# Package Notes

This chapter contains information about how to use and maintain the Nix expressions for a number of specific packages, such as the Linux kernel or X.org.

## Linux kernel

The Nix expressions to build the Linux kernel are in `pkgs/os-specific/linux/kernel`.

The function that builds the kernel has an argument `kernelPatches` which should be a list of `{name, patch, extraConfig}` attribute sets, where `name` is the name of the patch (which is included in the kernel’s `meta.description` attribute), `patch` is the patch itself (possibly compressed), and `extraConfig` (optional) is a string specifying extra options to be concatenated to the kernel configuration file (`.config`).

The kernel derivation exports an attribute `features` specifying whether optional functionality is or isn’t enabled. This is used in NixOS to implement kernel-specific behaviour. For instance, if the kernel has the `iwlwifi` feature (i.e. has built-in support for Intel wireless chipsets), then NixOS doesn’t have to build the external `iwlwifi` package:

    modulesTree = [kernel]
      ++ pkgs.lib.optional (!kernel.features ? iwlwifi) kernelPackages.iwlwifi
      ++ ...;

How to add a new (major) version of the Linux kernel to Nixpkgs:

1. Copy the old Nix expression (e.g. `linux-2.6.21.nix`) to the new one (e.g. `linux-2.6.22.nix`) and update it.

2. Add the new kernel to `all-packages.nix` (e.g., create an attribute `kernel_2_6_22`).

3. Now we’re going to update the kernel configuration. First unpack the kernel. Then for each supported platform (`i686`, `x86_64`, `uml`) do the following:

   1. Make an copy from the old config (e.g. `config-2.6.21-i686-smp`) to the new one (e.g. `config-2.6.22-i686-smp`).

   2. Copy the config file for this platform (e.g. `config-2.6.22-i686-smp`) to `.config` in the kernel source tree.

   3. Run `make oldconfig ARCH={i386,x86_64,um}` and answer all questions. (For the uml configuration, also add `SHELL=bash`.) Make sure to keep the configuration consistent between platforms (i.e. don’t enable some feature on `i686` and disable it on `x86_64`).

   4. If needed you can also run `make menuconfig`:

          $ nix-env -i ncurses
          $ export NIX_CFLAGS_LINK=-lncurses
          $ make menuconfig ARCH=arch

   5. Copy `.config` over the new config file (e.g. `config-2.6.22-i686-smp`).

4. Test building the kernel: `nix-build -A kernel_2_6_22`. If it compiles, ship it! For extra credit, try booting NixOS with it.

5. It may be that the new kernel requires updating the external kernel modules and kernel-dependent packages listed in the `linuxPackagesFor` function in `all-packages.nix` (such as the NVIDIA drivers, AUFS, etc.). If the updated packages aren’t backwards compatible with older kernels, you may need to keep the older versions around.

## X.org

The Nix expressions for the X.org packages reside in `pkgs/servers/x11/xorg/default.nix`. This file is automatically generated from lists of tarballs in an X.org release. As such it should not be modified directly; rather, you should modify the lists, the generator script or the file `pkgs/servers/x11/xorg/overrides.nix`, in which you can override or add to the derivations produced by the generator.

The generator is invoked as follows:

    $ cd pkgs/servers/x11/xorg
    $ cat tarballs-7.5.list extra.list old.list \
      | perl ./generate-expr-from-tarballs.pl

For each of the tarballs in the `.list` files, the script downloads it, unpacks it, and searches its `configure.ac` and `*.pc.in` files for dependencies. This information is used to generate `default.nix`. The generator caches downloaded tarballs between runs. Pay close attention to the `NOT FOUND: name` messages at the end of the run, since they may indicate missing dependencies. (Some might be optional dependencies, however.)

A file like `tarballs-7.5.list` contains all tarballs in a X.org release. It can be generated like this:

    $ export i="mirror://xorg/X11R7.4/src/everything/"
    $ cat $(PRINT_PATH=1 nix-prefetch-url $i | tail -n 1) \
      | perl -e 'while (<>) { if (/(href|HREF)="([^"]*.bz2)"/) { print "$ENV{'i'}$2\n"; }; }' \
      | sort > tarballs-7.4.list

`extra.list` contains libraries that aren’t part of X.org proper, but are closely related to it, such as `libxcb`. `old.list` contains some packages that were removed from X.org, but are still needed by some people or by other packages (such as `imake`).

If the expression for a package requires derivation attributes that the generator cannot figure out automatically (say, `patches` or a `postInstall` hook), you should modify `pkgs/servers/x11/xorg/overrides.nix`.

## Eclipse

The Nix expressions related to the Eclipse platform and IDE are in `pkgs/applications/editors/eclipse`.

Nixpkgs provides a number of packages that will install Eclipse in its various forms, these range from the bare-bones Eclipse Platform to the more fully featured Eclipse SDK or Scala-IDE packages and multiple version are often available. It is possible to list available Eclipse packages by issuing the command:

    $ nix-env -f '<nixpkgs>' -qaP -A eclipses --description

Once an Eclipse variant is installed it can be run using the `eclipse` command, as expected. From within Eclipse it is then possible to install plugins in the usual manner by either manually specifying an Eclipse update site or by installing the Marketplace Client plugin and using it to discover and install other plugins. This installation method provides an Eclipse installation that closely resemble a manually installed Eclipse.

If you prefer to install plugins in a more declarative manner then Nixpkgs also offer a number of Eclipse plugins that can be installed in an Eclipse environment. This type of environment is created using the function `eclipseWithPlugins` found inside the `nixpkgs.eclipses` attribute set. This function takes as argument `{ eclipse, plugins ? [], jvmArgs ? [] }` where `eclipse` is a one of the Eclipse packages described above, `plugins` is a list of plugin derivations, and `jvmArgs` is a list of arguments given to the JVM running the Eclipse. For example, say you wish to install the latest Eclipse Platform with the popular Eclipse Color Theme plugin and also allow Eclipse to use more RAM. You could then add

    packageOverrides = pkgs: {
      myEclipse = with pkgs.eclipses; eclipseWithPlugins {
        eclipse = eclipse-platform;
        jvmArgs = [ "-Xmx2048m" ];
        plugins = [ plugins.color-theme ];
      };
    }

to your Nixpkgs configuration (`~/.nixpkgs/config.nix`) and install it by running `nix-env -f '<nixpkgs>' -iA myEclipse` and afterward run Eclipse as usual. It is possible to find out which plugins are available for installation using `eclipseWithPlugins` by running

    $ nix-env -f '<nixpkgs>' -qaP -A eclipses.plugins --description

If there is a need to install plugins that are not available in Nixpkgs then it may be possible to define these plugins outside Nixpkgs using the `buildEclipseUpdateSite` and `buildEclipsePlugin` functions found in the `nixpkgs.eclipses.plugins` attribute set. Use the `buildEclipseUpdateSite` function to install a plugin distributed as an Eclipse update site. This function takes `{ name, src }` as argument where `src` indicates the Eclipse update site archive. All Eclipse features and plugins within the downloaded update site will be installed. When an update site archive is not available then the `buildEclipsePlugin` function can be used to install a plugin that consists of a pair of feature and plugin JARs. This function takes an argument `{ name, srcFeature, srcPlugin }` where `srcFeature` and `srcPlugin` are the feature and plugin JARs, respectively.

Expanding the previous example with two plugins using the above functions we have

    packageOverrides = pkgs: {
      myEclipse = with pkgs.eclipses; eclipseWithPlugins {
        eclipse = eclipse-platform;
        jvmArgs = [ "-Xmx2048m" ];
        plugins = [
          plugins.color-theme
          (plugins.buildEclipsePlugin {
            name = "myplugin1-1.0";
            srcFeature = fetchurl {
              url = "http://…/features/myplugin1.jar";
              sha256 = "123…";
            };
            srcPlugin = fetchurl {
              url = "http://…/plugins/myplugin1.jar";
              sha256 = "123…";
            };
          });
          (plugins.buildEclipseUpdateSite {
            name = "myplugin2-1.0";
            src = fetchurl {
              stripRoot = false;
              url = "http://…/myplugin2.zip";
              sha256 = "123…";
            };
          });
        ];
      };
    }

## Elm

The Nix expressions for Elm reside in `pkgs/development/compilers/elm`. They are generated automatically by `update-elm.rb` script. One should specify versions of Elm packages inside the script, clear the `packages` directory and run the script from inside it. `elm-reactor` is special because it also has Elm package dependencies. The process is not automated very much for now -- you should get the `elm-reactor` source tree (e.g. with `nix-shell`) and run `elm2nix.rb` inside it. Place the resulting `package.nix` file into `packages/elm-reactor-elm.nix`.