I have been a big fan of Nix for a few years now, however I never ended up learning it properly. It has been (still is) something that I will repeatedly attempt to use for a job I know it is made for, but end up not succeeding because my lack of understanding. Of course there have been minor successes along the way when I managed to make it cooperate, but often times I just did not know how to describe what I wanted in terms of what Nix makes available to me.

What Exactly is Nix?#

Nix is a package manager. And a programming/markup language. And there is an entire OS (a Linux distro) built around it. But you can use it on any Linux or Mac machine.

Nixpkgs, the software repository the Nix package manager uses, is probably the biggest binary repository that exists with over 80,000 packages. Yes, I specifically said binary so that the AUR does not qualify. However, the humongous amount of packages pre-built is not the reason I love Nix. Although, it is very nice.

It is a package manager which claims to have solved dependency hell. If two programs need different versions of the same library, it will simply install both versions, and just make it work. The “and just make it work” part is a lot more complicated than what I am making it sound like, but generally it just works.

Prerequisites#

I will not go in-depth about the syntax of the Nix language. Although it is not difficult to read, pre se, it does require a basic understanding of common functional programming concepts and it does use some unusual syntax in places.

Basic understanding of the Nix language is recommended, although not strictly necessary.

Building a Package with Nix#

Oh, did I mention that Nix is also a build tool? Well, it is. Let’s do a deep dive. Here is an example of using Nix to build GNU Hello:

hello.nix:

{ pkgs ? import <nixpkgs> { } }:

pkgs.stdenv.mkDerivation rec {
  pname = "hello";
  version = "2.12.1";

  src = pkgs.fetchurl {
    url = "https://ftp.gnu.org/gnu/hello/hello-${version}.tar.gz";
    sha256 = "086vqwk2wl8zfs47sq2xpjc9k066ilmb8z6dn0q6ymwjzlm196cd";
  };
}

It specifies to use the Nixpkgs version that is used on the current system, then it creates a “derivation”. A derivation is a thing that describes how to build a package.

But all that is specified is the package name (pname), the version (version) and the source (src). How does it know how to actually, y’know, build it?

The answer is generic builders. Nixpkgs has a bunch of generic builders for common build tools. GNU Hello is built with autotools, so it just automatically figures out how to build a package with autotools. This leads to very elegant package descriptions, since the steps for building for common build tools do not need to be specified.

Otherwise, if Nixpkgs does not already have a generic builder for the build tool a project uses, the build instructions can be specified as Bash snippets, or a single bash script file.

Enough talk, let’s actually build it.

$ nix-build hello.nix -o hello
this derivation will be built:
  /nix/store/bfw39x992w52r4sqml563zkzwn4aczcp-hello-2.12.1.drv
building '/nix/store/bfw39x992w52r4sqml563zkzwn4aczcp-hello-2.12.1.drv'...
Running phase: unpackPhase
unpacking source archive /nix/store/pa10z4ngm0g83kx9mssrqzz30s84vq7k-hello-2.12.1.tar.gz
source root is hello-2.12.1
setting SOURCE_DATE_EPOCH to timestamp 1653865426 of file hello-2.12.1/ChangeLog
Running phase: patchPhase
Running phase: updateAutotoolsGnuConfigScriptsPhase
Updating Autotools / GNU config script to a newer upstream version: ./build-aux/config.sub
Updating Autotools / GNU config script to a newer upstream version: ./build-aux/config.guess
Running phase: configurePhase
[...] # Omitted the rest of the 1353 lines of build output
/nix/store/sc0rz1y1qy03xn40wg3n2i80hbv1r4s7-hello-2.12.1

There are some interesting things here, other then the fact that it is a lot of lines. First, this all happens under /nix/store. This is how Nix makes sure the builds stay isolated, so it can be as reproducible as possible.

The last line (which is the only one actually printed to stdout, the rest were on stderr) shows us where the binary has been built to. It is not in the current directory, but it gets symlinked there.

$ ls -F
hello@  hello.nix
$ file hello
hello: symbolic link to /nix/store/sc0rz1y1qy03xn40wg3n2i80hbv1r4s7-hello-2.12.1
$ file /nix/store/sc0rz1y1qy03xn40wg3n2i80hbv1r4s7-hello-2.12.1
/nix/store/sc0rz1y1qy03xn40wg3n2i80hbv1r4s7-hello-2.12.1: directory

Oh, alright, so that is not an executable, but a directory. Let’s look at its contents.

$ ls hello
bin  share
$ ls hello/bin
hello

Aha! That is where it is. Let’s see if it works

$ ./hello/bin/hello 
Hello, world!

Seems to work just fine. It also has a share/ directory since the packages are all self-contained under their store path. These files would be traditionally added to /usr/local/share or equivalent. Nix, however, keeps it under the store path of the package in /nix/store/<hash>-package/share. Upon install this path gets added to XDG_DATA_DIRS so that packages work as intended. Similarly, the bin/ directory gets added to PATH to be able to run the binaries like any other. Pretty clever.

We can delete it with the help of nix-store --delete:

$ realpath hello
/nix/store/sc0rz1y1qy03xn40wg3n2i80hbv1r4s7-hello-2.12.1
$ rm hello
$ nix-store --delete /nix/store/sc0rz1y1qy03xn40wg3n2i80hbv1r4s7-hello-2.12.1
finding garbage collector roots...
removing stale link from '/nix/var/nix/gcroots/auto/q1zlrwh5qg28kcfi517np5m0kmrz2wmm' to '/home/kin/blog/contributing-to-nixpkgs/hello'
deleting '/nix/store/sc0rz1y1qy03xn40wg3n2i80hbv1r4s7-hello-2.12.1'
deleting unused links...
note: currently hard linking saves -0.00 MiB
1 store paths deleted, 0.19 MiB freed

Unfortunately I do not know of a simpler way of deleting the build output of a single package. Alternatively, rm hello && nix-collect-garbage would have also worked, but that would also remove all other packages that are not explicitly installed. – or are not GC roots themselves or dependencies of GC roots

Building the VLC BitTorrent Plugin#

Enough exploring, let’s package vlc-bittorent with Nix. Oddly enough, it is not part of Nixpkgs, even though it can be found under the same name in the Debian/Ubuntu and the Fedora repos. So let’s add it.

First Attempt#

We create a file named, let’s say, vlc-bittorrent.nix. Model it after the GNU Hello package with small changes. We use fetchGit to download a given revision (commit) from a git repo instead.

{ pkgs ? import <nixpkgs> { } }:

pkgs.stdenv.mkDerivation {
  pname = "vlc-bittorrent";
  version = "2.15.0";

  src = fetchGit {
    url = "https://github.com/johang/vlc-bittorrent";
    rev = "6810d479e6c1f64046d3b30efe78774b49d1c95b";
  };
}

Let’s try building it too:

$ nix-build vlc-bittorent.nix 
this derivation will be built:
  /nix/store/0117p9yhqs5qcnhlkhxz15rf3ka1yx9y-vlc-bittorrent-2.15.0.drv
building '/nix/store/0117p9yhqs5qcnhlkhxz15rf3ka1yx9y-vlc-bittorrent-2.15.0.drv'...
Running phase: unpackPhase
unpacking source archive /nix/store/c9w5y8cl1pw4szy5s4i0k8pxxpw34fc1-source
source root is source
Running phase: patchPhase
Running phase: updateAutotoolsGnuConfigScriptsPhase
Running phase: configurePhase
no configure script, doing nothing
Running phase: buildPhase
no Makefile or custom buildPhase, doing nothing
Running phase: installPhase
no Makefile or custom installPhase, doing nothing
Running phase: fixupPhase
error: builder for '/nix/store/0117p9yhqs5qcnhlkhxz15rf3ka1yx9y-vlc-bittorrent-2.15.0.drv' failed to produce output path for output 'out' at '/nix/store/0117p9yhqs5qcnhlkhxz15rf3ka1yx9y-vlc-bittorrent-2.15.0.drv.chroot/nix/store/qhzg57m6l2zymyf85ysx6w91sk23xz4c-vlc-bittorrent-2.15.0'

Alright, that is not gonna work, I’ll actually need to specify a bit more to Nix. For this package it clearly could not just figure out how to build it.

The error message explains what happened and with a bit of understanding about how Nix builds packages we can decipher what went wrong. Nothing actually got written to the build directory (out). In other words, it did not build anything. Very sensible that this results in an error.

Second Attempt – Specify Build Steps#

It seems that here we will actually need to specify the build steps. Shouldn’t be too bad. First, let’s check the vlc-bittorrent GitHub for the official build instructions:

sudo apt-get install autoconf autoconf-archive automake libtool make \
  libvlc-dev libvlccore-dev libtorrent-rasterbar-dev g++
git clone https://github.com/johang/vlc-bittorrent.git vlc-bittorrent
cd vlc-bittorrent
autoreconf -i
./configure --prefix=/tmp/vlc
make
make install

Let’s copy over the build and install commands into the derivation. We already have cloning the repo figured out declaratively with fetchGit.

{ pkgs ? import <nixpkgs> { } }:

pkgs.stdenv.mkDerivation {
  pname = "vlc-bittorrent";
  version = "2.15.0";

  src = fetchGit {
    url = "https://github.com/johang/vlc-bittorrent";
    rev = "6810d479e6c1f64046d3b30efe78774b49d1c95b";
  };

  buildPhase = ''
    autoreconf -i
    ./configure --prefix=$out
    make
  '';

  installPhase = ''
    make install
  '';
}

We separate the commands into the build and install phases. People might want to only build a package and not yet install it, for example. Nix supports this.

We also make a small change, we change ./configure --prefix=/tmp/vlc to ./configure --prefix=$out. This is because we want the build to happen in its own nix store path, and this is the way we can specify it. out is a shell variable Nix automatically assigns to the store path it builds the package under. In our case, this will be something like out=/nix/store/k4rxnw719z628brqinrhlxmfibw1cz2q-vlc-bittorrent-2.15.0.

$ nix-build vlc-bittorent.nix 
this derivation will be built:
  /nix/store/1i6fj0dmp4fapnigxbkfkws4pdk286nn-vlc-bittorrent-2.15.0.drv
building '/nix/store/1i6fj0dmp4fapnigxbkfkws4pdk286nn-vlc-bittorrent-2.15.0.drv'...
Running phase: unpackPhase
unpacking source archive /nix/store/c9w5y8cl1pw4szy5s4i0k8pxxpw34fc1-source
source root is source
Running phase: patchPhase
Running phase: updateAutotoolsGnuConfigScriptsPhase
Running phase: configurePhase
no configure script, doing nothing
Running phase: buildPhase
/nix/store/10i1kjjq5szjn1gp6418x8bc1hswqc90-stdenv-linux/setup: line 1553: autoreconf: command not found
error: builder for '/nix/store/1i6fj0dmp4fapnigxbkfkws4pdk286nn-vlc-bittorrent-2.15.0.drv' failed with exit code 127;
       last 9 log lines:
       > Running phase: unpackPhase
       > unpacking source archive /nix/store/c9w5y8cl1pw4szy5s4i0k8pxxpw34fc1-source
       > source root is source
       > Running phase: patchPhase
       > Running phase: updateAutotoolsGnuConfigScriptsPhase
       > Running phase: configurePhase
       > no configure script, doing nothing
       > Running phase: buildPhase
       > /nix/store/10i1kjjq5szjn1gp6418x8bc1hswqc90-stdenv-linux/setup: line 1553: autoreconf: command not found
       For full logs, run 'nix log /nix/store/1i6fj0dmp4fapnigxbkfkws4pdk286nn-vlc-bittorrent-2.15.0.drv'.

Looks a bit better. Now it actually tries to execute the relevant build steps. However the build tooling, such as autoreconf, is not present.

Third Attempt – Explicitly Declare the Dependencies#

We need to manually declare the dependencies. This will involve a bit of hand-translating package names from Debian to Nix. Luckily, the package names are similar, and I can just search for packages on search.nixos.org.

Debian Nix
autoconf autoconf
autoconf-archive autoconf-archive
automake automake
g++ gcc
libtool libtool
libtorrent-rasterbar-dev libtorrent-rasterbar
libvlc-dev libvlccore-dev libvlc
make gnumake

After listing these packages as build dependencies I have the following code:

{ pkgs ? import <nixpkgs> { } }:

pkgs.stdenv.mkDerivation {
  pname = "vlc-bittorrent";
  version = "2.15.0";

  nativeBuildInputs = with pkgs; [
    autoconf
    autoconf-archive
    automake
    gcc
    libtool
    gnumake
  ];

  buildInputs = with pkgs; [
    libtorrent-rasterbar
    libvlc
  ];

  strictDeps = true;

  src = fetchGit {
    url = "https://github.com/johang/vlc-bittorrent";
    rev = "6810d479e6c1f64046d3b30efe78774b49d1c95b";
  };

  buildPhase = ''
    autoreconf -i
    ./configure --prefix=$out
    make
  '';

  installPhase = ''
    make install
  '';
}

We separate the dependencies needed for building the package (nativeBuildInputs) from the ones that are also needed to run it (buildInputs). I just used my intuition about which one should go where. The packages listed as buildInputs are present both at build time, and they also get installed as dependencies of the package.

It makes sense to separate the two. As the user, you wouldn’t want to install automake and libtool just to get vlc-bittorrent. It does not need them to run, so it would be simply a waste of bandwidth to download those as well.

We need to set strictDeps to true for Nix to actually consider compile time and runtime dependencies separately. Without it, strictDeps defaults to false, and then it just merges nativeBuildInputs into buildInputs, effectively nullifying our efforts.

Note: Technically, even without strictDeps = true it would be fine. Nix looks at all dependencies listed in buildInputs and later tries to prune the ones that are not needed at runtime. Thus it automatically separates packages based on whether they are needed at runtime, and only installs those for the end user. But as you might expect, this is not infallible. It’s better to specify ourselves both for human readability and for correctness.

Okay, let’s build it, hopefully that is enough.

$ nix-build vlc-bittorent.nix 
this derivation will be built:
  /nix/store/ggyycrj3dl6kimwq4bw2y4q2n7xy0myf-vlc-bittorrent-2.15.0.drv
building '/nix/store/ggyycrj3dl6kimwq4bw2y4q2n7xy0myf-vlc-bittorrent-2.15.0.drv'...
Running phase: unpackPhase
unpacking source archive /nix/store/c9w5y8cl1pw4szy5s4i0k8pxxpw34fc1-source
source root is source
Running phase: patchPhase
Running phase: updateAutotoolsGnuConfigScriptsPhase
Running phase: configurePhase
no configure script, doing nothing
Running phase: buildPhase
aclocal: warning: couldn't open directory 'm4': No such file or directory
libtoolize: putting auxiliary files in AC_CONFIG_AUX_DIR, 'build-aux'.
libtoolize: copying file 'build-aux/ltmain.sh'
libtoolize: putting macros in AC_CONFIG_MACRO_DIRS, 'm4'.
libtoolize: copying file 'm4/libtool.m4'
libtoolize: copying file 'm4/ltoptions.m4'
libtoolize: copying file 'm4/ltsugar.m4'
libtoolize: copying file 'm4/ltversion.m4'
libtoolize: copying file 'm4/lt~obsolete.m4'
libtoolize: Consider adding '-I m4' to ACLOCAL_AMFLAGS in Makefile.am.
configure.ac:32: installing 'build-aux/compile'
configure.ac:32: installing 'build-aux/config.guess'
configure.ac:32: installing 'build-aux/config.sub'
configure.ac:14: installing 'build-aux/install-sh'
configure.ac:14: installing 'build-aux/missing'
configure.ac:7: installing 'build-aux/tap-driver.sh'
src/Makefile.am: installing 'build-aux/depcomp'
checking whether the C++ compiler works... yes
checking for C++ compiler default output file name... a.out
checking for suffix of executables... 
checking whether we are cross compiling... no
checking for suffix of object files... o
checking whether the compiler supports GNU C++... yes
checking whether g++ accepts -g... yes
checking for g++ option to enable C++11 features... none needed
checking for gawk... gawk
checking for a BSD-compatible install... /nix/store/rk067yylvhyb7a360n8k1ps4lb4xsbl3-coreutils-9.3/bin/install -c
checking whether build environment is sane... yes
checking for a race-free mkdir -p... /nix/store/rk067yylvhyb7a360n8k1ps4lb4xsbl3-coreutils-9.3/bin/mkdir -p
checking whether make sets $(MAKE)... yes
checking whether make supports the include directive... yes (GNU style)
checking whether make supports nested variables... yes
checking dependency style of g++... gcc3
./configure: line 4320: syntax error near unexpected token `VLC_PLUGIN,'
./configure: line 4320: `PKG_CHECK_MODULES(VLC_PLUGIN, vlc-plugin >= 3.0.0)'
error: builder for '/nix/store/ggyycrj3dl6kimwq4bw2y4q2n7xy0myf-vlc-bittorrent-2.15.0.drv' failed with exit code 2;
       last 10 log lines:
       > checking for gawk... gawk
       > checking for a BSD-compatible install... /nix/store/rk067yylvhyb7a360n8k1ps4lb4xsbl3-coreutils-9.3/bin/install -c
       > checking whether build environment is sane... yes
       > checking for a race-free mkdir -p... /nix/store/rk067yylvhyb7a360n8k1ps4lb4xsbl3-coreutils-9.3/bin/mkdir -p
       > checking whether make sets $(MAKE)... yes
       > checking whether make supports the include directive... yes (GNU style)
       > checking whether make supports nested variables... yes
       > checking dependency style of g++... gcc3
       > ./configure: line 4320: syntax error near unexpected token `VLC_PLUGIN,'
       > ./configure: line 4320: `PKG_CHECK_MODULES(VLC_PLUGIN, vlc-plugin >= 3.0.0)'
       For full logs, run 'nix log /nix/store/ggyycrj3dl6kimwq4bw2y4q2n7xy0myf-vlc-bittorrent-2.15.0.drv'.

Oh no… Syntax error near unexpected token? And in ./configure of all places? Not a very helpful error. The full logs nix log would give us are identical to what has already been printed, so it is not particularly helpful here.

Inspect What Went Wrong#

We can attempt inspecting the build steps ourselves, maybe that will help? Nix also provides a tool called nix-shell, which allows us to enter the build environment and have access to the build steps.

$ nix-shell vlc-bittorent.nix 
(nix-shell) $ cd `mktemp -d`
(nix-shell) $ pwd
/run/user/1000/tmp.974kkEVXOl
(nix-shell) $ unpackPhase 
unpacking source archive /nix/store/c9w5y8cl1pw4szy5s4i0k8pxxpw34fc1-source
source root is source
(nix-shell) $ cd source/
(nix-shell) $ ls
configure.ac  data  LICENSE  Makefile.am  README.md  scripts  src  test

Nice, now we are in the build environment and we have the source code. After entering a Nix shell, all the packages specified in BuildInputs and nativeBuildInputs are available on the PATH. So we can manually run autoreconf, for example.

(nix-shell) $ autoreconf -i
aclocal: warning: couldn't open directory 'm4': No such file or directory
libtoolize: putting auxiliary files in AC_CONFIG_AUX_DIR, 'build-aux'.
libtoolize: copying file 'build-aux/ltmain.sh'
libtoolize: putting macros in AC_CONFIG_MACRO_DIRS, 'm4'.
libtoolize: copying file 'm4/libtool.m4'
libtoolize: copying file 'm4/ltoptions.m4'
libtoolize: copying file 'm4/ltsugar.m4'
libtoolize: copying file 'm4/ltversion.m4'
libtoolize: copying file 'm4/lt~obsolete.m4'
libtoolize: Consider adding '-I m4' to ACLOCAL_AMFLAGS in Makefile.am.
configure.ac:32: installing 'build-aux/compile'
configure.ac:32: installing 'build-aux/config.guess'
configure.ac:32: installing 'build-aux/config.sub'
configure.ac:14: installing 'build-aux/install-sh'
configure.ac:14: installing 'build-aux/missing'
configure.ac:7: installing 'build-aux/tap-driver.sh'
src/Makefile.am: installing 'build-aux/depcomp'

After running ./configure we get the exact same error again.

$ ./configure 
checking whether the C++ compiler works... yes
checking for C++ compiler default output file name... a.out
checking for suffix of executables... 
checking whether we are cross compiling... no
checking for suffix of object files... o
checking whether the compiler supports GNU C++... yes
checking whether g++ accepts -g... yes
checking for g++ option to enable C++11 features... none needed
checking for gawk... gawk
checking for a BSD-compatible install... /nix/store/rk067yylvhyb7a360n8k1ps4lb4xsbl3-coreutils-9.3/bin/install -c
checking whether build environment is sane... yes
checking for a race-free mkdir -p... /nix/store/rk067yylvhyb7a360n8k1ps4lb4xsbl3-coreutils-9.3/bin/mkdir -p
checking whether make sets $(MAKE)... yes
checking whether make supports the include directive... yes (GNU style)
checking whether make supports nested variables... yes
checking dependency style of g++... gcc3
./configure: line 4320: syntax error near unexpected token `VLC_PLUGIN,'
./configure: line 4320: `PKG_CHECK_MODULES(VLC_PLUGIN, vlc-plugin >= 3.0.0)'

I look at those lines very quick with vim ./configure +4320, and be greeted with shellcheck, a shell script linter, complaining about those lines too. ./configure is simply a shell script, it starts with the usual shebang #!/bin/sh. Line 4319 and 4320 are interesting:

PKG_CHECK_MODULES(VLC_PLUGIN, vlc-plugin >= 3.0.0)
# Diagnostics:
# 1. Couldn't parse this function. Fix to allow more checks. [SC1073]
# 2. Trying to declare parameters? Don't. Use () and refer to params as $1, $2.. [SC1065]
PKG_CHECK_MODULES(LIBTORRENT, libtorrent-rasterbar >= 1.0.0)
# Diagnostics:
# 1. Expected a { to open the function definition. [SC1064]
# 2.  Fix any mentioned problems and try again. [SC1072]

The comments are what shellcheck tells me. Interesting. That is definitely NOT the syntax for calling a shell function. It is probably not trying to define shell functions that take arguments, rather call something.

At this point something clicks: Isn’t this some pkg-config shenanigans? Let’s try adding it to nativeBuildInputs and see if it solves it. First exit the previous Nix shell and then create a new one.

$ nix-shell vlc-bittorent.nix 
(nix-shell) $ cd `mktemp -d` && unpackPhase && cd source && autoreconf -i
unpacking source archive /nix/store/c9w5y8cl1pw4szy5s4i0k8pxxpw34fc1-source
source root is source
[...]
(nix-shell) $ ./configure 
[...]
(nix-shell) $ echo $?
0

That solved it! Great. Let’s retry with nix-build now (again, after exit-ing out of that Nix Shell).

$ nix-build vlc-bittorent.nix 
[...]
In file included from /nix/store/rfiiq50w7mpkhsjsq28gzm7g3insy05p-libtorrent-rasterbar-2.0.9-dev/include/libtorrent/time.hpp:36,
                 from /nix/store/rfiiq50w7mpkhsjsq28gzm7g3insy05p-libtorrent-rasterbar-2.0.9-dev/include/libtorrent/alert.hpp:67,
                 from download.h:32,
                 from metadata.cpp:27:
/nix/store/rfiiq50w7mpkhsjsq28gzm7g3insy05p-libtorrent-rasterbar-2.0.9-dev/include/libtorrent/config.hpp:52:10: fatal error: boost/config.hpp: No such file or directory
   52 | #include <boost/config.hpp>
      |          ^~~~~~~~~~~~~~~~~~
compilation terminated.
make[2]: *** [Makefile:533: libaccess_bittorrent_plugin_la-metadata.lo] Error 1
make[2]: Leaving directory '/build/source/src'
make[1]: *** [Makefile:397: all] Error 2
make[1]: Leaving directory '/build/source/src'
make: *** [Makefile:404: all-recursive] Error 1
error: builder for '/nix/store/m170j5qrkk31x7b5nfbwans0byj7zqsr-vlc-bittorrent-2.15.0.drv' failed with exit code 2;
       last 10 log lines:
       >                  from metadata.cpp:27:
       > /nix/store/rfiiq50w7mpkhsjsq28gzm7g3insy05p-libtorrent-rasterbar-2.0.9-dev/include/libtorrent/config.hpp:52:10: fatal error: boost/config.hpp: No such file or directory
       >    52 | #include <boost/config.hpp>
       >       |          ^~~~~~~~~~~~~~~~~~
       > compilation terminated.
       > make[2]: *** [Makefile:533: libaccess_bittorrent_plugin_la-metadata.lo] Error 1
       > make[2]: Leaving directory '/build/source/src'
       > make[1]: *** [Makefile:397: all] Error 2
       > make[1]: Leaving directory '/build/source/src'
       > make: *** [Makefile:404: all-recursive] Error 1
       For full logs, run 'nix log /nix/store/m170j5qrkk31x7b5nfbwans0byj7zqsr-vlc-bittorrent-2.15.0.drv'.

Okay, we need to add boost as well. That one is a runtime dependency too so add it to buildInputs. Try again.

$ nix-build vlc-bittorent.nix 
[...]
/nix/store/rfiiq50w7mpkhsjsq28gzm7g3insy05p-libtorrent-rasterbar-2.0.9-dev/include/libtorrent/ssl.hpp:56:10: fatal error: openssl/opensslv.h: No such file or directory
   56 | #include <openssl/opensslv.h> // for OPENSSL_VERSION_NUMBER
      |          ^~~~~~~~~~~~~~~~~~~~
[...]

Okay, we also need to add openssl. Again, runtime. Once more.

$ nix-build vlc-bittorent.nix 
[...]
/nix/store/8nmq9hzpmsxh7sz2ai1z3c56q0w7ij69-vlc-bittorrent-2.15.0
$ echo $?
0

IT WORKED! I am very happy with this, but let’s check if it actually works before we celebrate.

Watching a Movie Testing the Package#

Previously, for GNU Hello, I added the -o flag to nix-build to specify what the output should be called. If omitted, it creates a symlink called result. Now to add this plugin to VLC all we need to do is put it on the VLC_PLUGIN_PATH. It works like PATH, but it is for VLC plugins, pretty self explanatory.

VLC_PLUGIN_PATH="$PWD/result" vlc 'magnet:?xt=urn:btih:dd8255ecdc7ca55fb0bbf81323d87062db1f6d1c&dn=Big+Buck+Bunny&tr=udp%3A%2F%2Fexplodie.org%3A6969&tr=udp%3A%2F%2Ftracker.coppersurfer.tk%3A6969&tr=udp%3A%2F%2Ftracker.empire-js.us%3A1337&tr=udp%3A%2F%2Ftracker.leechers-paradise.org%3A6969&tr=udp%3A%2F%2Ftracker.opentrackr.org%3A1337&tr=wss%3A%2F%2Ftracker.btorrent.xyz&tr=wss%3A%2F%2Ftracker.fastcast.nz&tr=wss%3A%2F%2Ftracker.openwebtorrent.com&ws=https%3A%2F%2Fwebtorrent.io%2Ftorrents%2F&xs=https%3A%2F%2Fwebtorrent.io%2Ftorrents%2Fbig-buck-bunny.torrent'

That magnet link of course points to Big Buck Bunny, a banger open movie by the Blender Institute. Okay, maybe it is not the best short movie you will ever see in your life, but it has a lot of charm.

VLC playing Big Buck Bunny over BitTorrent

After watching the whole thing, I can confirm: IT REALLY DOES WORK, WOOO! 🎉

Contributing it to Nixpkgs#

Now, all that is left to do is to make a Pull Request to the Nixpkgs repo. Okay, maybe a couple things need to be changed before that:

  • Adding the metadata (such as the homepage, the license etc)
  • Making sure it conforms to the Nixpkgs style guide
  • Rewriting the expression to have a signature Nixpkgs can work with
  • Yada-yada

If you are interested in how that went, then you can check out the PR linked above. Spoilers: There were indeed things that needed to be changed.

For example, there is autoreconfHook. I would have needed to add that to the nativeBuildInputs, and Nix would have figured out all the build and install steps. Yeah, that would have been simpler…

This is what it could look like:

{ pkgs ? import <nixpkgs> { } }:

pkgs.stdenv.mkDerivation {
  pname = "vlc-bittorrent";
  version = "2.15.0";

  nativeBuildInputs = with pkgs; [
    autoconf-archive
    autoreconfHook # This handles building
    pkg-config
  ];

  buildInputs = with pkgs; [
    libtorrent-rasterbar
    libvlc
    boost
    openssl
  ];

  strictDeps = true;

  src = fetchGit {
    url = "https://github.com/johang/vlc-bittorrent";
    rev = "6810d479e6c1f64046d3b30efe78774b49d1c95b";
  };
}

Oh well ¯\_(ツ)_/¯