# 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 `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.

## 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`.