Building your own project with nix

On the face of it, it seems attractive and natural to keep nix files for a project in the same repo! That's what we do with build instructions (CMakeLists.txt etc)

In other words, use callPackage!

1. callPackage supports cross-compilation (out of the box)
2. callPackage supports parameterization (i.e. overrides)
3. If you already have nix build setup for nixpkgs, then there's nothing new to do if you want to upload into nixpkgs (or incorporate into your fork thereof) later

Do use flakes! They're great for version pinning and specifying configurations

However, if you put build instructions in flakes (i.e. call outside callPackage), then you lose the ability to easily compose and adapt that the nixpkgs tools give you.

For packaging our own software we will use flake to do these things (and nothing else):

1. pin nixpkgs.
2. fetch repos for local projects (things we aren't getting from nixpkgs)
3. specifying dependendency sets for local projects.

In particular we exclude from flake:

1. stdenv.mkDerivation calls
2. specifying buildInputs or cousins

Diving into some of the contents of these two .nix files:

Use result just like a regular flake

Verify flake:

$ cd cmake-examples-nix
$ nix flake check

Example flake.lock:

{
  "nodes": {
    "cmake-examples-ex23-path": {
      "flake": false,
      "locked": {
        "lastModified": 1709699013,
        "narHash": "sha256-Polpd2+DiZF615sih6HLOtzj0LaaFkQxrN6Jobdnq7M=",
        "owner": "Rconybea",
        "repo": "cmake-examples",
        "rev": "6d14f4146f2c7fc2011dd6790947ea261575304e",
        "type": "github"
      },
      "original": {
        "owner": "Rconybea",
        "ref": "ex23",
        "repo": "cmake-examples",
        "type": "github"
      }
    },
    "nixpkgs": {
      "locked": {
        "narHash": "sha256-mBXQ65IrCJbNgTrj0+6xdXpD9/U31AWPKdwGlOufhtI=",
        "type": "tarball",
        "url": "https://github.com/NixOS/nixpkgs/archive/4dd376f7943c64b522224a548d9cab5627b4d9d6.tar.gz"
      },
      "original": {
        "type": "tarball",
        "url": "https://github.com/NixOS/nixpkgs/archive/4dd376f7943c64b522224a548d9cab5627b4d9d6.tar.gz"
      }
    },
    ...
  },
  "root": "root",
  "version": 7
}

Build (from pinned revisions)

$ nix build -L --print-build-logs .#cmake-examples-ex23

Result

$ tree ./result
result
├── 3.11
│   ├── pyzstream.cpython-311-x86_64-linux-gnu.so
│   └── zstream.py
├── bin
│   ├── hello
│   └── myzip
├── include
│   ├── compression
│   │   ├── base_zstream.hpp
│   │   ├── buffer.hpp
│   │   ├── buffered_deflate_zstream.hpp
│   │   ├── buffered_inflate_zstream.hpp
│   │   ├── compression.hpp
│   │   ├── deflate_zstream.hpp
│   │   ├── hex.hpp
│   │   ├── inflate_zstream.hpp
│   │   ├── span.hpp
│   │   └── tostr.hpp
│   └── zstream
│       ├── xfilebuf.hpp
│       ├── zstream.hpp
│       └── zstreambuf.hpp
├── lib
│   ├── libcompression.so -> libcompression.so.2.3
│   ├── libcompression.so.2
│   └── libcompression.so.2.3 -> libcompression.so.2
└── share
    └── doc
        └── cmake-examples
            └── html
                 ...

Resources

• https://jade.fyi/blog/flakes-arent-real/ wonderful Jade Lovelace blog – inspiration for this article!
• https://github.com/vlktomas/nix-examples Tomas Vlk nix examples
• https://nixos.org/guides/nix-pills/ Nix pills (nix tutorial, from the ground up)
• https://ianthehenry.com/posts/how-to-learn-nix/ Ian Henry's "nix travel diary"
• https://ryantm.github.io/nixpkgs/stdenv/stdenv Nix standard environment docs
• https://github.com/Rconybea/cmake-examples my progressive series of cmake examples, using as example of a local project
• https://github.com/Rconybea/cmake-examples-nix nix build for cmake-examples