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Building Nix

This section provides some notes on how to start hacking on Nix. To get the latest version of Nix from GitHub:

$ git clone https://github.com/NixOS/nix.git
$ cd nix

Note

The following instructions assume you already have some version of Nix installed locally, so that you can use it to set up the development environment. If you don't have it installed, follow the installation instructions.

To build all dependencies and start a shell in which all environment variables are set up so that those dependencies can be found:

$ nix-shell

To get a shell with one of the other supported compilation environments:

$ nix-shell --attr devShells.x86_64-linux.native-clangStdenvPackages

Note

You can use native-ccacheStdenvPackages to drastically improve rebuild time. By default, ccache keeps artifacts in ~/.cache/ccache/.

To build Nix itself in this shell:

[nix-shell]$ mesonFlags+=" --prefix=$(pwd)/outputs/out"
[nix-shell]$ dontAddPrefix=1 mesonConfigurePhase
[nix-shell]$ ninjaBuildPhase

To test it:

[nix-shell]$ mesonCheckPhase

To install it in $(pwd)/outputs:

[nix-shell]$ ninjaInstallPhase
[nix-shell]$ ./outputs/out/bin/nix --version
nix (Nix) 2.12

To build a release version of Nix for the current operating system and CPU architecture:

$ nix-build

You can also build Nix for one of the supported platforms.

Building Nix with flakes

This section assumes you are using Nix with the flakes and nix-command experimental features enabled.

To build all dependencies and start a shell in which all environment variables are set up so that those dependencies can be found:

$ nix develop

This shell also adds ./outputs/bin/nix to your $PATH so you can run nix immediately after building it.

To get a shell with one of the other supported compilation environments:

$ nix develop .#native-clangStdenvPackages

Note

Use ccacheStdenv to drastically improve rebuild time. By default, ccache keeps artifacts in ~/.cache/ccache/.

To build Nix itself in this shell:

[nix-shell]$ mesonConfigurePhase
[nix-shell]$ ninjaBuildPhase

To test it:

[nix-shell]$ mesonCheckPhase

To install it in $(pwd)/outputs:

[nix-shell]$ ninjaInstallPhase
[nix-shell]$ nix --version
nix (Nix) 2.12

For more information on running and filtering tests, see testing.md.

To build a release version of Nix for the current operating system and CPU architecture:

$ nix build

You can also build Nix for one of the supported platforms.

Platforms

Nix can be built for various platforms, as specified in flake.nix:

  • x86_64-linux
  • x86_64-darwin
  • i686-linux
  • aarch64-linux
  • aarch64-darwin
  • armv6l-linux
  • armv7l-linux
  • riscv64-linux

In order to build Nix for a different platform than the one you're currently on, you need a way for your current Nix installation to build code for that platform. Common solutions include [remote build machines] and binary format emulation (only supported on NixOS).

Given such a setup, executing the build only requires selecting the respective attribute. For example, to compile for aarch64-linux:

$ nix-build --attr packages.aarch64-linux.default

or for Nix with the flakes and nix-command experimental features enabled:

$ nix build .#packages.aarch64-linux.default

Cross-compiled builds are available for:

  • armv6l-linux
  • armv7l-linux
  • riscv64-linux Add more system types to crossSystems in flake.nix to bootstrap Nix on unsupported platforms.

Building for multiple platforms at once

It is useful to perform multiple cross and native builds on the same source tree, for example to ensure that better support for one platform doesn't break the build for another. Meson thankfully makes this very easy by confining all build products to the build directory --- one simple shares the source directory between multiple build directories, each of which contains the build for Nix to a different platform.

Nixpkgs's mesonConfigurePhase always chooses build in the current directory as the name and location of the build. This makes having multiple build directories slightly more inconvenient. The good news is that Meson/Ninja seem to cope well with relocating the build directory after it is created.

Here's how to do that

  1. Configure as usual

    mesonConfigurePhase

  2. Rename the build directory

    cd .. # since `mesonConfigurePhase` cd'd inside
mv build build-linux # or whatever name we want
cd build-linux

  3. Build as usual

    ninjaBuildPhase

N.B. nixpkgs#335818 tracks giving mesonConfigurePhase proper support for custom build directories. When it is fixed, we can simplify these instructions and then remove this notice.

System type

Nix uses a string with the following format to identify the system type or platform it runs on:

<cpu>-<os>[-<abi>]

It is set when Nix is compiled for the given system, and based on the output of config.guess (upstream):

<cpu>-<vendor>-<os>[<version>][-<abi>]

When Nix is built such that ./configure is passed any of the --host, --build, --target options, the value is based on the output of config.sub (upstream):

<cpu>-<vendor>[-<kernel>]-<os>

For historic reasons and backward-compatibility, some CPU and OS identifiers are translated from the GNU Autotools naming convention in configure.ac as follows:

config.guess Nix
amd64 x86_64
i*86 i686
arm6 arm6l
arm7 arm7l
linux-gnu* linux
linux-musl* linux

Compilation environments

Nix can be compiled using multiple environments:

  • stdenv: default;
  • gccStdenv: force the use of gcc compiler;
  • clangStdenv: force the use of clang compiler;
  • ccacheStdenv: enable [ccache], a compiler cache to speed up compilation.

To build with one of those environments, you can use

$ nix build .#nix-ccacheStdenv

for flake-enabled Nix, or

$ nix-build --attr nix-ccacheStdenv

for classic Nix.

You can use any of the other supported environments in place of nix-ccacheStdenv.

Editor integration

The clangd LSP server is installed by default on the clang-based devShells. See supported compilation environments and instructions how to set up a shell with flakes or in classic Nix.

To use the LSP with your editor, you will want a compile_commands.json file telling clangd how we are compiling the code. Meson's configure always produces this inside the build directory.

Configure your editor to use the clangd from the .#native-clangStdenvPackages shell. You can do that either by running it inside the development shell, or by using nix-direnv and the appropriate editor plugin.

Note

For some editors (e.g. Visual Studio Code), you may need to install a special extension for the editor to interact with clangd. Some other editors (e.g. Emacs, Vim) need a plugin to support LSP servers in general (e.g. lsp-mode for Emacs and vim-lsp for vim). Editor-specific setup is typically opinionated, so we will not cover it here in more detail.

Formatting and pre-commit hooks

You may run the formatters as a one-off using:

./maintainers/format.sh

If you'd like to run the formatters before every commit, install the hooks:

pre-commit-hooks-install

This installs pre-commit using cachix/git-hooks.nix.

When making a commit, pay attention to the console output. If it fails, run git add --patch to approve the suggestions and commit again.

To refresh pre-commit hook's config file, do the following:

  1. Exit the development shell and start it again by running nix develop.
  2. If you also use the pre-commit hook, also run pre-commit-hooks-install again.