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

opam-cross-android

This repository contains an up-to-date Android 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 x86 and ARM Android.

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.

The compiled toolchain requires about 5G of disk space.

Installation

Add this repository to OPAM:

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

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

Configure the compiler for ARM:

ARCH=arm SUBARCH=armv7 SYSTEM=linux_eabi \
  CCARCH=arm TOOLCHAIN=arm-linux-androideabi-4.9 \
  TRIPLE=arm-linux-androideabi LEVEL=24 \
  STLVER=4.9 STLARCH=armeabi \
  opam install conf-android

Alternatively, configure the compiler for AArch64:

ARCH=arm64 SUBARCH=arm64 SYSTEM=linux_eabi \
  CCARCH=arm64 TOOLCHAIN=aarch64-linux-android-4.9 \
  TRIPLE=aarch64-linux-android LEVEL=24 \
  STLVER=4.9 STLARCH=arm64-v8a \
  opam install conf-android

Alternatively, configure the compiler for x86:

ARCH=i386 SUBARCH=default SYSTEM=linux_elf \
  CCARCH=x86 TOOLCHAIN=x86-4.9 \
  TRIPLE=i686-linux-android LEVEL=24 \
  STLVER=4.9 STLARCH=x86 \
  opam install conf-android

Some options can be tweaked:

  • SUBARCH (on ARM) specifies the ARM architecture version, e.g. armv5te or armv7;
  • SYSTEM (on ARM) specifies the ABI: linux_eabi for soft-float and linux_eabihf for hard-float;
  • LEVEL specifies the Android API level and defaults to latest available API.

The options above (ARCH, SUBARCH, SYSTEM, LEVEL, TOOLCHAIN and TRIPLE) are recorded inside the conf-android package, so make sure to reinstall that package if you wish to switch to a different toolchain. Otherwise, it is not necessary to supply them while upgrading the ocaml-android* packages.

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

opam install conf-flambda-android

Install the compiler:

opam install ocaml-android

Build some code:

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

Install some packages:

opam install re-android

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 android ocamlopt -package re.pcre -linkpkg test_pcre.ml -o test_pcre

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

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

Make a shared object out of it:

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

With opam-android, cross-compilation is easy!

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 -android suffix while updating any dependencies it could have, and update the build script. Don't forget to add ocaml-android 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 Android toolchain; it is often enough to add ["env" "OCAMLFIND_TOOLCHAIN=android" make ...] to the build command in the opam file.

For projects using OASIS, the following steps will work:

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

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}%/android-sysroot" "pkg.install"]]
remove: [["ocamlfind" "-toolchain" "android" "remove" "pkg"]]
depends: ["ocaml-android" ...]

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 Android toolchain are embedded inside ocamlc and ocamlopt; thus, no knowledge of the Android 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.

Acknowledgements

Some of the tricks in this repository were inspired by Jerome Vouillon's opam-android-repository. However, no code was reused.

References

See also opam-cross-windows and opam-cross-ios.