Cross Compiling: Difference between revisions

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Cross compiling is well supported in Nixpkgs since 18.09^{[citation needed]}.

Getting Started

There are two main entry points to configure a Nixpkgs instance for cross compilation. The simplest one is to leverage pkgs.pkgsCross:

let
  pkgs = import <nixpkgs> {};
in pkgs.pkgsCross.aarch64-multiplatform.hello

The above will provide a derivation result for the hello derivation that can run on an aarch64 system. A slightly less simple, but "purer" (i.e. reproducible) snippet involves configuring Nixpkgs as you import it:

let
  pkgs = import <nixpkgs> {
    # localSystem will always default to your current system, meaning it is not essential to define here.
    # It is however, encouraged if you are trying to follow nix's purity model.

    localSystem = "x86_64-linux"; # This should match the system you are compiling from (your running system)
    crossSystem = "aarch64-linux"; # This should match the system you are compiling for (your target system)
  };
in pkgs.hello # The hello derivation will be built for aarch64-linux

You can perform the same operations using the CLI, though you will probably prefer to use pkgsCross in that case, as its less verbose:

nix-build '<nixpkgs>' -A pkgsCross.aarch64-multiplatform.hello # nix-legacy
nix build nixpkgs#pkgsCross.aarch64-multiplatform.hello # nix3

All of the above snippets will resolve to the exact same derivation result, which will provide a binary for GNU Hello that can execute only on an aarch64 system. There are many other systems pkgsCross has defined, you can see an exhaustive list of all of them on your system:

$ nix-instantiate --eval --expr 'builtins.attrNames (import <nixpkgs> {}).pkgsCross' --json | nix-shell -p jq --command 'jq' # nix-legacy
$ nix eval --impure --expr 'builtins.attrNames (import <nixpkgs> {}).pkgsCross' --json | nix run nixpkgs#jq # nix3

If you instead prefer to write your systems directly, through localSystem and crossSystem, you can refer to nixpkgs/lib/systems/examples.nix for a list of platforms exposed as attributes (though it will be easier to use pkgsCross in this case). These can be directly used in-place for the aforementioned arguments:

let
  lib = import <nixpkgs/lib>;
  pkgs = import <nixpkgs> {
    crossSystem = lib.systems.examples.aarch64-multiplatform;
  };
in pkgs.hello

Development

Using the same ideas as above, we can create development environments which provide us with a compilation suite that can perform cross-compilation for us. A very simple development shell (colloquially called a "devshell") can be written as:

# shell.nix
{
  pkgs ? import <nixpkgs> {
    # localSystem = "x86_64-linux"; # not necessary but encouraged
    crossSystem = "aarch64-linux";
  },
}:
pkgs.mkShell {
  # By default this provides gcc, ar, ld, and some other bare minimum tools
}

Entering this development shell via nix-shell shell.nix will add the relevant compiler tools to your PATH temporarily. Similar to other Linux systems, all cross-compiling tools are prefixed with relevant platform prefixes, which means simply typing gcc will not work. However, the provided mkShell will introduce environment variables for your devshell, such as $CC, $AR, $LD, and more. At the time of writing, official documentation on an exhaustive list of these variables does not exist, but you can view them for your devshell through the command-line:

$ $EDITOR $(nix-build ./shell.nix) # opens your EDITOR with a massive bash script full of declare -x ...

Given these environment variables, you can run compile your software using the exact same commands with fairly minimal changes (changing hardcoded gcc values into $CC, for example):

$ $CC -o main src/main.c
$ file main
main: ELF 64-bit LSB executable, ARM aarch64, version 1 (SYSV), dynamically linked, interpreter /nix/store/qa51m8r8rjnigk5hf7sxv0hw7qr7l4bc-glibc-aarch64-unknown-linux-gnu-2.39-52/lib/ld-linux-aarch64.so.1, for GNU/Linux 3.10.0, not stripped

The above snippet will have minor differences depending on when you run it, but the main thing to notice is ARM aarch64, which tells us our software was able to successfully cross compile.

Declaring dependencies

If you try to declare build-time dependencies within the devshell (such as pkgs.cmake), you will quickly realize that these derivations are actually being built for the crossSystem, making them unusable on your system architecture (see #49526). There are ways around this, but in general once you've gotten to this point you should prefer writing a derivation, which will make it not only easier to write both derivations, but will allow you to follow the recommended practices for using Nix.

If you would prefer to continue building within the devshell, you can use callPackage, which will magically resolve the dependencies for the correct architecture, provided you place them in the correct attributes:

# shell.nix
{
  pkgs ? import <nixpkgs> {
    crossSystem = "aarch64-linux";
  },
}:
pkgs.callPackage (
  {
    mkShell,

    pkg-config,
    libGL,
  }:
  mkShell {
    # Derivations that must run on the localSystem, referred to as the buildPlatform.
    nativeBuildInputs = [
      pkg-config
    ];

    # Derivations that must link with the crossSystem, referred to as the targetPlatform.
    buildInputs = [
      libGL
    ];
  }
) {}

The above snippet will drop you into a devshell that provides pkg-config as a native binary (accessible through $PKG_CONFIG), while also allowing linking to a valid libGL for the crossSystem.

For more information regarding the above, namely the usage of nativeBuildInputs and buildInputs, see stdenv dependencies for a in-depth explanation. Alternatively, a simplified explanation can be found in a comment on the nixpkgs repo.

Tips and tricks

Executing cross compiled binaries

By using QEMU, we can natively execute a cross-compiled binary through an emulation layer. This will result in degraded performance but is very suitable for testing the functionality of a binary.

If you are on NixOS, this functionality can be provided automatically on any cross-compiled binary by setting boot.binfmt.emulatedSystems in your configuration. After rebuilding, attempting to run a cross-compiled binary will automatically invoke qemu indirectly.

$ ./result
Hello World!
$ ./result-aarch64-linux
Hello World!

Otherwise, you can use the pkgs.qemu-user to download qemu user space programs (or use any installed by your distro) to run your package easily.

$ ./result
Hello World!
$ qemu-aarch64 ./result-aarch64-linux
Hello World!

Leveraging the binary cache

You will likely have noticed that resolving derivations through either pkgsCross or a configured Nixpkgs instance results in your system needing to build the binary. This is because cross-compiled binaries are not cached on the official binary cache. However, for some systems, natively compiled binaries are provided. At the time of writing, it only supports aarch64-linux, aarch64-darwin, i686-linux, x86_64-linux, and x86_64-darwin. As a result, this section is only applicable to a very small number of cross-compilation situations.

You can leverage the binary cache to correctly substitute some applicable derivations without causing a local build.

Please note that this is not recommended, as it hacks around some internal details of Nixpkgs which are subject to change at any time.

An example of this using pkgs.SDL2:

let
  # this will use aarch64 binaries from binary cache, so no need to build those
  pkgsArm = import <nixpkgs> {
    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 = "aarch64-linux";
  };
in pkgsCross.SDL2.override { 
  # These should 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
    ;
}