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Markdown
303 lines
11 KiB
Markdown
---
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myst:
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html_meta:
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"description lang=en": "Cross compilation tutorial using Nix"
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"keywords": "Nix, cross compilation, cross-compile, Nix"
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---
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(cross-compilation)=
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# Cross compilation
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Nixpkgs offers powerful tools to cross-compile software for various system types.
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## What do you need?
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- Experience using C compilers
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- Basic knowledge of the [Nix language](<reading-nix-language>)
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## Platforms
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When compiling code, we can distinguish between the **build platform**, where the executable is *built*, and the **host platform**, where the compiled executable *runs*. [^id3]
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**Native compilation** is the special case where those two platforms are the same.
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**Cross compilation** is the general case where those two platforms are not.
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Cross compilation is needed when the host platform has limited resources (such as CPU) or when it's not easily accessible for development.
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The `nixpkgs` package collection has world-class support for cross compilation, after many years of hard work by the Nix community.
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[^id3]: Terminology for cross compilation platforms differs between build systems.
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We have chosen to follow
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[autoconf terminology](https://www.gnu.org/software/autoconf/manual/autoconf-2.69/html_node/Hosts-and-Cross_002dCompilation.html).
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## What's a target platform?
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There is a third concept for a platform we call a **target platform**.
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The target platform is relevant to cases where you want to build a compiler binary.
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In such cases, you would build a compiler on the *build platform*, run it to compile code on the *host platform*, and run the final executable on the *target platform*.
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Since this is rarely needed, we will assume that the target is identical to the host.
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## Determining the host platform config
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The build platform is determined automatically by Nix during the configure phase.
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The host platform is best determined by running this command on the host platform:
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```shell-session
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$ $(nix-build '<nixpkgs>' -I nixpkgs=channel:nixos-22.11 -A gnu-config)/config.guess
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aarch64-unknown-linux-gnu
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```
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In case this is not possible (for example, when the host platform is not easily accessible for development), the platform config has to be constructed manually via the following template:
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```
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<cpu>-<vendor>-<os>-<abi>
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```
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This string representation is used in `nixpkgs` for historic reasons.
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Note that `<vendor>` is often `unknown` and `<abi>` is optional.
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There's also no unique identifier for a platform, for example `unknown` and `pc` are interchangeable (which is why the script is called `config.guess`).
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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.
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Some other common examples of platform configs:
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- aarch64-apple-darwin14
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- aarch64-pc-linux-gnu
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- x86_64-w64-mingw32
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- aarch64-apple-ios
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:::{note}
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macOS/Darwin is a special case, as not the whole OS is open-source.
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It's only possible to cross compile between `aarch64-darwin` and `x86_64-darwin`.
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`aarch64-darwin` support was recently added, so cross compilation is barely tested.
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:::
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## Choosing the host platform with Nix
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`nixpkgs` comes with a set of predefined host platforms for cross compilation called `pkgsCross`.
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It is possible to explore them in `nix repl`:
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```shell-session
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$ nix repl '<nixpkgs>' -I nixpkgs=channel:nixos-22.11
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Welcome to Nix version 2.3.12. Type :? for help.
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Loading '<nixpkgs>'...
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Added 14200 variables.
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nix-repl> pkgsCross.<TAB>
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pkgsCross.aarch64-android pkgsCross.musl-power
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pkgsCross.aarch64-android-prebuilt pkgsCross.musl32
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pkgsCross.aarch64-darwin pkgsCross.musl64
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pkgsCross.aarch64-embedded pkgsCross.muslpi
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pkgsCross.aarch64-multiplatform pkgsCross.or1k
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pkgsCross.aarch64-multiplatform-musl pkgsCross.pogoplug4
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pkgsCross.aarch64be-embedded pkgsCross.powernv
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pkgsCross.amd64-netbsd pkgsCross.ppc-embedded
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pkgsCross.arm-embedded pkgsCross.ppc64
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pkgsCross.armhf-embedded pkgsCross.ppc64-musl
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pkgsCross.armv7a-android-prebuilt pkgsCross.ppcle-embedded
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pkgsCross.armv7l-hf-multiplatform pkgsCross.raspberryPi
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pkgsCross.avr pkgsCross.remarkable1
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pkgsCross.ben-nanonote pkgsCross.remarkable2
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pkgsCross.fuloongminipc pkgsCross.riscv32
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pkgsCross.ghcjs pkgsCross.riscv32-embedded
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pkgsCross.gnu32 pkgsCross.riscv64
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pkgsCross.gnu64 pkgsCross.riscv64-embedded
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pkgsCross.i686-embedded pkgsCross.scaleway-c1
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pkgsCross.iphone32 pkgsCross.sheevaplug
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pkgsCross.iphone32-simulator pkgsCross.vc4
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pkgsCross.iphone64 pkgsCross.wasi32
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pkgsCross.iphone64-simulator pkgsCross.x86_64-embedded
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pkgsCross.mingw32 pkgsCross.x86_64-netbsd
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pkgsCross.mingwW64 pkgsCross.x86_64-netbsd-llvm
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pkgsCross.mmix pkgsCross.x86_64-unknown-redox
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pkgsCross.msp430
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```
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These attribute names for cross compilation packages have been chosen somewhat freely over the course of time.
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They usually do not match the corresponding platform config string.
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You can retrieve the platform string from `pkgsCross.<platform>.stdenv.hostPlatform.config`:
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```shell-session
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nix-repl> pkgsCross.aarch64-multiplatform.stdenv.hostPlatform.config
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"aarch64-unknown-linux-gnu"
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```
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If the host platform you seek hasn't been defined yet, please [contribute it upstream](https://github.com/NixOS/nixpkgs/blob/master/lib/systems/examples.nix).
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## Specifying the host platform
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The mechanism for setting up cross compilation works as follows:
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1. Take the build platform configuration and apply it to the current package set, called `pkgs` by convention.
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The build platform is implied in `pkgs = import <nixpkgs> {}` to be the current system.
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This produces a build environment `pkgs.stdenv` with all the dependencies present to compile on the build platform.
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2. Apply the appropriate host platform configuration to all the packages in `pkgsCross`.
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Taking `pkgs.pkgsCross.<host>.hello` will produce the package `hello` compiled on the build platform to run on the `<host>` platform.
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There are multiple equivalent ways to access packages targeted to the host platform.
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1. Explicitly pick the host platform package from within the build platform environment:
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```nix
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let
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nixpkgs = fetchTarball "https://github.com/NixOS/nixpkgs/tarballs/release-22.11";
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pkgs = import <nixpkgs> {};
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in
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pkgs.pkgsCross.aarch64-multiplatform.hello
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```
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2. Pass the host platform to `crossSystem` when importing `<nixpkgs>`:
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```nix
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let
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nixpkgs = fetchTarball "https://github.com/NixOS/nixpkgs/tarballs/release-22.11";
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# configure `nixpkgs` such that all its packages are build for the host platform
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pkgs = import nixpkgs { crossSystem = { config = "aarch64-unknown-linux-gnu"; }; };
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in
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pkgs.hello
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```
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Equivalently, you can pass the host platform as an argument to `nix-build`:
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```sh
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$ nix-build '<nixpkgs>' -A hello --arg crossSystem '{ config = "aarch64-unknown-linux-gnu"; }'
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```
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## Cross compiling for the first time
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To cross compile a package like [hello](https://www.gnu.org/software/hello/), pick the platform attribute — `aarch64-multiplatform` in our case — and run:
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```shell-session
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$ nix-build '<nixpkgs>' -I nixpkgs=channel:nixos-22.11 \
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-A pkgsCross.aarch64-multiplatform.hello
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...
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/nix/store/nqy5dlzzkbq6bvz5wknjpb8d64jl7g9x-hello-aarch64-unknown-linux-gnu-2.12.1
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```
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[Search for a package](https://search.nixos.org/packages) attribute name to find the one you're interested in building.
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## Real-world cross compiling of a Hello World example
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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](https://en.wikipedia.org/wiki/Emulator).
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```nix
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let
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nixpkgs = fetchTarball "https://github.com/NixOS/nixpkgs/tarballs/release-22.11";
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pkgs = import <nixpkgs> {};
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# Create a C program that prints Hello World
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helloWorld = pkgs.writeText "hello.c" ''
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#include <stdio.h>
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int main (void)
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{
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printf ("Hello, world!\n");
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return 0;
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}
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'';
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# A function that takes host platform packages
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crossCompileFor = hostPkgs:
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# Run a simple command with the compiler available
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hostPkgs.runCommandCC "hello-world-cross-test" {} ''
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# Wine requires home directory
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HOME=$PWD
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# Compile our example using the compiler specific to our host platform
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$CC ${helloWorld} -o hello
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# Run the compiled program using user mode emulation (Qemu/Wine)
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# buildPackages is passed so that emulation is built for the build platform
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${hostPkgs.stdenv.hostPlatform.emulator hostPkgs.buildPackages} hello > $out
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# print to stdout
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cat $out
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'';
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in {
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# Statically compile our example using the two platform hosts
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rpi = crossCompileFor pkgs.pkgsCross.raspberryPi;
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windows = crossCompileFor pkgs.pkgsCross.mingwW64;
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}
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```
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If we build this example and print both resulting derivations, we should see "Hello, world!" for each:
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```shell-session
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$ cat $(nix-build cross-compile.nix)
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Hello, world!
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Hello, world!
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```
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## Developer environment with a cross compiler
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In the {ref}`tutorial for declarative reproducible environments <declarative-reproducible-envs>`, we looked at how Nix helps us provide tooling and system libraries for our project.
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It's also possible to provide an environment with a compiler configured for **cross-compilation to static binaries using musl**.
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Given we have a `shell.nix`:
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```nix
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let
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nixpkgs = fetchTarball "https://github.com/NixOS/nixpkgs/tarballs/release-22.11";
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pkgs = (import nixpkgs {}).pkgsCross.aarch64-multiplatform;
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in
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# callPackage is needed due to https://github.com/NixOS/nixpkgs/pull/126844
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pkgs.pkgsStatic.callPackage ({ mkShell, zlib, pkg-config, file }: mkShell {
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# these tools run on the build platform, but are configured to target the host platform
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nativeBuildInputs = [ pkg-config file ];
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# libraries needed for the host platform
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buildInputs = [ zlib ];
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}) {}
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```
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And `hello.c`:
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```{code-block} c hello.c
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#include <stdio.h>
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int main (void)
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{
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printf ("Hello, world!\n");
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return 0;
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}
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```
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We can cross compile it:
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```shell-session
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$ nix-shell --run '$CC hello.c -o hello' cross-compile-shell.nix
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```
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And confirm it's aarch64:
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```shell-session
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$ nix-shell --run 'file hello' cross-compile-shell.nix
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hello: ELF 64-bit LSB executable, ARM aarch64, version 1 (SYSV), statically linked, with debug_info, not stripped
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```
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## Next steps
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- The [official binary cache](https://cache.nixos.org) has a limited number of binaries for packages that are cross compiled, so to save time recompiling, configure {ref}`your own binary cache and CI with GitHub Actions <github-actions>`.
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- While many compilers in Nixpkgs support cross compilation, not all of them do.
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Additionally, 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.
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[A detailed explanation how of cross compilation is implemented in Nix](https://nixos.org/manual/nixpkgs/stable/#chap-cross) can help with fixing those issues.
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- The Nix community has a [dedicated Matrix room](https://matrix.to/#/#cross-compiling:nixos.org) for help with cross compiling.
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