 builds do have several integrated supported Clang build environments on Windows (e.g. Visual Studio, Cygwin; see Release Notes).

I've just run a successful test with

• LLVM-3.9.0-r273898-win32.exe from http://llvm.org/builds/
• cmake-3.6.0-rc4-win64-x64.msi from https://cmake.org/download/
• Microsoft VS2015 Community Edition Version 14.0.23107.0

All installed to their standard paths with their bin directories in the global PATH environment.

The part you need to know is setting the right toolset with the CMake -T"LLVM-vs2014" command line option. During the configuration process CMake will let you know which compiler it has found/taken.

CMakeLists.txt

cmake_minimum_required(VERSION 3.6)

project(HelloWorld)

file(
    WRITE main.cpp 
        "#include <iostream>\n"
        "int main() { std::cout << \"Hello World!\" << std::endl; return 0; }"
)
add_executable(${PROJECT_NAME} main.cpp)

Windows Console

...> mkdir VS2015
...> cd VS2015
...\VS2015> cmake -G"Visual Studio 14 2015" -T"LLVM-vs2014" ..
-- The C compiler identification is Clang 3.9.0
-- The CXX compiler identification is Clang 3.9.0
-- Check for working C compiler: C:/Program Files (x86)/LLVM/msbuild-bin/cl.exe
-- Check for working C compiler: C:/Program Files (x86)/LLVM/msbuild-bin/cl.exe -- works
-- Detecting C compiler ABI info
-- Detecting C compiler ABI info - done
-- Check for working CXX compiler: C:/Program Files (x86)/LLVM/msbuild-bin/cl.exe
-- Check for working CXX compiler: C:/Program Files (x86)/LLVM/msbuild-bin/cl.exe -- works
-- Detecting CXX compiler ABI info
-- Detecting CXX compiler ABI info - done
-- Detecting CXX compile features
-- Detecting CXX compile features - done
-- Configuring done
-- Generating done
-- Build files have been written to: .../VS2015
...\VS2015> cmake --build . 
Microsoft (R)-Buildmodul, Version 14.0.23107.0
[...]
...\VS2015> Debug\HelloWorld.exe
Hello World!

Installation Hints

Please note that I have added LLVM to my search paths during setup:

And you can crosscheck the available "Platform Toolsets" in any VS project's property page:

References

• What is the -D define to tell Cmake where to find nmake?
• Linker for Clang?
• Switching between GCC and Clang/LLVM using CMake

Answer 2

As an update for Visual Studio 2019 and 2022, you can install the clang-cl toolchain via the Visual Studio Installer and use this to generate a .sln file:

> mkdir build && cd build
> cmake .. -G "Visual Studio 16 2019" -T ClangCL -A x64

or

> mkdir build && cd build
> cmake .. -G "Visual Studio 17 2022" -T ClangCL -A x64

However a much better workflow is to simply open the folder containing the CMakeLists.txt file directly in VS and it will recognise it as a CMake project and provide the ability to set the compiler etc. directly.

The only drawback I could find to this approach is that the Visual Studio Graphics Debugger workflow doesn't work with it, which will affect you if you are developing with DirectX etc.

Answer 3

Follow these instructions:

Install choco if you don't have it: https://chocolatey.org/install

choco install ninja -y
choco install cmake -y
choco install llvm -y

Reset your shell so environment variables are set properly (you can check if bin folders for each are added to your Path).

Using Ninja

From PowerShell

$env:CC="C:\Program Files\LLVM\bin\clang.exe"
$env:CXX="C:\Program Files\LLVM\bin\clang++.exe"
cmake -S ./ -B ./build -G "Ninja-Multi-Config"
cmake --build ./build --config Release

From GUI

Go to your project and run:

cmake-gui .

From the upper menu select Tools/Configure and follow these settings:

Choose "Ninja Multi-Config" and Specify native compilers:

Give the path to the compilers:

Finally, run

cmake --build ./build --config Release

Using Visual Studio

Using GUI

In some folder install llvm-utils:

git clone https://github.com/zufuliu/llvm-utils.git
cd llvm-utils/VS2017
.\install.bat

Go to your project and run:

cmake-gui .

From the upper menu select Tools/Configure and follow these settings:

Choose Visual Studio 2019 and 2nd option (specify native compilers). Set LLVM_v142 for Visual Studio 2019 and above. See here for older versions for others.

Give the path to the compilers:

Finally, run

cmake --build ./build --config Release