Clang C++ Cross Compiler - Generating Windows Executable from Mac OS X

Here are step-by-step instructions for building a Hello World .exe using llvm/clang on Mac OS X.

Cross-compile Hello World for Windows using Clang/LLVM on Mac OS X

Install llvm with homebrew. This will include the clang and the llvm linker.

brew install llvm

You'll need access to Visual Studio C++ libraries and headers, which are available through Visual Studio 2017 on a Windows 10 Virtual Machine (VM) or on a Windows 10 Computer. Install Visual Studio on Window, and include the following 'Individual Components' through the Visual Studio Installer:

• Windows Universal CRT SDK
• Windows Universal C Runtime
• Windows 10 SDK (X.X.X.X) for UWP: C++
• VC++ 2017 vXXX toolset (x86,x64)
• Visual C++ 2017 Redistributable Update
• C++/CLI support

Get access to the MSVC libraries and headers from your Mac.

• (Option 1) Use your Windows VM and create a shared folder between host and guest.
• (Option 2) Create a remote share on your Windows computer and connect to it from your Mac.
• (Option 3) Copy the libraries and headers to your Mac, following any licensing terms.

Find the specific directories on your llvm+MSVC install that correspond to the following:

// LLVM:INCLUDES: /usr/local/Cellar/llvm/5.0.0/include// MSVC:INCLUDES: "C:\Program Files (x86)\Microsoft Visual Studio\2017\Community\VC\Tools\MSVC\14.11.25503\include"LIBS: "C:\Program Files (x86)\Microsoft Visual Studio\2017\Community\VC\Tools\MSVC\14.11.25503\lib\x86"// C Runtime Library (CRT):INCLUDES: "C:\Program Files (x86)\Windows Kits\10\Include\10.0.15063.0\ucrt"LIBS: "C:\Program Files (x86)\Windows Kits\10\Include\10.0.15063.0\ucrt"// User-Mode Library (UM):INCLUDES: "C:\Program Files (x86)\Windows Kits\10\Include\10.0.15063.0\um"LIBS: "C:\Program Files (x86)\Windows Kits\10\Lib\10.0.15063.0\um\x86"// 'Shared' includes:INCLUDES: "C:\Program Files (x86)\Windows Kits\10\Include\10.0.15063.0\shared"// WinRT includes:INCLUDES: "C:\Program Files (x86)\Windows Kits\10\Include\10.0.15063.0\winrt"// Figure out your MSC 'version', e.g.Visual C++ 2012 (11.0)   -->     MSC_VER=1700Visual C++ 2013 (12.0)   -->     MSC_VER=1800Visual C++ 2015 (14.0)   -->     MSC_VER=1900Visual C++ 2017 (15.0)   -->     MSC_VER=1910

Create your Hello World src:

// hello.cc#include <cstdio>int main(int argc, char* argv[]) {  printf("Hello, World!\n");  return 0;}

Compile with clang:

clang -target i686-pc-win32 \  -fms-compatibility-version=19 \  -fms-extensions \  -fdelayed-template-parsing \   -fexceptions \  -mthread-model posix \  -fno-threadsafe-statics \  -Wno-msvc-not-found \  -DWIN32 \  -D_WIN32 \  -D_MT \  -D_DLL \  -Xclang -disable-llvm-verifier \  -Xclang '--dependent-lib=msvcrt' \  -Xclang '--dependent-lib=ucrt' \  -Xclang '--dependent-lib=oldnames' \  -Xclang '--dependent-lib=vcruntime' \  -D_CRT_SECURE_NO_WARNINGS \  -D_CRT_NONSTDC_NO_DEPRECATE \  -U__GNUC__ \  -U__gnu_linux__ \  -U__GNUC_MINOR__ \  -U__GNUC_PATCHLEVEL__ \  -U__GNUC_STDC_INLINE__  \  -I/usr/local/Cellar/llvm/5.0.0/include \  -I/c/Program\ Files\ (x86)/Microsoft\ Visual\ Studio/2017/Community/VC/Tools/MSVC/14.11.25503/include \  -I/c/Program\ Files\ (x86)/Windows\ Kits/10/Include/10.0.15063.0/ucrt \  -I/c/Program\ Files\ (x86)/Windows\ Kits/10/Include/10.0.15063.0/shared \  -I/c/Program\ Files\ (x86)/Windows\ Kits/10/Include/10.0.15063.0/winrt \  -c hello.cc -o hello.o

Link with the lld linker, driven by clang:

clang -fuse-ld=lld -target i686-pc-win32 -Wl,-machine:x86 -fmsc-version=1900 \  -o hello.exe hello.o \  -L/external/code8-cc/cc/msvctoolchain/x86/lib/msvc \  -L/external/code8-cc/cc/msvctoolchain/x86/lib/um \  -L/code8-cc/cc/msvctoolchain/x86/lib/ucrt  -nostdlib -lmsvcrt -Wno-msvc-not-found 

Copy hello.exe to your Windows computer or Windows VM and run in PowerShell:

.\hello.exe

To build 64-bit versions, change to '-target x86_64-pc-win32', '-Wl,-machine:x64', and link to x64 libraries.

Clang can in principle be used as a cross compiler: unlike most compilers clang/LLVM includes components (such as the codegen, assembler, and linker) for different platforms in the same binary.

However you'll run into a number of problems trying to use it as such in a production capacity:

• You need platform libraries and headers. To generate an executable that will work on Windows you need Windows headers and Windows libraries you want to link to, either import libs if you're dynamically linking or static libs for static linking. You should be able to get these from an installation of Visual Studio.

• Many C++ features such as name mangling and RTTI support are not complete on Windows. You'd have these same problems compiling for Windows on Windows with Clang. Windows C++ support is pretty much complete these days.

• The LLVM project includes the lld linker, which is apparently far enough along that it can self host on x86 Windows and so might work for you as a cross-platform linker, however lld is not yet a standard part of clang distributions. Clang on OS X still uses the OS X platform linker ld by default as does Clang on Windows (link.exe). You'll need to get lld and figure out how to link with it, or find some other cross-platform linker.

• The clang driver isn't written as a cross-platform compiler driver. You'll likely have to do a lot more hands-on work to run a cross-platform compilation. Take a look at the output of clang -###: the clang driver constructs that command for you, but you may have to do much of the same work as the clang driver by hand. And since clang gets much less testing at cross-platform compilation you're likely to run into more bugs.

• Xcode is not going to help you with any of this. It can configure clang to build for OS X or iOS, but you'll have to manually configure cross-platform builds to Windows.

I'm relatively confident that one could cobble together an LLVM based environment to build a C "Hello, World" Windows exe on OS X or Linux, but it's not quite ready for Xcode to add a "Windows" item to the list of possible target platforms.

If you're not a compiler developer you're probably best off just copying your source code to a Windows machine and building with Visual Studio. If you are, or want to be, a compiler developer then by all means, help push Clang's cross-compilation abilities forward. I think the Clang universal driver project is exciting and I would really like to see progress continue.

I've successfully done the opposite cross-compilation: compiling a Mac OS X executable on Windows. This turned out to be quite easy to do manually on a small program, i.e. directly compiling a .cpp file.

First, Mac OS X development tools come with "SDKs" which contain all the system libraries and headers for a particular OS. The largest challenge here was figuring out how to transfer the SDK to Windows while preserving all the symbolic links in the SDK. (For some reason creating symbolic links on Windows requires elevated privileges, so after producing a tar.gz on OS X with the symbolic links I had to run 7zip on Windows as an administrator to correctly expand the archive.)

Once the SDK is available on Windows there's a single flag to tell clang where to get all the system dependencies: -isysroot. This combined with the -target flag were all that I needed to tell clang how to produce complete object files for OS X.

For linking I manually used lld, as the compiler driver didn't seem support using cross linking with lld. lld supports similar flags for determining the target system libraries.

The final step was simply copying the produced executable to an OS X machine, enabling the execute permission (Windows doesn't support the same file permissions so the execute bit doesn't get set when building) and running the result.

Consider using MinGW on Mac OS X to compile Windows binaries. Here are instructions on how to do it on Linux: http://www.blogcompiler.com/2010/07/11/compile-for-windows-on-linux/

You'll have to adapt them for Mac OS X and you may have to compile MinGW yourself.

http://www.mingw.org