NixOS custom ".so" derivations

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Hi, NixOS community, newbie here!

I’m trying to build my project using nix flakes and fail to expose custom .so libraries to CMake.

The CMake error message: Could not find DAHUA_NET_SDK using the following names: dhnetsdk. Where dhnetsdk corresponds to libdhnetsd.so.

Obviously CMake’s find_library() can’t simply locate this foreign .so.

What I’ve done:

  1. Created a flake.nix in my project that looks like this (showing only outputs section):
outputs = { self, nixpkgs }:
    let
      system = "x86_64-linux";
      pkgs = nixpkgs.legacyPackages.${system};
    in
    {
      packages.${system}.default = (pkgs.callPackage ./default.nix { inherit pkgs; });
      devShells.${system}.default = { ... };
      ...
    };
  1. Created a default.nix to host my project derivation:
{ pkgs ? import <nixpkgs> { } }:

pkgs.llvmPackages_latest.stdenv.mkDerivation rec {
  pname = "lithium";
  version = "0.2.9";

  src = ./.;

  nativeBuildInputs = with pkgs; [ cmake ];
  buildInputs = with pkgs; [
    cmake
    clang
    jsoncpp
    zlib
    libuuid
    (pkgs.callPackage ./dahua-package.nix { }).dahua-avnetsdk
    (pkgs.callPackage ./dahua-package.nix { }).dahua-dhnetsdk
    (pkgs.callPackage ./dahua-package.nix { }).dahua-dhconfigsdk
  ];

  cmakeFlags = [
    "-DLITHIUM_USE_SUBDMODS=ON"
    ...
  ];
}
  1. And finally created dahua-package.nix to store a set of derivations with my custom .so files:
{ pkgs ? import <nixpkgs> { } }:

let
  basename = "dahua";
  origin = "https://github.com/danny-mhlv/dahua-c-wrapper/releases/download/0.0.1";
in
with pkgs;
{
  "${basename}-dhnetsdk" = pkgs.stdenv.mkDerivation rec {
    pname = "libdhnetsdk";
    version = "1.0.0";

    dontUnpack = true;
    src = fetchurl {
      url = "${origin}/${pname}.so";
      hash = "sha256-tzXLd2Mv/HoouN/eSfnJGPUPywxRrPZgPTx5QkmWpUA=";
    };
    phases = [ "unpackPhase" "installPhase" ];
    installPhase = ''
      mkdir -p $out
      cp $src $out
    '';
  };

  ...
}

As you can see for my .so dependencies - I simply fetch them from the web and copy to the $out.

You can see from the configurations provided, I’m including my derivations via callPackage in buildInputs section of my project’s derivation.

I’ve tried to investigate it myself, but couldn’t find any examples where some 3rd party, God forbidden .so files would be pulled into a C++ project.

So the question is: how can I use .so files fetched from the web in my project build?