Cross compilation

Dune allows for cross compilation by defining build contexts with multiple targets. Targets are specified by adding a targets field to the definition of a build context.

targets takes a list of target name. It can be either:

  • native which means using the native tools that can build binaries that run on the machine doing the build
  • the name of an alternative toolchain

Note that at the moment, there is no official support for cross-compilation in OCaml. Dune supports the opam-cross-x repositories from the ocaml-cross organization on github, such as:

In particular:

  • to build Windows binaries using opam-cross-windows, write windows in the list of targets
  • to build Android binaries using opam-cross-android, write android in the list of targets
  • to build IOS binaries using opam-cross-ios, write ios in the list of targets

For example, the following workspace file defines three different targets for the default build context:

(context (default (targets (native windows android))))

This configuration defines three build contexts:

  • default
  • default.windows
  • default.android

Note that the native target is always implicitly added when not present. However, when implicitly added dune build @install will skip this context, i.e. default will only be used for building executables needed by the other contexts.

With such a setup, calling dune build @install will build all the packages three times.

Note that instead of writing a dune-workspace file, you can also use the -x command line option. Passing -x foo to dune without having a dune-workspace file is the same as writing the following dune-workspace file:

(context (default (targets (foo))))

If you have a dune-workspace and pass a -x foo option, foo will be added as target of all context stanzas.

How does it work?

In such a setup, binaries that need to be built and executed in the default.windows or default.android contexts as part of the build, will no longer be executed. Instead, all the binaries that will be executed will come from the default context. One consequence of this is that all preprocessing (ppx or otherwise) will be done using binaries built in the default context.

To clarify this with an example, let’s assume that you have the following src/dune file:

(executable (name foo))
(rule (with-stdout-to blah (run ./foo.exe)))

When building _build/default/src/blah, dune will resolve ./foo.exe to _build/default/src/foo.exe as expected. However, for _build/default.windows/src/blah dune will resolve ./foo.exe to _build/default/src/foo.exe

Assuming that the right packages are installed or that your workspace has no external dependencies, dune will be able to cross-compile a given package without doing anything special.

Some packages might still have to be updated to support cross-compilation. For instance if the foo.exe program in the previous example was using Sys.os_type, it should instead take it as a command line argument:

(rule (with-stdout-to blah (run ./foo.exe -os-type %{os_type})))