Packaging existing software with Nix#

One of Nix’s primary use-cases is in addressing common difficulties encountered with packaging software, such as specifying and obtaining dependencies.

In the long term, Nix helps tremendously with alleviating such problems. But when first packaging existing software with Nix, it’s common to encounter errors that seem inscrutable.

Introduction#

In this tutorial, you’ll create your first Nix derivations to package C/C++ software, taking advantage of the Nixpkgs Standard Environment (stdenv), which automates much of the work involved.

What will you learn?#

The tutorial begins with hello, an implementation of “hello world” which only requires dependencies already provided by stdenv. Next, you will build more complex packages with their own dependencies, leading you to use additional derivation features.

You’ll encounter and address Nix error messages, build failures, and a host of other issues, developing your iterative debugging techniques along the way.

What do you need?#

  • Familiarity with the Unix shell and plain text editors

  • You should be confident with reading the Nix language. Feel free to go back and work through the tutorial first.

How long does it take?#

Going through all the steps carefully will take around 60 minutes.

Your first package#

Note

A package is a loosely defined concept that refers to either a collection of files and other data, or a Nix expression representing such a collection before it comes into being. Packages in Nixpkgs have a conventional structure, allowing them to be discovered in searches and composed in environments alongside other packages.

For the purposes of this tutorial, a “package” is a Nix language function that will evaluate to a derivation. It will enable you or others to produce an artifact for practical use, as a consequence of having “packaged existing software with Nix”.

To start, consider this skeleton derivation:

1{ stdenv }:
2
3stdenv.mkDerivation {	}

This is a function which takes an attribute set containing stdenv, and produces a derivation (which currently does nothing).

A package function#

GNU Hello is an implementation of the “hello world” program, with source code accessible from the GNU Project’s FTP server.

To begin, add a pname attribute to the set passed to mkDerivation. Every package needs a name and a version, and Nix will throw error: derivation name missing without.

stdenv.mkDerivation {
+ pname = "hello";
+ version = "2.12.1";

Next, you will declare a dependency on the latest version of hello, and instruct Nix to use fetchzip to download the source code archive.

Note

fetchzip can fetch more archives than just zip files!

The hash cannot be known until after the archive has been downloaded and unpacked. Nix will complain if the hash supplied to fetchzip is incorrect. It is common practice to supply a fake one with lib.fakeSha256 and change the derivation definition after Nix reports the correct hash:

 1# hello.nix
 2{
 3  lib,
 4  stdenv,
 5  fetchzip,
 6}:
 7
 8stdenv.mkDerivation {
 9  pname = "hello";
10  version = "2.12.1";
11
12  src = fetchzip {
13    url = "https://ftp.gnu.org/gnu/hello/hello-2.12.1.tar.gz";
14    sha256 = lib.fakeSha256;
15  };
16}

Save this file to hello.nix and run nix-build to observe your first build failure:

$ nix-build hello.nix
error: cannot evaluate a function that has an argument without a value ('lib')
       Nix attempted to evaluate a function as a top level expression; in
       this case it must have its arguments supplied either by default
       values, or passed explicitly with '--arg' or '--argstr'. See
       https://nix.dev/manual/nix/2.18/language/constructs.html#functions.

       at /home/nix-user/hello.nix:2:3:

            1| # hello.nix
            2| { lib
             |   ^
            3| , stdenv

Problem: the expression in hello.nix is a function, which only produces its intended output if it is passed the correct arguments.

Building with nix-build#

lib is available from nixpkgs, which must be imported with another Nix expression in order to pass it as an argument to this derivation.

The recommended way to do this is to create a default.nix file in the same directory as hello.nix, with the following contents:

1# default.nix
2let
3  nixpkgs = fetchTarball "https://github.com/NixOS/nixpkgs/tarball/nixos-22.11";
4  pkgs = import nixpkgs { config = {}; overlays = []; };
5in
6{
7  hello = pkgs.callPackage ./hello.nix { };
8}

This allows you to run nix-build -A hello to realize the derivation in hello.nix, similar to the current convention used in Nixpkgs.

Note

callPackage automatically passes attributes from pkgs to the given function, if they match attributes required by that function’s argument attribute set.

In this case, callPackage will supply lib, stdenv, and fetchzip to the function defined in hello.nix.

Now run the nix-build command with the new argument:

$ nix-build -A hello
error:
...
       … while evaluating attribute 'src' of derivation 'hello'

         at /home/nix-user/hello.nix:9:3:

            8|
            9|   src = fetchzip {
             |   ^
           10|     url = "https://ftp.gnu.org/gnu/hello/hello-2.12.1.tar.gz";

       error: hash mismatch in file downloaded from 'https://ftp.gnu.org/gnu/hello/hello-2.12.1.tar.gz':
         specified: sha256:0000000000000000000000000000000000000000000000000000
         got:       sha256:0xw6cr5jgi1ir13q6apvrivwmmpr5j8vbymp0x6ll0kcv6366hnn

Finding the file hash#

As expected, the incorrect file hash caused an error, and Nix helpfully provided the correct one. In hello.nix, replace lib.fakeSha256 with the correct hash:

 1# hello.nix
 2{
 3  lib,
 4  stdenv,
 5  fetchzip,
 6}:
 7
 8stdenv.mkDerivation {
 9  pname = "hello";
10  version = "2.12.1";
11
12  src = fetchzip {
13    url = "https://ftp.gnu.org/gnu/hello/hello-2.12.1.tar.gz";
14    sha256 = "0xw6cr5jgi1ir13q6apvrivwmmpr5j8vbymp0x6ll0kcv6366hnn";
15  };
16}

Now run the previous command again:

$ nix-build -A hello
this derivation will be built:
  /nix/store/rbq37s3r76rr77c7d8x8px7z04kw2mk7-hello.drv
building '/nix/store/rbq37s3r76rr77c7d8x8px7z04kw2mk7-hello.drv'...
...
configuring
...
configure: creating ./config.status
config.status: creating Makefile
...
building
... <many more lines omitted>

Great news: the derivation built successfully!

The console output shows that configure was called, which produced a Makefile that was then used to build the project. It wasn’t necessary to write any build instructions in this case because the stdenv build system is based on GNU Autoconf, which automatically detected the structure of the project directory.

Build result#

Check your working directory for the result:

$ ls
default.nix hello.nix  result

This result is a symbolic link to a Nix store location containing the built binary; you can call ./result/bin/hello to execute this program:

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

Congratulations, you have successfully packaged your first program with Nix!

Next, you’ll package another piece of software with external-to-stdenv dependencies that present new challenges, requiring you to make use of more mkDerivation features.

A package with dependencies#

Now you will package a somewhat more complicated program, icat, which allows you to render images in your terminal.

Change the default.nix from the previous section by adding a new attribute for icat:

1# default.nix
2let
3  nixpkgs = fetchTarball "https://github.com/NixOS/nixpkgs/tarball/nixos-22.11";
4  pkgs = import nixpkgs { config = {}; overlays = []; };
5in
6{
7  hello = pkgs.callPackage ./hello.nix { };
8  icat = pkgs.callPackage ./icat.nix { };
9}

Copy hello.nix to a new file icat.nix, and update the pname and version attributes in that file:

 1# icat.nix
 2{
 3  lib,
 4  stdenv,
 5  fetchzip,
 6}:
 7
 8stdenv.mkDerivation {
 9  pname = "icat";
10  version = "v0.5";
11
12  src = fetchzip {
13    # ...
14  };
15}

Now to download the source code. icat’s upstream repository is hosted on GitHub, so you should replace the previous source fetcher. This time you will use fetchFromGitHub instead of fetchzip, by updating the argument attribute set to the function accordingly:

 1# icat.nix
 2{
 3  lib,
 4  stdenv,
 5  fetchFromGitHub,
 6}:
 7
 8stdenv.mkDerivation {
 9  pname = "icat";
10  version = "v0.5";
11
12  src = fetchFromGitHub {
13    # ...
14  };
15}

Fetching source from GitHub#

While fetchzip required url and sha256 arguments, more are needed for fetchFromGitHub.

The source URL is https://github.com/atextor/icat, which already gives the first two arguments:

  • owner: the name of the account controlling the repository

    owner = "atextor";

  • repo: the name of the repository to fetch

    repo = "icat";

Navigate to the project’s Tags page to find a suitable Git revision (rev), such as the Git commit hash or tag (e.g. v1.0) corresponding to the release you want to fetch.

In this case, the latest release tag is v0.5.

As in the hello example, a hash must also be supplied. This time, instead of using lib.fakeSha256 and letting nix-build report the correct one in an error, you can fetch the correct hash in the first place with the nix-prefetch-url command.

You need the SHA256 hash of the contents of the tarball (as opposed to the hash of the tarball file itself). Therefore pass the --unpack and --type sha256 arguments:

$ nix-prefetch-url --unpack https://github.com/atextor/icat/archive/refs/tags/v0.5.tar.gz --type sha256
path is '/nix/store/p8jl1jlqxcsc7ryiazbpm7c1mqb6848b-v0.5.tar.gz'
0wyy2ksxp95vnh71ybj1bbmqd5ggp13x3mk37pzr99ljs9awy8ka

Set the correct hash for fetchFromGitHub:

 1# icat.nix
 2{
 3  lib,
 4  stdenv,
 5  fetchFromGitHub,
 6}:
 7
 8stdenv.mkDerivation {
 9  pname = "icat";
10  version = "v0.5";
11
12  src = fetchFromGitHub {
13    owner = "atextor";
14    repo = "icat";
15    rev = "v0.5";
16    sha256 = "0wyy2ksxp95vnh71ybj1bbmqd5ggp13x3mk37pzr99ljs9awy8ka";
17  };
18}

Missing dependencies#

Running nix-build with the new icat attribute, an entirely new issue is reported:

$ nix-build -A icat
these 2 derivations will be built:
  /nix/store/86q9x927hsyyzfr4lcqirmsbimysi6mb-source.drv
  /nix/store/l5wz9inkvkf0qhl8kpl39vpg2xfm2qpy-icat.drv
...
error: builder for '/nix/store/l5wz9inkvkf0qhl8kpl39vpg2xfm2qpy-icat.drv' failed with exit code 2;
       last 10 log lines:
       >                  from /nix/store/hkj250rjsvxcbr31fr1v81cv88cdfp4l-glibc-2.37-8-dev/include/stdio.h:27,
       >                  from icat.c:31:
       > /nix/store/hkj250rjsvxcbr31fr1v81cv88cdfp4l-glibc-2.37-8-dev/include/features.h:195:3: warning: #warning "_BSD_SOURCE and _SVID_SOURCE are deprecated, use _DEFAULT_SOURCE" [8;;https://gcc.gnu.org/onlinedocs/gcc/Warning-Options.html#index-Wcpp-Wcpp8;;]
       >   195 | # warning "_BSD_SOURCE and _SVID_SOURCE are deprecated, use _DEFAULT_SOURCE"
       >       |   ^~~~~~~
       > icat.c:39:10: fatal error: Imlib2.h: No such file or directory
       >    39 | #include <Imlib2.h>
       >       |          ^~~~~~~~~~
       > compilation terminated.
       > make: *** [Makefile:16: icat.o] Error 1
       For full logs, run 'nix log /nix/store/l5wz9inkvkf0qhl8kpl39vpg2xfm2qpy-icat.drv'.

A compiler error! The icat source was pulled from GitHub, and Nix tried to build what it found, but compilation failed due to a missing dependency: the imlib2 header.

If you search for imlib2 on search.nixos.org, you’ll find that imlib2 is already in Nixpkgs.

Add this package to your build environment by adding imlib2 to the arguments of the function in icat.nix. Then add the argument’s value imlib2 to the list of buildInputs in stdenv.mkDerivation:

 1# icat.nix
 2{
 3  lib,
 4  stdenv,
 5  fetchFromGitHub,
 6  imlib2,
 7}:
 8
 9stdenv.mkDerivation {
10  pname = "icat";
11  version = "v0.5";
12
13  src = fetchFromGitHub {
14    owner = "atextor";
15    repo = "icat";
16    rev = "v0.5";
17    sha256 = "0wyy2ksxp95vnh71ybj1bbmqd5ggp13x3mk37pzr99ljs9awy8ka";
18  };
19
20  buildInputs = [ imlib2 ];
21}

Run nix-build -A icat again and you’ll encounter another error, but compilation proceeds further this time:

$ nix-build -A icat
this derivation will be built:
  /nix/store/bw2d4rp2k1l5rg49hds199ma2mz36x47-icat.drv
...
error: builder for '/nix/store/bw2d4rp2k1l5rg49hds199ma2mz36x47-icat.drv' failed with exit code 2;
       last 10 log lines:
       >                  from icat.c:31:
       > /nix/store/hkj250rjsvxcbr31fr1v81cv88cdfp4l-glibc-2.37-8-dev/include/features.h:195:3: warning: #warning "_BSD_SOURCE and _SVID_SOURCE are deprecated, use _DEFAULT_SOURCE" [8;;https://gcc.gnu.org/onlinedocs/gcc/Warning-Options.html#index-Wcpp-Wcpp8;;]
       >   195 | # warning "_BSD_SOURCE and _SVID_SOURCE are deprecated, use _DEFAULT_SOURCE"
       >       |   ^~~~~~~
       > In file included from icat.c:39:
       > /nix/store/4fvrh0sjc8sbkbqda7dfsh7q0gxmnh9p-imlib2-1.11.1-dev/include/Imlib2.h:45:10: fatal error: X11/Xlib.h: No such file or directory
       >    45 | #include <X11/Xlib.h>
       >       |          ^~~~~~~~~~~~
       > compilation terminated.
       > make: *** [Makefile:16: icat.o] Error 1
       For full logs, run 'nix log /nix/store/bw2d4rp2k1l5rg49hds199ma2mz36x47-icat.drv'.

You can see a few warnings which should be corrected in the upstream code. But the important bit for this tutorial is fatal error: X11/Xlib.h: No such file or directory: another dependency is missing.

Finding packages#

Determining from where to source a dependency is currently somewhat involved, because package names don’t always correspond to library or program names.

You will need the Xlib.h headers from the X11 C package, the Nixpkgs derivation for which is libX11, available in the xorg package set. There are multiple ways to figure this out:

search.nixos.org#

Tip

The easiest way to find what you need is on search.nixos.org/packages.

Unfortunately in this case, searching for x11 produces too many irrelevant results because X11 is ubiquitous. On the left side bar there is a list package sets, and selecting xorg shows something promising.

In case all else fails, it helps to become familiar with searching the Nixpkgs source code for keywords.

Adding package sets as dependencies#

Add this to your derivation’s input attribute set and to buildInputs:

 1# icat.nix
 2{
 3  lib,
 4  stdenv,
 5  fetchFromGitHub,
 6  imlib2,
 7  xorg,
 8}:
 9
10stdenv.mkDerivation {
11  pname = "icat";
12  version = "v0.5";
13
14  src = fetchFromGitHub {
15    owner = "atextor";
16    repo = "icat";
17    rev = "v0.5";
18    sha256 = "0wyy2ksxp95vnh71ybj1bbmqd5ggp13x3mk37pzr99ljs9awy8ka";
19  };
20
21  buildInputs = [ imlib2 xorg.libX11 ];
22}

Note

Only add the top-level xorg derivation to the input attrset, rather than the full xorg.libX11, as the latter would cause a syntax error.

Because Nix is lazily-evaluated, using xorg.libX11 means that we only include the libX11 attribute and the derivation doesn’t actually include all of xorg into the build context.

Fixing build failures#

Run the last command again:

$ nix-build -A icat
this derivation will be built:
  /nix/store/x1d79ld8jxqdla5zw2b47d2sl87mf56k-icat.drv
...
error: builder for '/nix/store/x1d79ld8jxqdla5zw2b47d2sl87mf56k-icat.drv' failed with exit code 2;
       last 10 log lines:
       >   195 | # warning "_BSD_SOURCE and _SVID_SOURCE are deprecated, use _DEFAULT_SOURCE"
       >       |   ^~~~~~~
       > icat.c: In function 'main':
       > icat.c:319:33: warning: ignoring return value of 'write' declared with attribute 'warn_unused_result' [8;;https://gcc.gnu.org/onlinedocs/gcc/Warning-Options.html#index-Wunused-result-Wunused-result8;;]
       >   319 |                                 write(tempfile, &buf, 1);
       >       |                                 ^~~~~~~~~~~~~~~~~~~~~~~~
       > gcc -o icat icat.o -lImlib2
       > installing
       > install flags: SHELL=/nix/store/8fv91097mbh5049i9rglc73dx6kjg3qk-bash-5.2-p15/bin/bash install
       > make: *** No rule to make target 'install'.  Stop.
       For full logs, run 'nix log /nix/store/x1d79ld8jxqdla5zw2b47d2sl87mf56k-icat.drv'.

The missing dependency error is solved, but there is now another problem: make: *** No rule to make target 'install'.  Stop.

installPhase#

stdenv is automatically working with the Makefile that comes with icat. The console output shows that configure and make are executed without issue, so the icat binary is compiling successfully.

The failure occurs when the stdenv attempts to run make install. The Makefile included in the project happens to lack an install target. The README in the icat repository only mentions using make to build the tool, leaving the installation step up to users.

To add this step to your derivation, use the installPhase attribute. It contains a list of command strings that are executed to perform the installation.

Because make finishes successfully, the icat executable is available in the build directory. You only need to copy it from there to the output directory.

In Nix, the output directory is stored in the $out variable. That variable is accessible in the derivation’s builder execution environment. Create a bin directory within the $out directory and copy the icat binary there:

 1# icat.nix
 2{
 3  lib,
 4  stdenv,
 5  fetchFromGitHub,
 6  imlib2,
 7  xorg,
 8}:
 9
10stdenv.mkDerivation {
11  pname = "icat";
12  version = "v0.5";
13
14  src = fetchFromGitHub {
15    owner = "atextor";
16    repo = "icat";
17    rev = "v0.5";
18    sha256 = "0wyy2ksxp95vnh71ybj1bbmqd5ggp13x3mk37pzr99ljs9awy8ka";
19  };
20
21  buildInputs = [ imlib2 xorg.libX11 ];
22
23  installPhase = ''
24    mkdir -p $out/bin
25    cp icat $out/bin
26  '';
27}

Phases and hooks#

Nixpkgs stdenv.mkDerivation derivations are separated into phases. Each is intended to control some aspect of the build process.

Earlier you observed how stdenv.mkDerivation expected the project’s Makefile to have an install target, and failed when it didn’t. To fix this, you defined a custom installPhase containing instructions for copying the icat binary to the correct output location, in effect installing it. Up to that point, the stdenv.mkDerivation automatically determined the buildPhase information for the icat package.

During derivation realisation, there are a number of shell functions (“hooks”, in Nixpkgs) which may execute in each derivation phase. Hooks do things like set variables, source files, create directories, and so on.

These are specific to each phase, and run both before and after that phase’s execution. They modify the build environment for common operations during the build.

It’s good practice when packaging software with Nix to include calls to these hooks in the derivation phases you define, even when you don’t make direct use of them. This facilitates easy overriding of specific parts of the derivation later. And it keeps the code tidy and makes it easier to read.

Adjust your installPhase to call the appropriate hooks:

 1# icat.nix
 2
 3# ...
 4
 5  installPhase = ''
 6    runHook preInstall
 7    mkdir -p $out/bin
 8    cp icat $out/bin
 9    runHook postInstall
10  '';
11
12# ...

A successful build#

Running the nix-build command once more will finally do what you want, repeatably. Call ls in the local directory to find a result symlink to a location in the Nix store:

$ ls
default.nix hello.nix icat.nix result

result/bin/icat is the executable built previously. Success!

References#

Next steps#