Cross compiling to windows using mingw

[emillon]

Hi.

There are several ways to cross-compile ocaml code to windows using mingw:

• either using opam and https://github.com/ocaml-cross/opam-cross-windows
• or using dune on sources directly, possibly with duniverse to fetch them.

On the other hand, nix also has native support for cross-compilation and this overlay exposes some ways to cross compile to, say, aarch64 through pkgsCross.

It seems possible to me to extend that to cross-compiling ocaml to windows using mingw.

My questions are:

• has this every been tried?
• are there known blockers to this approach?

Thanks

[anmonteiro]

i haven't tried it, personally, I can't think of any blockers immediately other than adding the correct incantations to our overlay (right now we only overlay pkgsCross.{musl64,aarch64-multiplatform,aarch64-multiplaform-musl}).

Would you like to try?

[emillon]

Thanks. I've started experimenting in that direction but that gives me a flexdll error for now:

error: builder for '/nix/store/zycsnwgankm00yl9kqdrd4wv5pkl9537-ocaml+flambda-i686-w64-mingw32-5.2.1.drv' failed with exit code 1;
       last 25 log lines:
       > checking for objdir... .libs
       > checking if i686-w64-mingw32-gcc supports -fno-rtti -fno-exceptions... no
       > checking for i686-w64-mingw32-gcc option to produce PIC... -DDLL_EXPORT -DPIC
       > checking if i686-w64-mingw32-gcc PIC flag -DDLL_EXPORT -DPIC works... yes
       > checking if i686-w64-mingw32-gcc static flag -static works... yes
       > checking if i686-w64-mingw32-gcc supports -c -o file.o... yes
       > checking if i686-w64-mingw32-gcc supports -c -o file.o... (cached) yes
       > checking whether the i686-w64-mingw32-gcc linker (i686-w64-mingw32-ld) supports shared libraries... yes
       > checking whether -lc should be explicitly linked in... yes
       > checking dynamic linker characteristics... Win32 ld.exe
       > checking how to hardcode library paths into programs... immediate
       > checking whether stripping libraries is possible... yes
       > checking if libtool supports shared libraries... yes
       > checking whether to build shared libraries... yes
       > checking whether to build static libraries... no
       > checking how to run the C preprocessor... i686-w64-mingw32-gcc -E
       > checking C compiler vendor... mingw-12-0-gcc-14-3
       > checking whether host executables can be run in the build... no
       > ./configure: line 13769: cygpath: command not found
       > checking whether #! works in shell scripts... yes
       > checking whether C compiler accepts -Wold-style-declaration... yes
       > checking for flexdll sources... no
       > checking for flexlink... no
       > checking for flexdll.h... no
       > configure: error: flexdll.h is required for native Win32
       For full logs, run:
         nix-store -l /nix/store/zycsnwgankm00yl9kqdrd4wv5pkl9537-ocaml+flambda-i686-w64-mingw32-5.2.1.drv
error: 1 dependencies of derivation '/nix/store/1d93mmhazbz6f62zv98wmjyswdb16vmi-ocaml5.2.1-ocamlformat-0.27.0-i686-w64-mingw32.drv' failed to build

I'll check how it's done in opam-cross-windows, since I've had success with it a couple years back.

[emillon]

So that error is caused by a faulty --host being passed to ocaml's ./configure, I'll check what's going on.

[emillon]

I'm making some progress, but I don't understand where the value of hostPlatform comes from.

I'm setting this:

let
  pkgs =
    self.makePkgs                                                               
     {                                                                           
       system = "x86_64-linux";                                                  
       config.allowUnsupportedSystem = true;
    };
  xpkgs = pkgs.pkgsCross.mingwW64;
in
xpkgs.ocaml-ng.ocamlPackages_5_3.ocamlformat

But then in the derivation for the compiler, the values I have are:

BUILD = x86_64-unknown-linux-gnu
HOST = x86_64-w64-mingw32
TARGET = x86_64-w64-mingw32

Which describe a compiler built on linux that will run on windows to produce windows binaries.
I don't want that; I'm after a compiler build on linux that will run on linux to produce windows binaries.
In other words, stdenv.hostPlatform should match its buildPlatform and not its targetPlatform.

That's probably only the case for the compiler itself and not the other ocaml packages.

Ongoing interrogations:

• why wasn't this an issue with x86 -> arm cross compilation (or other supported paths)?
• would it be easier to first add support for 5.4? (which has a bunch of patches already merged)

[emillon]

I've posted a branch and a summary at #2091.