An OCaml cross-toolchain for Windows and several useful libraries
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README.md

opam-cross-windows

This repository contains an up-to-date Windows toolchain featuring OCaml 4.04.0, as well as some commonly used packages.

The supported build systems are 32-bit and 64-bit x86 Linux. The supported target systems are 32-bit and 64-bit x86 Windows.

If you need support for other platforms or versions, please open an issue.

Prerequisites

On 64-bit Linux build systems, 32-bit libraries must be installed. On Debian derivatives they are provided in the gcc-multilib package.

A C cross-compiler targeting the appropriate Windows platform must be installed. On Debian derivatives they are provided in the gcc-mingw-w64-i686 (for 32-bit x86 Windows targets) or gcc-mingw-w64-x86_64 (for 64-bit x86 Windows targets) packages. Alternatively, the MXE environment can be used.

Installation

Add this repository to OPAM:

opam repository add windows git://github.com/whitequark/opam-cross-windows

On 64-bit build systems, switch to 32-bit compiler when compiling for 32-bit targets:

opam switch 4.04.0+32bit
eval `opam config env`

Otherwise, use a regular compiler; its version must match the version of the cross-compiler:

opam switch 4.04.0
eval `opam config env`

Pin some prerequisite packages that don't yet have fixes merged upstream:

opam pin add ocamlbuild https://github.com/ocaml/ocamlbuild.git
opam pin add topkg https://github.com/whitequark/topkg.git

If desired, request the compiler to be built with flambda optimizers:

opam install conf-flambda-windows

Install the compiler:

opam install ocaml-windows

The compiler version is selected automatically based on the current OPAM switch; either ocaml-windows32 or ocaml-windows64 can be installed in any single one.

Alternatively, specify the path to the C toolchain explicitly:

TOOLPREF32=~/mxe/usr/bin/i686-w64-mingw32.static- opam install ocaml-windows
TOOLPREF64=~/mxe/usr/bin/x86_64-w64-mingw32.static- opam install ocaml-windows

The options have the following meaning:

  • TOOLPREF32 and TOOLPREF64 specify the compiler path prefix. The tools named ${TOOLPREF*}gcc, ${TOOLPREF*}as, ${TOOLPREF*}ar, ${TOOLPREF*}ranlib and ${TOOLPREF*}ld must be possible to locate via PATH.

    The values above are suitable for use with the MXE environment located in ~/mxe after running make gcc.

The TOOLPREF* options are recorded inside the conf-gcc-windows* packages, so make sure to reinstall those if you wish to switch to a different toolchain. Otherwise, it is not necessary to supply them while upgrading the ocaml-windows* packages.

Build some code:

echo 'let () = print_endline "Hello, world!"' >helloworld.ml
ocamlfind -toolchain windows ocamlc helloworld.ml -o helloworld.byte
ocamlfind -toolchain windows ocamlopt helloworld.ml -o helloworld.native

Run it:

wine cmd /c "set PATH=Z:/$(ocamlfind -toolchain windows printconf path)/../bin;%PATH%
             && ./helloworld.byte"
wine ./helloworld.native

Install some packages:

opam install re-windows

Write some code using them:

let () =
  let regexp = Re_pcre.regexp {|\b([a-z]+)\b|} in
  let result = Re.exec regexp "Hello, world!" in
  Format.printf "match: %s\n" (Re.get result 1)

Build it:

ocamlfind -toolchain windows ocamlopt -package re.pcre -linkpkg test_pcre.ml -o test_pcre

Make an object file out of it and link it with your native project (you'll need to call caml_startup(argv) to run OCaml code; see this article):

ocamlfind -toolchain windows ocamlopt -package re.pcre -linkpkg -output-complete-obj test_pcre.ml -o test_pcre.o

Make a DLL out of it:

ocamlfind -toolchain windows ocamlopt -package re.pcre -linkpkg -output-obj -cclib -shared test_pcre.ml -o test_pcre.dll

With opam-windows-cross, cross-compilation is easy!

External dependencies

opam-windows-cross is designed to use native dependencies from the MXE environment. It is possible to automatically install all required dependencies for an OPAM package, e.g. camlbz2-windows, using one short command within an MXE checkout:

make `opam list --short --recursive --external=mxe camlbz2-windows`

Porting packages

OCaml packages often have components that execute at compile-time (camlp4 or ppx syntax extensions, cstubs, OASIS, ...). Thus, it is not possible to just blanketly cross-compile every package in the OPAM repository; sometimes you would even need a cross-compiled and a non-cross-compiled package at once. The package definitions also often need package-specific modification in order to work.

As a result, if you want a package to be cross-compiled, you have to copy the definition from opam-repository, rename the package to add -windows suffix while updating any dependencies it could have, and update the build script. Don't forget to add ocaml-windows as a dependency!

Findlib 1.5.4 adds a feature that makes porting packages much simpler; namely, an OCAMLFIND_TOOLCHAIN environment variable that is equivalent to the -toolchain command-line flag. Now it is not necessary to patch the build systems of the packages to select the Windows toolchain; it is often enough to add ["env" "OCAMLFIND_TOOLCHAIN=windows" make ...] to the build command in the opam file.

For projects using OASIS, the following steps will work:

build: [
  ["env" "OCAMLFIND_TOOLCHAIN=windows"
   "ocaml" "setup.ml" "-configure" "--prefix" "%{prefix}%/windows-sysroot"
                                   "--override" "ext_dll" ".dll"]
  ["env" "OCAMLFIND_TOOLCHAIN=windows"
   "ocaml" "setup.ml" "-build"]
]
install: [
  ["env" "OCAMLFIND_TOOLCHAIN=windows"
   "ocaml" "setup.ml" "-install"]
]
remove: [["ocamlfind" "-toolchain" "windows" "remove" "pkg"]]
depends: ["ocaml-windows" ...]

The output of the configure script will be entirely wrong, referring to the host configuration rather than target configuration. Thankfully, it is not actually used in the build process itself, so it doesn't matter.

For projects installing the files via OPAM's .install files (e.g. topkg), the following steps will work:

build: [["ocaml" "pkg/pkg.ml" "build" "--pinned" "%{pinned}%" "--toolchain" "windows" ]]
install: [["opam-installer" "--prefix=%{prefix}%/windows-sysroot" "pkg.install"]]
remove: [["ocamlfind" "-toolchain" "windows" "remove" "pkg"]]
depends: ["ocaml-windows" ...]

Internals

The aim of this repository is to build a cross-compiler while altering the original codebase in the minimal possible way. (Indeed, only about 50 lines are changed.) There are no attempts to alter the configure script; rather, the configuration is provided directly. The resulting cross-compiler has several interesting properties:

  • All paths to the Windows toolchain are embedded inside ocamlc and ocamlopt; thus, no knowledge of the Windows toolchain is required even for packages that have components in C, provided they use the OCaml driver to compile the C code. (This is usually the case.)
  • The build system makes several assumptions that are not strictly valid while cross-compiling, mainly the fact that the bytecode the cross-compiler has just built can be ran by the ocamlrun on the build system. Thus, the requirement for a 32-bit build compiler for 32-bit targets, as well as for the matching versions.
  • The .opt versions of the compiler are built using itself, which doesn't work while cross-compiling, so all provided tools are bytecode-based.

License

All files contained in this repository are licensed under the CC0 1.0 Universal license.

References

See also ocaml-cross-android and ocaml-cross-ios.