By the way, I use Linux

1998 2008 2018 2022 will be the year of the Linux desktop. Definitely sometimes before 2028. Maybe before 2038? Ok, maybe not. In any case, I am the most comfortable working (and studying, and procrastinating, and gaming, and procrastinating) on Linux (yeah, yeah, GNU/Linux or GNU+Linux, I know). Actually I am a weakling running Debian with the non-tiling WM Xfce4 (I could as well install Xubuntu at this point).

Anyway, the point is that I still need to target Windows as a platform for the 96 % of users who have not yet seen the light. But I do not really want to have to reboot to switch to a different operating system and maintain multiple developing environment. Besides, I need to build my project in the CI/CD pipeline for automatic testing and release building, but I really do not want to deal with a Windows virtual machine in the cloud.

If only I could build for Windows without running Windows…

Cross-Compiling

Well, it’s a thing. Actually, compiling for X without running on X is a thing. It’s even pretty much mandatory when you cannot run a compiler on the target system. So, if you are developing for Arduino, you are most likely compiling from amd64 (Intel and AMD microprocessors) towards avr (Atmel’s microcontrollers). It’s called cross-compiling.

What makes cross-compiling not completely trivial is that the file hierarchy and dependency management are mostly built around the assumption that compiled code will be run on the current system. So, on my computer, binary libraries in /usr/lib are amd64, /usr/bin/gcc compiles to amd64 and apt installs amd64 binaries by default. Debian does support various architectures, so, I can tell apt to install arm64 libraries, and it will install them in /usr/lib/aarch64-linux-gnu/. Binaries of different architectures, however, will conflict: both amd64 and arm64 Firefox will want to be /usr/bin/firefox. But, in any case, I do not want a gcc compiled for arm64! I want a gcc compiled for amd64 that will compile to arm64. For this, I have to install gcc-aarch64-linux-gnu. With this, I can easily set up a working environment where I will be compiling for arm64 from my amd64 computer.

Cross-Compiling to Windows

But wait, I do not want to cross-compile to arm64! I actually want to cross-compile from amd64 to amd64, but for Windows! Does that even make sense? It does! If I install mingw-w64, I get a new gcc at /usr/bin/x86_64-w64-mingw32-gcc (actually a symbolic link to /usr/bin/x86_64-w64-mingw32-gcc-win32). And then:

$ cat hw.c
#include <stdio.h>
int main(void) {
    puts("Hello World!");
}
$ x86_64-w64-mingw32-gcc hw.c
$ file a.exe 
a.exe: PE32+ executable (console) x86-64, for MS Windows

I actually do get a PE file, an executable for Windows!

Good, I can compile code for Windows. Now I need to install my project’s dependencies, such as GLFW3. But there is no such thing as libglfw3-mingw-w64-dev. In fact, there are only a few libraries available for this platform in the Debian repositories:

• libassuan-mingw-w64-dev
• libgcrypt-mingw-w64-dev
• libgpg-error-mingw-w64-dev
• libksba-mingw-w64-dev
• libz-mingw-w64-dev
• libnpth-mingw-w64-dev

For everything else, you’ll need to compile the dependencies yourself and install them in /usr/local/x86_64-w64-mingw32/. Yup, no magic wand here. And of course, you will need to cross-compile them to Windows. And fix the bugs you will encounter while doing so.

Pro-tip: write yourself a script to build and install these dependencies. It will serve as documentation for when someone (probably future you) wants to install the development environment. And you will have to do it one way or another if you want to cross-compile in Docker for your CI/CD pipeline.

Running Windows binaries from Linux

How do we run the “Hello World!” example from earlier? Easy!

$ wine a.exe
[useless warnings]
Hello World!

Yup, that’s it.

Okay, okay, it is still useful to test the Windows build on an actual Windows or, at the very least a Windows VM since it sometimes behave differently.

Anyway, that’s all for today. And don’t think too much about floating point determinism when cross-compiling.