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Tools for cross-compiling standalone applications using Nix.
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.gitignore Add the 'support/manage gcroots' command: finally a good solution for… Feb 13, 2019
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README.md

nixcrpkgs

www.pololu.com

nixcrpkgs is a collection of tools for cross-compiling statically-linked, standalone software applications. With nixcrpkgs, you can specify what platforms you want to target, what libraries and build tools you depend on, and the commands that build your software. When you build your software, nixcrpkgs will automatically take care of building or retrieving everything you need, including cross-compilers and libraries.

nixcrpkgs primarily consists of Nix expressions, which are recipes for building software with Nix, the purely functional package manager. These recipes build on top of the Nix Packages collection (Nixpkgs).

Features

Getting started

To get started, you should first install Nix on a Linux machine by following the instructions on the Nix website.

Next, run df -h to make sure you have enough disk space.

  • The filesystem that holds /nix should have many gigabytes of free space. Each cross-compiler can take about 1 GB while each Qt installation takes about 500 MB.
  • The filesystem that holds /tmp should have at least 4 GB of free space, which will be needed while building cross-compilers. If that is not the case on your system, you can set the TMPDIR environment variable to tell nix-build to perform its builds in a different directory on a filesystem with more free space.

Next, clone or download this repository and use cd to change into the top-level directory.

To build a simple "Hello, World!" program for Windows, run:

nix-build -A win32.hello

The first time you run this command, it will take a while because Nix has to build a cross-compiling toolchain. When nix-build is done, it will print the name of a directory in /nix/store that holds the resulting program, and it will create a symbolic link in the current directory named result that points to that directory.

If you copy result/bin/hello.exe to a Windows machine and run it, you should see a message box appear that says "Hello, World!".

If you run nix-build -A win32.hello a second time, Nix will detect that nothing about the build recipes has changed, so it will simply print the directory name and remake the symbolic link.

To see how the hello package is specified in nixcrpkgs, you can look in pkgs.nix and the pkgs/hello directory. To see how the GCC cross-compiler for Windows was specified, you can look in the mingw-w64 directory. If you change any of the build recipes for hello or its dependencies and then run the nix-build command again, Nix will automatically rebuild those dependencies and anything that depends on them, ensuring that you always get a consistent build.

Obtaining the macOS SDK

If you are trying to build software for macOS, you will need to get a macOS SDK tarball and put it in the the right place. Otherwise, you will get an error like this:

error: getting attributes of path '/home/yourname/nixcrpkgs/macos/MacOSX.sdk.tar.xz': No such file or directory

To generate the tarball, follow these steps:

  1. On a macOS machine, install Xcode.
  2. Download this repository to the machine.
  3. In a Terminal window, run the macos/gen_sdk_package.sh script from this repository.
  4. After several minutes, the current directory should have a tarball with a name like MacOSX10.12.sdk.tar.xz and a size of about 25 MB.
  5. Copy the SDK tarball file to the machine where you will be building software, and put it in the macos directory.
  6. The nixcrpkgs build recipe for the SDK is hardcoded to look for a file named MacOSX.sdk.tar.xz, so rename the tarball to that.
  7. Consider keeping a backup of the tarball so you can always rebuild any software you made with it.

Now you should be able to build your software for macOS.

Integrating nixcrpkgs into your project

The instructions above show how to cross-compile a "Hello, World!" program that is included with nixcrpkgs. Instead of including your project in nixcrpkgs like the hello program, you will probably want to just use nixcrpkgs as a tool in your project. To get an idea of how to do that, you can look at other projects that have done the same. In the projects listed below, you should look for a file named default.nix in the top-level directory and look for build instructions that explain what nix-build commands to run.

Updating package versions

Each build recipe in nixcrpkgs specifies a version number for the software that it builds. It is relatively easy to update the recipes even if you have not worked with Nix before. The general procedure is:

  1. Find the build recipe you want to update. For example, if you wanted to update the version of GCC used to build Linux programs, you would update the build recipe in linux/gcc/default.nix.
  2. Find the part of the build recipe where the software sources are downloaded from the internet. It is usually a fetchurl command with two parameters: url and sha256. The url parameter usually refers to a version string defined nearby, so update that version string and/or the url parameter as desired.
  3. In a shell, run nix-prefetch-url URL, where URL is the new URL specified in your modified build recipe with all version variables fully expanded). This command will download the URL you specified, store it in the Nix store, and output the hash of it in the proper format for Nix build recipes.
  4. Update the sha256 hash string in the build recipe by replacing it with the hash that was printed in the output of nix-prefetch-url. Updating the hash in the build recipe is important: Nix uses it to determine whether you already downloaded the right file, so if you don't update the hash then Nix might use the wrong file (e.g. an older version of the software that you downloaded earlier).
  5. Run the usual nix-build command that you use to build your software. For example, you could go to the top-level directory of nixcrpkgs and run nix-build -A rpi.hello to build a "Hello world" program for the Raspberry Pi, or you could run nix-build -A rpi.gcc to just build the cross-compiler.
  6. Fix any error messages that happen, one at a time. (Tip: to make a .patch file, run diff -ur old new where old and new are directories that contain the unpatched and patched versions of the source code, respectively.)
  7. Once things are working, consider publishing your work on Github so others can benefit from what you figured out.

Freeing disk space

You should occasionally run nix-collect-garbage to remove items that are no longer needed and reclaim your disk space. However, note that Nix will typically remove all of your cross compilers and libraries when you run this command, which could require you to do a length mass rebuild the next time you want to build your software.

There is a method you can use to prevent Nix from garbage collecting the most important items used by nixcrpkgs. First of all, edit or create your nix.conf file and add this line to it:

keep-outputs = true

Now run this command from the nixcrpkgs directory (you'll need Ruby installed):

./manage gcroots

This makes a symbolic link in the nixcrpkgs directory called gcroots.drv, which points to a derivation (a compiled recipe for building some software) in the Nix store which depends on all the derivations that are most important for nixcrpkgs. As long as gcroots.drv remains in place, those derivations and their outputs cannot be garbage collected.

In the future, if you update nixcrpkgs and then run the command to update gcroots.drv, then items used by the old version of nixcrpkgs that are no longer needed can be garbage collected. You can also simply delete gcroots.drv if you want to garbage collect everything needed by nixcrpkgs.

The Nix manual has more information about Nix garbage collection.

Updating recipes

You should occasionally run nix-channel --update to update to the latest version of Nixpkgs. However, when doing this, be aware that the new version of Nixpkgs might require you to do a mass rebuild.

You should occasionally update your checkout of the nixcrpkgs repository to get the latest versions of build tools, new features, and bug fixes. Once again, this might require a mass rebuild.

If you want your builds to be very stable and reliable, you could make forks of nixcrpkgs and/or Nixpkgs and update them at your own pace, carefully considering any changes made by others before merging them in. That's one of the beauties of Nix when compared to other package management systems: you will never be forced to upgrade your build tools, and using old tools is just as easy as using new ones. You can use the NIX_PATH environment variable to tell nix-build to use your forked versions.

Related projects

  • osxcross: Cross-compiling toolchain targeting macOS.
  • musl-cross-make: Makefile-based build tool for creating cross-compilers targeting musl.
  • musl_nix_arm: A fork of nixcrpkgs with a focus on building Docker images for ARM Linux.
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