Nix Cook Book


                           tropf


                          ABSTRACT

          My personal recipes when using nix ⟨https://
     nixos.org/⟩.


1.  Intro

     Nix refers to the operating system NixOS,  the  package
manager  and  associated  repository  nixpkgs, and the func‐
tional language nix itself.  I use all three, because I like
its focus towards reproducability.

     It  declarative  approach  breaks  with many habits and
also tools, so in this document I attempt to make some notes
how to tackle challenges I encountered in using nix.

2.  Parameterized Packages in Flakes

     To  create multiple versions of the same package (deri‐
vation) with different parameters use callPackage.  Note the
trailing {} to immedeately create a valid derivation.


     mypkg = pkgs.callPackage ({doCheck ? false}: stdenv.mkDerivation rec {
       inherit doCheck;
       checkTarget = "test";

       cmakeFlags = (if doCheck then [
         "‐DBUILD_TESTING=TRUE"
       ] else []);
     }) {};

     mypkg_with_tests = selfpkgs.mypkg.override {doCheck = true;};


3.  Include nix CFLAGS in Exported Compile Commands

     When  exporting  cmake compile commands the nix‐defined
compiler flags (stored in environment variables) will not be
included.  You can manually add them with a (hacky) sed com‐
mand.





                         29 May 2023





                             ‐2‐


     › This modifies JSON with sed and is evil. Use  at
     your own risk.


     nix develop && cd build
     cmake ‐DCMAKE_EXPORT_COMPILE_COMMANDS=ON .
     sed ‐e "s,$(which g++),$(which g++) $NIX_CFLAGS_COMPILE,g" ‐i compile_commands.json

     # note: this is maybe already called automatically by your editor
     rc ‐J .


4.  Creating Debuggable Builds

     By  default,  nix  will  enable  all sorts of optimiza‐
tions/security options ("fortifications") in C/C++ programs.
This  can  make  debugging  very  hard, as it optimizes code
out/reorders your code, even using Debug cmake build type.

     To disable these modifications set: hardeningDisable  =
[ "all" ]
In full context this may be used as such:


     buildInputs = [
         cmake
         gcc
     ] ++ (if doCheck then [
         catch2
         cmakeCurses
         gdb
     ] else []);

     cmakeFlags = (if doCheck then [
         "‐DBUILD_TESTING=TRUE"
         "‐DCMAKE_BUILD_TYPE=Debug"
     ] else []);

     hardeningDisable = if doCheck then [ "all" ] else [];


5.  Build without Tests, but Develop with Tests

     You can specify different targets for nix build and nix
develop such that they refer to different deviations:


     defaultPackage.x86_64‐linux = mypkg;
     devShells.x86_64‐linux.default = mypkg_with_tests;








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                             ‐3‐


6.  Ensure Locales Exist

     The CI infrastructure I use lacks some  locales.   When
building  plots  etc., the font handling doesn’t work due to
these missing locales.  Disturbingly, my local nix build in‐
vocation  finds the locales and looks completely fine, while
the CI version does not.  To  fix  this  set  the  following
variables inside the stdenv invocation:


     LOCALE_ARCHIVE = "${glibcLocales.override {allLocales = false; locales=["en_US.UTF‐8/UTF‐8"];}}/lib/locale/locale‐archive";
     LC_ALL = "en_US.utf8";













































                         29 May 2023