.. _cross-compilation: .. meta:: :description: Cross compilation tutorial using Nix :keywords: Nix, cross compilation, cross-compile, Nix Cross compilation ================= When compiling code, we can distinguish between the **build platform**, where the executable is *built*, and the **host platform**, where the compiled executable *runs*. [#]_ **Native compilation** is the special case where those two platforms are the same. **Cross compilation** is the general case where those two platforms are not. Cross compilation needed when the host platform has limited resources (such as CPU) or when it's not easily accessible for development. The Nix community has world-class support for cross compilation, after many years of hard work. .. [#] Terminology for cross compilation platforms differs between build systems. We have chosen to follow `autoconf terminology `_. What's a target platform? ------------------------- There's actually a third platform named the target platform. It matters in cases where you'd like to distribute a compiler binary, as you'd then like to build a compiler on the build platform, compile code on the host plaform and run the final executable on the target platform. Since that's rarely needed, we'll treat the target platform the same as the host. Pinning nixpkgs --------------- To ensure reproducibility of this tutorial as explained in :ref:`the pinning tutorial `: .. code:: shell-session $ NIX_PATH=https://github.com/NixOS/nixpkgs/archive/9420363b95521e65a76eb5153de1eaee4a2e41c6.tar.gz Determining the host platform config ------------------------------------ The build platform is determined automatically by Nix as it can just guess it during the configure phase. The host platform is best determined by running on the host platform: .. code:: shell-session $ bash $(nix-build '' -A gnu-config)/config.guess aarch64-unknown-linux-gnu In case that's not possible (when the host platform is not easily accessible for development), the platform config has to be constructed manually via the following template: .. code:: --- Note that ```` is often ``unknown`` and ```` is optional. There's also no unique identifier for a platform, for example ``unknown`` and ``pc`` are interchangeable (hence it's called config.guess). If you can't install Nix, find a way to run ``config.guess`` (usually comes with the autoconf package) from the OS you're able to run on the host platform. Some other common examples of platform configs: - aarch64-apple-darwin14 - aarch64-pc-linux-gnu - x86_64-w64-mingw32 - aarch64-apple-ios .. note:: macOS/Darwin is a special case, as not the whole OS is open-source. It's only possible to cross compile between ``aarch64-darwin`` and ``x86_64-darwin``. ``aarch64-darwin`` support was recently added, so cross compilation is barely tested. Choosing the host platform with Nix ----------------------------------- Nixpkgs comes with a set of predefined host platforms applied to all packages. It's possible to explore predefined attribute sets via ``nix repl````: .. code:: shell-session $ nix repl '' Welcome to Nix version 2.3.12. Type :? for help. Loading ''... Added 14200 variables. nix-repl> pkgsCross. pkgsCross.aarch64-android pkgsCross.musl-power pkgsCross.aarch64-android-prebuilt pkgsCross.musl32 pkgsCross.aarch64-darwin pkgsCross.musl64 pkgsCross.aarch64-embedded pkgsCross.muslpi pkgsCross.aarch64-multiplatform pkgsCross.or1k pkgsCross.aarch64-multiplatform-musl pkgsCross.pogoplug4 pkgsCross.aarch64be-embedded pkgsCross.powernv pkgsCross.amd64-netbsd pkgsCross.ppc-embedded pkgsCross.arm-embedded pkgsCross.ppc64 pkgsCross.armhf-embedded pkgsCross.ppc64-musl pkgsCross.armv7a-android-prebuilt pkgsCross.ppcle-embedded pkgsCross.armv7l-hf-multiplatform pkgsCross.raspberryPi pkgsCross.avr pkgsCross.remarkable1 pkgsCross.ben-nanonote pkgsCross.remarkable2 pkgsCross.fuloongminipc pkgsCross.riscv32 pkgsCross.ghcjs pkgsCross.riscv32-embedded pkgsCross.gnu32 pkgsCross.riscv64 pkgsCross.gnu64 pkgsCross.riscv64-embedded pkgsCross.i686-embedded pkgsCross.scaleway-c1 pkgsCross.iphone32 pkgsCross.sheevaplug pkgsCross.iphone32-simulator pkgsCross.vc4 pkgsCross.iphone64 pkgsCross.wasi32 pkgsCross.iphone64-simulator pkgsCross.x86_64-embedded pkgsCross.mingw32 pkgsCross.x86_64-netbsd pkgsCross.mingwW64 pkgsCross.x86_64-netbsd-llvm pkgsCross.mmix pkgsCross.x86_64-unknown-redox pkgsCross.msp430 Cross compilation package attribute names are made up, so it isn't always clear what is the corresponding platform config. It's possible to query the platform config using:: nix-repl> pkgsCross.aarch64-multiplatform.stdenv.hostPlatform.config "aarch64-unknown-linux-gnu" In case the host platform you seek hasn't been defined yet: a) `Contribute it upstream `_. b) Pass the host platforms to ``crossSystem`` when importing ````:: nix-repl> (import { crossSystem = { config = "aarch64-unknown-linux-gnu"; }; }).hello «derivation /nix/store/qjj23s25kg4vjqq19vxs4dg7k7h214ns-hello-aarch64-unknown-linux-gnu-2.10.drv» Or using passing it as an argument to ``nix-build``:: $ nix-build '' -A hello --arg crossSystem '{ config = "aarch64-unknown-linux-gnu"; }' Cross compiling for the first time! ----------------------------------- To cross compile a package like `hello `_, pick the platform attribute - ``aarch64-multiplatform`` in our case - and run: .. code:: shell-session $ nix-build '' -A pkgsCross.aarch64-multiplatform.hello ... /nix/store/pzi2h0d60nb4ydcl3nn7cbxxdnibw3sy-hello-aarch64-unknown-linux-gnu-2.10 `Search for a package `_ attribute name to find the one that you're interested in building. Real-world cross compiling of a Hello World example --------------------------------------------------- To show off the power of cross compilation in Nix, let's build our own Hello World program by cross compiling it as static executables to ``armv6l-unknown-linux-gnueabihf`` and ``x86_64-w64-mingw32`` (Windows) platforms and run the resulting executable with `an emulator `_. .. code:: nix { pkgs ? import {} }: let # Create a C program that prints Hello World helloWorld = pkgs.writeText "hello.c" '' #include int main (void) { printf ("Hello, world!\n"); return 0; } ''; # A function that takes host platform packages crossCompileFor = hostPkgs: # Run a simple command with the compiler available hostPkgs.runCommandCC "hello-world-cross-test" {} '' # Wine requires home directory HOME=$PWD # Compile our example using the compiler specific to our host platform $CC ${helloWorld} -o hello # Run the compiled program using user mode emulation (Qemu/Wine) # buildPackages is passed so that emulation is built for the build platform ${hostPkgs.stdenv.hostPlatform.emulator hostPkgs.buildPackages} hello > $out # print to stdout cat $out ''; in { # Statically compile our example using the two platform hosts rpi = crossCompileFor pkgs.pkgsCross.raspberryPi; windows = crossCompileFor pkgs.pkgsCross.mingwW64; } If we build this example and print both resulting derivations, we should see "Hello, world!" for each: .. code:: shell-session $ cat $(nix-build cross-compile.nix) Hello, world! Hello, world! Developer environment with a cross compiler ------------------------------------------- In the :ref:`tutorial for declarative reproducible environments `, we looked at how Nix helps us provide tooling and system libraries for our project. It's also possible to provide an environment with a compiler configured for **cross-compilation to static binaries using musl**. Given we have a ``shell.nix``: .. code:: nix { nixpkgs ? fetchTarball "https://github.com/NixOS/nixpkgs/archive/bba3474a5798b5a3a87e10102d1a55f19ec3fca5.tar.gz" , pkgs ? (import nixpkgs {}).pkgsCross.aarch64-multiplatform }: # callPackage is needed due to https://github.com/NixOS/nixpkgs/pull/126844 pkgs.pkgsStatic.callPackage ({ mkShell, zlib, pkg-config, file }: mkShell { # these tools run on the build platform, but are configured to target the host platform nativeBuildInputs = [ pkg-config file ]; # libraries needed for the host platform buildInputs = [ zlib ]; }) {} And ``hello.c``: .. code:: c #include int main (void) { printf ("Hello, world!\n"); return 0; } We can cross compile it: .. code:: shell-session $ nix-shell --run '$CC hello.c -o hello' cross-compile-shell.nix And confirm it's aarch64: .. code:: shell-session $ nix-shell --run 'file hello' cross-compile-shell.nix hello: ELF 64-bit LSB executable, ARM aarch64, version 1 (SYSV), statically linked, with debug_info, not stripped Next steps ---------- - The `official binary cache `_ has very limited number of binaries for packages that are cross compiled, so to save time recompiling, configure :ref:`a binary cache and CI (GitHub Actions and Cachix) `. - While many compilers in nixpkgs support cross compilation, not all of them do. On top of that, supporting cross compilation is not trivial work and due to many possible combinations of what would need to be tested, some packages might not build. `A detailed explanation how of cross compilation is implemented in Nix `_ can help with fixing those issues. - The Nix community has a `dedicated Matrix room `_ for help around cross compiling.