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Merge pull request #160 from nix-dot-dev/cross-compilation-tutorial
Tutorial for cross-compilation
This commit is contained in:
Domen Kožar
GitHub
commit
ec9298d348
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@ -383,6 +383,8 @@ linkcheck_ignore = [
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r'https://app.terraform.io',
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# Seems like README anchors aren't parsable?
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r'https://github.com/cachix/install-nix-action',
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# It's dynamic
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r'https://matrix.to'
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]
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# Anchors are not present in HTML
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@ -14,6 +14,7 @@ Welcome to nix.dev
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- Installing software from source code.
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- :ref:`Transparent build caching using binary caches <github-actions>`.
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- Strong support for software auditability.
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- First-class cross compilation support.
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- Remote builds.
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- Remote deployments.
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- Atomic upgrades and rollbacks.
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291
source/tutorials/cross-compilation.rst
Normal file
291
source/tutorials/cross-compilation.rst
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@ -0,0 +1,291 @@
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.. _ref-cross-compilation:
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Cross compilation
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=================
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When compiling code, we can distinguish between the **build platform**, where the executable
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is *built*, and the **host platform**, where the compiled executable *runs*. [#]_
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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 needed when the host platform has limited resources (such as CPU)
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or when it's not easily accessible for development.
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The Nix community has world-class support for cross compilation,
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after many years of hard work.
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.. [#] 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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-------------------------
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There's actually a third platform named the target platform.
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It matters in cases where you'd like to distribute a compiler binary,
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as you'd then like to build a compiler on the build platform, compile code on the
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host plaform and run the final executable on the target platform.
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Since that's rarely needed, we'll treat the target platform the same as the host.
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Pinning nixpkgs
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---------------
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To ensure reproducability of this tutorial as explained in :ref:`the pinning tutorial <pinning-nixpkgs>`:
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.. code:: shell-session
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$ NIX_PATH=https://github.com/NixOS/nixpkgs/archive/9420363b95521e65a76eb5153de1eaee4a2e41c6.tar.gz
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Determining the host platform config
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------------------------------------
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The build platform is determined automatically by Nix
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as it can just guess it during the configure phase.
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The host platform is best determined by running on the host platform:
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.. code:: shell-session
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$ bash $(nix-build '<nixpkgs>' -A gnu-config)/config.guess
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aarch64-unknown-linux-gnu
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In case that's not possible (when the host platform is not easily accessible
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for development), the platform config has to be constructed manually via the following template:
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.. code::
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<cpu>-<vendor>-<os>-<abi>
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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
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``pc`` are interchangeable (hence it's called config.guess).
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If you can't install Nix, find a way to run ``config.guess`` (usually comes with
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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:: 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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Choosing the host platform with Nix
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-----------------------------------
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Nixpkgs comes with a set of predefined host platforms applied to all packages.
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It's possible to explore predefined attribute sets via ``nix repl````:
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.. code:: shell-session
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$ nix repl '<nixpkgs>'
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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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Cross compilation package attribute names are made up, so it isn't always clear
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what is the corresponding platform config.
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It's possible to query the platform config using::
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nix-repl> pkgsCross.aarch64-multiplatform.stdenv.hostPlatform.config
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"aarch64-unknown-linux-gnu"
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In case the host platform you seek hasn't been defined yet:
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a) `Contribute it upstream <https://github.com/NixOS/nixpkgs/blob/master/lib/systems/examples.nix>`_.
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b) Pass the host platforms to ``crossSystem`` when importing ``<nixpkgs>``::
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nix-repl> (import <nixpkgs> { crossSystem = { config = "aarch64-unknown-linux-gnu"; }; }).hello
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«derivation /nix/store/qjj23s25kg4vjqq19vxs4dg7k7h214ns-hello-aarch64-unknown-linux-gnu-2.10.drv»
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Or using passing it as an argument to ``nix-build``::
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$ nix-build '<nixpkgs>' -A hello --arg crossSystem '{ config = "aarch64-unknown-linux-gnu"; }'
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Cross compiling for the first time!
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-----------------------------------
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To cross compile a package like `hello <https://www.gnu.org/software/hello/>`_,
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pick the platform attribute - ``aarch64-multiplatform`` in our case - and run:
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.. code:: shell-session
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$ nix-build '<nixpkgs>' -A pkgsCross.aarch64-multiplatform.hello
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...
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/nix/store/pzi2h0d60nb4ydcl3nn7cbxxdnibw3sy-hello-aarch64-unknown-linux-gnu-2.10
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`Search for a package <https://search.nixos.org/packages>`_ attribute name to find the
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one that you're interested in building.
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Real-world cross compiling of a Hello World example
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---------------------------------------------------
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To show off the power of cross compilation in Nix, let's build our own Hello World program
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by cross compiling it as static executables to ``armv6l-unknown-linux-gnueabihf``
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and ``x86_64-w64-mingw32`` (Windows) platforms and run the resulting executable
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with `an emulator <https://en.wikipedia.org/wiki/Emulator>`_.
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.. code:: nix
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{ pkgs ? import <nixpkgs> {}
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}:
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let
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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 $out
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'';
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in {
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# Statically compile our example using the two platform hosts
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rpi = crossCompileFor pkgs.pkgsCross.raspberryPi;
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windows = crossCompileFor pkgs.pkgsCross.mingwW64;
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}
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If we build this example and print both resulting derivations, we should see "Hello, world!" for each:
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.. code:: shell-session
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$ cat $(nix-build cross-compile.nix)
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Hello, world!
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Hello, world!
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Developer environment with a cross compiler
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-------------------------------------------
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In the :ref:`tutorial for declarative reproducible environments <declarative-reproducible-envs>`,
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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
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to static binaries using musl**.
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Given we have a ``shell.nix``:
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.. code:: nix
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{ nixpkgs ? fetchTarball "https://github.com/NixOS/nixpkgs/archive/bba3474a5798b5a3a87e10102d1a55f19ec3fca5.tar.gz"
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, pkgs ? (import nixpkgs {}).pkgsCross.aarch64-multiplatform
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}:
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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 target platform
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nativeBuildInputs = [ pkg-config file ];
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# libraries needed for the target platform
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buildInputs = [ zlib ];
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}) {}
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And ``hello.c``:
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.. code:: 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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We can cross compile it:
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.. code:: shell-session
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$ nix-shell --run '$CC hello.c -o hello' cross-compile-shell.nix
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And confirm it's aarch64:
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.. code:: shell-session
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$ 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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Next steps
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----------
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- The `official binary cache <https://cache.nixos.org>`_ has very limited number of binaries
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for packages that are cross compiled, so to save time recompiling, configure
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:ref:`a binary cache and CI (GitHub Actions and Cachix) <github-actions>`.
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- While many compilers in nixpkgs support cross compilation,
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not all of them do.
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On top of that, supporting cross compilation is not trivial
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work and due to many possible combinations of what would
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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>`_
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for help around cross compiling.
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@ -14,4 +14,5 @@ Tutorials
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deploying-nixos-using-terraform.rst
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installing-nixos-on-a-raspberry-pi.rst
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integration-testing-using-virtual-machines.rst
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cross-compilation.rst
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contributing.rst
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