6. 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.

6.1. Linux kernel

The Nix expressions to build the Linux kernel are in `pkgs/os-specific/linux/kernel <https://github.com/NixOS/nixpkgs/blob/master/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=`` 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=

    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 kernelPackagesFor 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.

6.2. 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: `` 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.