# Package-specific usage notes

These chapters includes some notes that apply to specific packages and should answer some of the frequently asked questions related to Nixpkgs use.

Some useful information related to package use can be found in package-specific development notes.

## OpenGL

Packages that use OpenGL have NixOS desktop as their primary target. The current solution for loading the GPU-specific drivers is based on `libglvnd` and looks for the driver implementation in `LD_LIBRARY_PATH`. If you are using a non-NixOS GNU/Linux/X11 desktop with free software video drivers, consider launching OpenGL-dependent programs from Nixpkgs with Nixpkgs versions of `libglvnd` and `mesa_drivers` in `LD_LIBRARY_PATH`. For proprietary video drivers you might have luck with also adding the corresponding video driver package.

## Locales

To allow simultaneous use of packages linked against different versions of `glibc` with different locale archive formats Nixpkgs patches `glibc` to rely on `LOCALE_ARCHIVE` environment variable.

On non-NixOS distributions this variable is obviously not set. This can cause regressions in language support or even crashes in some Nixpkgs-provided programs. The simplest way to mitigate this problem is exporting the `LOCALE_ARCHIVE` variable pointing to `${glibcLocales}/lib/locale/locale-archive`. The drawback (and the reason this is not the default) is the relatively large (a hundred MiB) size of the full set of locales. It is possible to build a custom set of locales by overriding parameters `allLocales` and `locales` of the package.

## Emacs

### Configuring Emacs

The Emacs package comes with some extra helpers to make it easier to configure. `emacsWithPackages` allows you to manage packages from ELPA. This means that you will not have to install that packages from within Emacs. For instance, if you wanted to use `company`, `counsel`, `flycheck`, `ivy`, `magit`, `projectile`, and `use-package` you could use this as a `~/.config/nixpkgs/config.nix` override:

    {
      packageOverrides = pkgs: with pkgs; {
        myEmacs = emacsWithPackages (epkgs: (with epkgs.melpaStablePackages; [
          company
          counsel
          flycheck
          ivy
          magit
          projectile
          use-package
        ]));
      }
    }

You can install it like any other packages via `nix-env -iA myEmacs`. However, this will only install those packages. It will not `configure` them for us. To do this, we need to provide a configuration file. Luckily, it is possible to do this from within Nix! By modifying the above example, we can make Emacs load a custom config file. The key is to create a package that provide a `default.el` file in `/share/emacs/site-start/`. Emacs knows to load this file automatically when it starts.

    {
      packageOverrides = pkgs: with pkgs; rec {
        myEmacsConfig = writeText "default.el" ''
    ;; initialize package

    (require 'package)
    (package-initialize 'noactivate)
    (eval-when-compile
      (require 'use-package))

    ;; load some packages

    (use-package company
      :bind ("<C-tab>" . company-complete)
      :diminish company-mode
      :commands (company-mode global-company-mode)
      :defer 1
      :config
      (global-company-mode))

    (use-package counsel
      :commands (counsel-descbinds)
      :bind (([remap execute-extended-command] . counsel-M-x)
             ("C-x C-f" . counsel-find-file)
             ("C-c g" . counsel-git)
             ("C-c j" . counsel-git-grep)
             ("C-c k" . counsel-ag)
             ("C-x l" . counsel-locate)
             ("M-y" . counsel-yank-pop)))

    (use-package flycheck
      :defer 2
      :config (global-flycheck-mode))

    (use-package ivy
      :defer 1
      :bind (("C-c C-r" . ivy-resume)
             ("C-x C-b" . ivy-switch-buffer)
             :map ivy-minibuffer-map
             ("C-j" . ivy-call))
      :diminish ivy-mode
      :commands ivy-mode
      :config
      (ivy-mode 1))

    (use-package magit
      :defer
      :if (executable-find "git")
      :bind (("C-x g" . magit-status)
             ("C-x G" . magit-dispatch-popup))
      :init
      (setq magit-completing-read-function 'ivy-completing-read))

    (use-package projectile
      :commands projectile-mode
      :bind-keymap ("C-c p" . projectile-command-map)
      :defer 5
      :config
      (projectile-global-mode))
        '';
        myEmacs = emacsWithPackages (epkgs: (with epkgs.melpaStablePackages; [
          (runCommand "default.el" {} ''
    mkdir -p $out/share/emacs/site-lisp
    cp ${myEmacsConfig} $out/share/emacs/site-lisp/default.el
    '')
          company
          counsel
          flycheck
          ivy
          magit
          projectile
          use-package
        ]));
      };
    }

This provides a fairly full Emacs start file. It will load in addition to the user's presonal config. You can always disable it by passing `-q` to the Emacs command.

Sometimes `emacsWithPackages` is not enough, as this package set has some priorities imposed on packages (with the lowest priority assigned to Melpa Unstable, and the highest for packages manually defined in `pkgs/top-level/emacs-packages.nix`). But you can't control this priorities when some package is installed as a dependency. You can override it on per-package-basis, providing all the required dependencies manually - but it's tedious and there is always a possibility that an unwanted dependency will sneak in through some other package. To completely override such a package you can use `overrideScope'`.

    overrides = self: super: rec {
      haskell-mode = self.melpaPackages.haskell-mode;
      ...
    };
    ((emacsPackagesNgGen emacs).overrideScope' overrides).emacsWithPackages (p: with p; [
      # here both these package will use haskell-mode of our own choice
      ghc-mod
      dante
    ])

## DLib

DLib is a modern, C++-based toolkit which provides several machine learning algorithms.

### Compiling without AVX support

Especially older CPUs don't support AVX (Advanced Vector Extensions) instructions that are used by DLib to optimize their algorithms.

On the affected hardware errors like `Illegal instruction` will occur. In those cases AVX support needs to be disabled:

    self: super: {
      dlib = super.dlib.override { avxSupport = false; };
    }

## Unfree software

All users of Nixpkgs are free software users, and many users (and developers) of Nixpkgs want to limit and tightly control their exposure to unfree software. At the same time, many users need (or want) to run some specific pieces of proprietary software. Nixpkgs includes some expressions for unfree software packages. By default unfree software cannot be installed and doesn’t show up in searches. To allow installing unfree software in a single Nix invocation one can export `NIXPKGS_ALLOW_UNFREE=1`. For a persistent solution, users can set `allowUnfree` in the Nixpkgs configuration.

Fine-grained control is possible by defining `allowUnfreePredicate` function in config; it takes the `mkDerivation` parameter attrset and returns `true` for unfree packages that should be allowed.

## Steam

### Steam in Nix

Steam is distributed as a `.deb` file, for now only as an i686 package (the amd64 package only has documentation). When unpacked, it has a script called `steam` that in Ubuntu (their target distro) would go to `/usr/bin` . When run for the first time, this script copies some files to the user's home, which include another script that is the ultimate responsible for launching the steam binary, which is also in $HOME.

Nix problems and constraints:

- We don't have `/bin/bash` and many scripts point there. Similarly for `/usr/bin/python` .

- We don't have the dynamic loader in `/lib` .

- The `steam.sh` script in $HOME can not be patched, as it is checked and rewritten by steam.

- The steam binary cannot be patched, it's also checked.

The current approach to deploy Steam in NixOS is composing a FHS-compatible chroot environment, as documented here. This allows us to have binaries in the expected paths without disrupting the system, and to avoid patching them to work in a non FHS environment.

### How to play

For 64-bit systems it's important to have

    hardware.opengl.driSupport32Bit = true;

in your `/etc/nixos/configuration.nix`. You'll also need

    hardware.pulseaudio.support32Bit = true;

if you are using PulseAudio - this will enable 32bit ALSA apps integration. To use the Steam controller or other Steam supported controllers such as the DualShock 4 or Nintendo Switch Pro, you need to add

    hardware.steam-hardware.enable = true;

to your configuration.

### Troubleshooting

Steam fails to start. What do I do?

  Try to run

      strace steam

  to see what is causing steam to fail.
Using the FOSS Radeon or nouveau (nvidia) drivers

  - The `newStdcpp` parameter was removed since NixOS 17.09 and should not be needed anymore.

  - Steam ships statically linked with a version of libcrypto that conflics with the one dynamically loaded by radeonsi_dri.so. If you get the error

        steam.sh: line 713: 7842 Segmentation fault (core dumped)

    have a look at this pull request.
Java

  1. There is no java in steam chrootenv by default. If you get a message like

         /home/foo/.local/share/Steam/SteamApps/common/towns/towns.sh: line 1: java: command not found

     You need to add

          steam.override { withJava = true; };

     to your configuration.

### steam-run

The FHS-compatible chroot used for steam can also be used to run other linux games that expect a FHS environment. To do it, add

    pkgs.(steam.override {
              nativeOnly = true;
              newStdcpp = true;
            }).run

to your configuration, rebuild, and run the game with

    steam-run ./foo

## Citrix Receiver

The Citrix Receiver is a remote desktop viewer which provides access to XenDesktop installations.

### Basic usage

The tarball archive needs to be downloaded manually as the license agreements of the vendor need to be accepted first. This is available at the download page at citrix.com. Then run `nix-prefetch-url file://$PWD/linuxx64-$version.tar.gz`. With the archive available in the store the package can be built and installed with Nix.

Note: it's recommended to install `Citrix Receiver` using `nix-env -i` or globally to ensure that the `.desktop` files are installed properly into `$XDG_CONFIG_DIRS`. Otherwise it won't be possible to open `.ica` files automatically from the browser to start a Citrix connection.

### Custom certificates

The `Citrix Receiver` in `nixpkgs` trusts several certificates from the Mozilla database by default. However several companies using Citrix might require their own corporate certificate. On distros with imperative packaging these certs can be stored easily in `$ICAROOT`, however this directory is a store path in `nixpkgs`. In order to work around this issue the package provides a simple mechanism to add custom certificates without rebuilding the entire package using `symlinkJoin`:

    with import <nixpkgs> { config.allowUnfree = true; };
    let extraCerts = [ ./custom-cert-1.pem ./custom-cert-2.pem /* ... */ ]; in
    citrix_receiver.override {
      inherit extraCerts;
    }