Cross Compiling

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For building arm software check out the Article NixOS on ARM
If you are looking for building 32bit software, check out Packaging/32bit Applications
Quick example to cross compile a package: Cheatsheet#Cross-compile_packages.

Cross-Compiling in nixpkgs unstable (pre-18.09)

A lot has happened in 2018 and now cross-compilation is well supported within the unstable branch of nixpkgs. The basic idea is to use pkgsCross.platform instead of pkgs:

nix build -f channel:nixos-unstable pkgsCross.raspberryPi.emacs

How to obtain a shell with a cross compiler

Create a file crossShell.nix as follows:

with import <nixpkgs> {
  crossSystem = {
    config = "aarch64-unknown-linux-gnu";
  };
};

mkShell {
  buildInputs = [ zlib ]; # your dependencies here
}

and then use it to obtain a shell:

nix-shell crossShell.nix

The resulting shell contains a cross toolchain and zlib in this example. Note that contrary to native shells, the compiler and some other tools are prefixed: there is no gcc but a aarch64-unknown-linux-gnu-gcc. Some convenience environment variables expand to the prefixed version of tools: $CC, $LD...

Examples of how to specify your target system can be found in lib/systems/examples.nix. If the exact system you are targeting is available in this file then you can use the existing definition as in the following example:

let pkgs = import <nixpkgs> {
    crossSystem = (import <nixpkgs/lib>).systems.examples.armv7l-hf-multiplatform;
};
in
...

The examples above do not work as is with build dependencies (nativeBuildInputs). A solution is to use callPackage to enable splicing:

let pkgs = import <nixpkgs> {
  crossSystem = {
    config = "aarch64-unknown-linux-gnu";
  };
};
in
  pkgs.callPackage (
    {mkShell, pkg-config, zlib}:
    mkShell {
      nativeBuildInputs = [ pkg-config ]; # you build dependencies here
      buildInputs = [ zlib ]; # your dependencies here
    }
  ) {}

See also #49526.

Lazy cross-compiling

If you target "aarch64-unknown-linux-gnu", there is a nice way to reduce amount of cross-compiling and side-step journey to fix cross errors. The idea is to fetch non-essential dependencies from binary cache of regular aarch64 binaries.

Say we are building SDL2.

let
    # this will use aarch64 binaries from binary cache, so no need to build those
    pkgsArm = import <nixpkgs> {
        config = {};
        overlays = [];
        system = "aarch64-linux";
    };

    # these will be your cross packages
    pkgsCross = import <nixpkgs> {

       overlays = [(self: super: {

         # we want to hack on SDL, don't want to hack on those. Some even don't cross-compile
         inherit (pkgsArm)
           xorg libpulseaudio libGL guile systemd libxkbcommon
         ;
 
       })];
       crossSystem = {
         config = "aarch64-unknown-linux-gnu";
       };
     };

in pkgsCross.SDL2.override { 
      # those shouldn't be neither pkgsCross, nor pkgsArm
      # because those trigger
      #     cannot execute binary file: Exec format error
      # in this case it was enough to just use buildPackages variants
      # but in general there may be problems
      inherit (pkgsCross.buildPackages) 
         wayland wayland-protocols
      ;
   }

How to specify dependencies

Depending in which if packages are required at build time or at runtime they need to go to different inputs the derivation.

• If it is used at build-time it's depsBuildXXX
  • compiler producing native binaries go to depsBuildBuild
  • compiler producing cross binaries, all setup hooks and programs executed by the builder go to depsBuildHost
    • common examples: pkgconfig, autoreconfHook, makeWrapper, intltool, bison, flex
• If it is used at run-time it's depsHostXXX. [Stack linking doesn't effect this, even if it allows us to forget where things came from.]
  • if it’s an interpreter that will be needed by an installed script, it should go in depsHostTarget.
  • otherwise it is probably only needed at build time and can go in depsBuildHost
• If it is a tool and "acts" (e.g. helps build) on build-time stuff, then it's depsXXXBuild
• If it is a tool and "acts" on run-time stuff, then it's depsXXXHost
• if it is not a tool, it's depsXXX(XXX+1)(build + 1 == host, host +1 == target) for backwards compatibility, use nativeBuildInputs instead of depsBuildHost and buildInputs instead of depsHostTarget.

Source: https://github.com/NixOS/nixpkgs/pull/50881#issuecomment-440772499

References

• Nixpkgs manual on cross compiling
• Nixpkgs manual on Specifying dependencies

• Introduction to Cross Compilation with nix by Matthew Bauer

Additional resources

• Slides from the cross-compilation workshop on 35c3
• 2018-08-03 discussion on #nixos (Mirror of chat on Matrix.org)