Are you using flakes? You need to use the legacyPackages output for your system, e.g.

  buildInputs = [ nixpkgs.legacyPackages."x64_64-linux".gcc ];

In practice, it usually looks something like this:

  let
    system = "x86_64-linux";
    pkgs = nixpkgs.legacyPackages.${system};
  in stdenv.mkDerivation {
    ...
    buildInputs = [ pkgs.gcc ];
    ...
  }

So there is more than one way to skin this cat. ‘nixpkgs’ as handed to you as part of a flakes ‘inputs’ is (jargon alert) partially applied, which is to say you have to put a battery in it. In this instance what you have to pass is ‘system’ because nixpkgs supports many systems (e.g. ‘linux-x86_64’ or ‘darwin-aarach64’). It sort of makes sense that it wouldn’t know what GCC you want without that information.

You can do this up top by hand, e.g. P = inputs.nixpkgs { inherit system; }, but more commonly you would use a library like ‘flake-utils’ or ‘flake-utils-plus’ to issue system configuration for all supported packages to all your packages/derivations/devShells.

Ah right, I saw that library but it didn’t have examples and I didn’t manage to use it. Do you know a page that would explain that?

The ‘flake-utils’ readme is a pretty good jumping off point: https://github.com/numtide/flake-utils

I have this or that nitpick with FL and FLP but overall it’s very solid stuff. FLP is a little more “magical”, and that’s not always the best starting out, but you really can’t go wrong with either.

Maybe you got confused (like I did every time until I stopped doing anything non-flakey) about when `nixpkgs` is called `nixpkgs` and when it's called `pkgs`? In the flakes world it's considerably easier because everything is explicitly named, so you take an actual decision about what to call it.

Is the convention to call it pkgs when you instantiate it with a system?

yes. other possible arguments are overlays and package selection modifiers (like whether to include non-free stuff or stuff that is marked as broken).

I don’t know what is the specific file you are working on, but shouldn’t it be pkgs.gcc?

That's strange. GCC is part of the stdenv on Linux and though always available.

To correct your coffee snippet: nativeBuildInputs = [ pkgs.gcc ];

I’m on mac, I managed to fix it with https://mimoo.github.io/nixbyexample/flakes-packaging.html

And now I’m trying to understand how devshells work in flakes and there’s pretty no doc

So a dev shell as consumed by a flake is typically just a derivation. And typically the most important thing in that derivation is ‘buildInputs’, which is just a list of ‘pkgs.thingIWant’. It’s not uncommon to hook other “phases” of the derivation, which are (oversimplified) just little shell scripts that run in some order (‘configurePhase’ before ‘installPhase’) and typically expressed using the nifty “” syntax. For example if Nix is like stuffing paths into a Makefile that you want to use in your dev shell, you might put that in installPhase. Protip: a Nix “package” (derivation) evaluated in a string context (foo = “${pkgs.gcc}”;) gives you a path to where the thing lives in /nix/store.

Ask your questions on discourse.nixos.org please - the documentation situation in Nix-land is generally exactly as you have noted, which means centralised question-asking in well-known locations is much much more helpful for Those Who Come After.

I say this with the utmost respect for those who work hard to answer questions on discourse, but I have found it singularly unhelpful.

I strongly encourage the Nix community to embrace a mainstream Q&A platform like StackOverflow or GitHub Issues.

Nix is “weird” enough for beginners already, the kind of janky interface and uneven moderation/quality on discourse is IMHO self-defeating for Nix.

Agree. I feel the same way about OCaml, and I have forced myself to ask every questions I have on SO to build the list of answers there. Yet, people tend to answer on SO with “you should ask on the OCaml discourse”…

Aside: I love the Discourse UI. It feels easy and thoroughly modern to me. What's janky about it, in your opinion?

I also do not like it for different reasons (one of them is that they hijack ctrl+F)

Ah yeah, that behavior is annoying. At least if you hit it twice, you get the normal in-browser thing