Packaging Dynamic Libraries

Packaging dynamic libs under nix is fairly easy usually:

• Build whatever you are packaging
• Copy it to $out/lib/libnameoflibrary.so
• Profit!

But this will not necessarily allow using it in other build tools, as lots of tools use pkg-config to discover locations of packages.

Nixpkgs support this form of discovery. The way you do that is as following:

On the dependent:

Wherever you plan on using libnameoflibrary.so you do the following:

stdenv.mkDerivation {
    buildInputs = [ pkg-config nameoflibrary ];
}

Fairly simple! Under the hood pkg-config registers an environment hook that looks through all build dependencies and looks for .pc files in $dep/lib/pkgconfig/. Those then get put into whatever env variables pkg-config needs and allows them to be discovered.

On the dependency:

When building nameoflibrary you do the usual building steps but you also need to place a .pc file at $out/lib/pkgconfig/libnameoflibrary.pc.

Luckily nixpkgs provides such a helper to create this:

makePkgconfigItem and copyPkgconfigItems.

The commit that added them has some info on how they are used: https://github.com/NixOS/nixpkgs/commit/7249b8a2f3318bb03c50429f5907015e99901c0b

Thanks Kiskae for the pointer!

[!warn] The method below works, but the above is easier.

Example content:

prefix=@out@
includedir=${prefix}/include
libdir=${prefix}/lib

Name: @pname@
Version: @version@
Description: A great library!

This file on its own is not ready yet, but we just need one other step:

stdenv.mkDerivation {
    # Your usual build stuff
    postInstall = ''
        substitute libnameoflibrary.pc $out/lib/pkgconfig/libnameoflibrary.pc \
            --subst-var out \
            --subst-var pname \
            --subst-var version
    '';
}

substitute will replace all those @out@/@pname@ occurrences with the corresponding env variable. (Which is what out and pname turn out to be)

substitute is part of the default building environment given by nix.

Now your downstream packages will be able to find your library!

Voila