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258 lines
9.1 KiB
ReStructuredText
258 lines
9.1 KiB
ReStructuredText
.. _ref-cross-compilation:
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Cross-compilation
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=================
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Cross-compilation is the act of **compiling code** on the **build platform**
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to the **host platform**, where the compiled **executable runs**. [#]_
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It's 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 years of hard work from our community.
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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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.. 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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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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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 $(nix-build '<nixpkgs>' -A gnu-config)/config.guess
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aarch64-unknown-linux-gnu
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In case that's not possible (when the host platform is not easily accessible
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for development), the platform config has to be constructed manually via the following template:
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.. code::
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<cpu>-<vendor>-<os>-<abi>
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Note that ``<vendor>`` is often ``unknown`` and ``<abi>`` is optional.
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There's also no unique identifier for a platform, for example ``unknown`` and
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``pc`` are interchangeable (hence it's called config.guess).
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Some other common examples of platform configs:
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- aarch64-apple-darwin14
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- aarch64-pc-linux-gnu
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- x86_64-w64-mingw32
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- aarch64-apple-ios
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Choosing the host platform with Nix
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-----------------------------------
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Nixpkgs comes with a set of predefined host platforms applied to all packages.
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It's possible to list predefined sets via shell completion:
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.. code:: shell-session
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$ nix-build '<nixpkgs>' -A pkgsCross.<TAB>
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pkgsCross.aarch64-android pkgsCross.musl32
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pkgsCross.aarch64-android-prebuilt pkgsCross.musl64
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pkgsCross.aarch64be-embedded pkgsCross.muslpi
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pkgsCross.aarch64-darwin pkgsCross.musl-power
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pkgsCross.aarch64-embedded pkgsCross.or1k
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pkgsCross.aarch64-multiplatform pkgsCross.pogoplug4
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pkgsCross.aarch64-multiplatform-musl pkgsCross.powernv
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pkgsCross.amd64-netbsd pkgsCross.ppc64
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pkgsCross.arm-embedded pkgsCross.ppc64-musl
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pkgsCross.armhf-embedded pkgsCross.ppc-embedded
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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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From the attribute name it isn't always immediately clear what the platform is.
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It's possible to query the platform config using:
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$ nix-instantiate '<nixpkgs>' -A pkgsCross.aarch64-darwin.hostPlatform.config --eval
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"aarch64-apple-darwin"
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.. note:: In case the plaforms hasn't been defined yet, feel free to contribute one
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by `adding it upstream <https://github.com/NixOS/nixpkgs/blob/master/lib/systems/examples.nix>`_.
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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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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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# pkgs.callPackage is needed due to https://github.com/NixOS/nixpkgs/pull/126844
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pkgs.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), dynamically linked, interpreter /nix/store/733hzlw1hixdm6dfdsb8dlwa2h8fl5qi-glibc-2.31-74-aarch64-unknown-linux-gnu/lib/ld-linux-aarch64.so.1, for GNU/Linux 2.6.32, 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>`_ doesn't come with binaries
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for packages that are cross-compiled, so it's important to set up
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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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