KeiruaProd

I’ve been meaning to give Nix a try for a while. I’m somewhat convinced that the declarative, reproducible-build approach it suggests is very promising. Getting started is not trivial tough. There’s a lot of vocabulary and things.

Julia Evans wrote about some of her first steps. Here are some of mine: using nix to compile a small C++ program, with an external library.

External dependencies are usually a PITA with C/C++ programs. Even though you can something more complex with cmake (here is an example with sdl+opengl), that’s for another day : before we compile a complex project we need to be able to build a simple one.

Here is how we want to lay the code out:

• a default.nix file with the build details: the dependencies, where the code is stored, how to compile.
• the src directory contains the C++ code
• we build inside result/, which is in fact a symlink to the nix store directory. With nix a lot of things are taken care of with symlinks.

├── default.nix
├── result -> /nix/store/74gp7qalrqxh11pzqqrdhl2hi18q09ri-sdl-sample
└── src
    └── main.cpp

First, here is the code. A minimalistic, black window opens. It relies on SDL2 being present.

// src/main.cpp
#include <SDL2/SDL.h>

int main(int arc, char ** argv) {
    if (SDL_Init( SDL_INIT_VIDEO ) < 0) {
        printf("SDL could not initialize!\n");
    } else {
        SDL_CreateWindow(
            "SDL2 Demo",
            SDL_WINDOWPOS_CENTERED, SDL_WINDOWPOS_CENTERED,
            800, 600,
            SDL_WINDOW_SHOWN
        );

        SDL_Delay(2000);
    }

    return 0;
}

Now for the build script. Here is what I understand:

• we list the dependencies (than we can find on the package website, on )
• we

with import <nixpkgs> {};

stdenv.mkDerivation {
  name = "sdl-sample";
  src = ./src;
  buildInputs = [ gcc SDL2 SDL2.dev ];
  buildPhase = "c++ -o main main.cpp -lSDL2";

  installPhase = ''
    mkdir -p $out/bin
    cp main $out/bin/
  '';
}

Now we can :

• build with nix-build. It assumes the build script is default.nix.
• run the compiled program (./program/bin/main)

There are a lot of other features we did not cover, but we got it to work \o/

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