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</part>
<part>
<title>Standard environment</title>
<xi:include href="stdenv/stdenv.xml" />
<xi:include href="stdenv/meta.xml" />
<xi:include href="stdenv/multiple-output.xml" />
<xi:include href="stdenv/stdenv.chapter.xml" />
<xi:include href="stdenv/meta.chapter.xml" />
<xi:include href="stdenv/multiple-output.chapter.xml" />
<xi:include href="stdenv/cross-compilation.chapter.xml" />
<xi:include href="stdenv/platform-notes.chapter.xml" />
</part>

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# Meta-attributes {#chap-meta}
Nix packages can declare *meta-attributes* that contain information about a package such as a description, its homepage, its license, and so on. For instance, the GNU Hello package has a `meta` declaration like this:
```nix
meta = with lib; {
description = "A program that produces a familiar, friendly greeting";
longDescription = ''
GNU Hello is a program that prints "Hello, world!" when you run it.
It is fully customizable.
'';
homepage = "https://www.gnu.org/software/hello/manual/";
license = licenses.gpl3Plus;
maintainers = [ maintainers.eelco ];
platforms = platforms.all;
};
```
Meta-attributes are not passed to the builder of the package. Thus, a change to a meta-attribute doesn’t trigger a recompilation of the package. The value of a meta-attribute must be a string.
The meta-attributes of a package can be queried from the command-line using `nix-env`:
```ShellSession
$ nix-env -qa hello --json
{
"hello": {
"meta": {
"description": "A program that produces a familiar, friendly greeting",
"homepage": "https://www.gnu.org/software/hello/manual/",
"license": {
"fullName": "GNU General Public License version 3 or later",
"shortName": "GPLv3+",
"url": "http://www.fsf.org/licensing/licenses/gpl.html"
},
"longDescription": "GNU Hello is a program that prints \"Hello, world!\" when you run it.\nIt is fully customizable.\n",
"maintainers": [
"Ludovic Court\u00e8s <ludo@gnu.org>"
],
"platforms": [
"i686-linux",
"x86_64-linux",
"armv5tel-linux",
"armv7l-linux",
"mips32-linux",
"x86_64-darwin",
"i686-cygwin",
"i686-freebsd",
"x86_64-freebsd",
"i686-openbsd",
"x86_64-openbsd"
],
"position": "/home/user/dev/nixpkgs/pkgs/applications/misc/hello/default.nix:14"
},
"name": "hello-2.9",
"system": "x86_64-linux"
}
}
```
`nix-env` knows about the `description` field specifically:
```ShellSession
$ nix-env -qa hello --description
hello-2.3 A program that produces a familiar, friendly greeting
```
## Standard meta-attributes {#sec-standard-meta-attributes}
It is expected that each meta-attribute is one of the following:
### `description` {#var-meta-description}
A short (one-line) description of the package. This is shown by `nix-env -q --description` and also on the Nixpkgs release pages.
Don’t include a period at the end. Don’t include newline characters. Capitalise the first character. For brevity, don’t repeat the name of package --- just describe what it does.
Wrong: `"libpng is a library that allows you to decode PNG images."`
Right: `"A library for decoding PNG images"`
### `longDescription` {#var-meta-longDescription}
An arbitrarily long description of the package.
### `branch` {#var-meta-branch}
Release branch. Used to specify that a package is not going to receive updates that are not in this branch; for example, Linux kernel 3.0 is supposed to be updated to 3.0.X, not 3.1.
### `homepage` {#var-meta-homepage}
The package’s homepage. Example: `https://www.gnu.org/software/hello/manual/`
### `downloadPage` {#var-meta-downloadPage}
The page where a link to the current version can be found. Example: `https://ftp.gnu.org/gnu/hello/`
### `changelog` {#var-meta-changelog}
A link or a list of links to the location of Changelog for a package. A link may use expansion to refer to the correct changelog version. Example: `"https://git.savannah.gnu.org/cgit/hello.git/plain/NEWS?h=v${version}"`
### `license` {#var-meta-license}
The license, or licenses, for the package. One from the attribute set defined in [`nixpkgs/lib/licenses.nix`](https://github.com/NixOS/nixpkgs/blob/master/lib/licenses.nix). At this moment using both a list of licenses and a single license is valid. If the license field is in the form of a list representation, then it means that parts of the package are licensed differently. Each license should preferably be referenced by their attribute. The non-list attribute value can also be a space delimited string representation of the contained attribute `shortNames` or `spdxIds`. The following are all valid examples:
- Single license referenced by attribute (preferred) `lib.licenses.gpl3Only`.
- Single license referenced by its attribute shortName (frowned upon) `"gpl3Only"`.
- Single license referenced by its attribute spdxId (frowned upon) `"GPL-3.0-only"`.
- Multiple licenses referenced by attribute (preferred) `with lib.licenses; [ asl20 free ofl ]`.
- Multiple licenses referenced as a space delimited string of attribute shortNames (frowned upon) `"asl20 free ofl"`.
For details, see [Licenses](#sec-meta-license).
### `maintainers` {#var-meta-maintainers}
A list of the maintainers of this Nix expression. Maintainers are defined in [`nixpkgs/maintainers/maintainer-list.nix`](https://github.com/NixOS/nixpkgs/blob/master/maintainers/maintainer-list.nix). There is no restriction to becoming a maintainer, just add yourself to that list in a separate commit titled “maintainers: add alice”, and reference maintainers with `maintainers = with lib.maintainers; [ alice bob ]`.
### `priority` {#var-meta-priority}
The *priority* of the package, used by `nix-env` to resolve file name conflicts between packages. See the Nix manual page for `nix-env` for details. Example: `"10"` (a low-priority package).
### `platforms` {#var-meta-platforms}
The list of Nix platform types on which the package is supported. Hydra builds packages according to the platform specified. If no platform is specified, the package does not have prebuilt binaries. An example is:
```nix
meta.platforms = lib.platforms.linux;
```
Attribute Set `lib.platforms` defines [various common lists](https://github.com/NixOS/nixpkgs/blob/master/lib/systems/doubles.nix) of platforms types.
### `tests` {#var-meta-tests}
::: warning
This attribute is special in that it is not actually under the `meta` attribute set but rather under the `passthru` attribute set. This is due to how `meta` attributes work, and the fact that they are supposed to contain only metadata, not derivations.
:::
An attribute set with as values tests. A test is a derivation, which builds successfully when the test passes, and fails to build otherwise. A derivation that is a test needs to have `meta.timeout` defined.
The NixOS tests are available as `nixosTests` in parameters of derivations. For instance, the OpenSMTPD derivation includes lines similar to:
```nix
{ /* ... */, nixosTests }:
{
# ...
passthru.tests = {
basic-functionality-and-dovecot-integration = nixosTests.opensmtpd;
};
}
```
### `timeout` {#var-meta-timeout}
A timeout (in seconds) for building the derivation. If the derivation takes longer than this time to build, it can fail due to breaking the timeout. However, all computers do not have the same computing power, hence some builders may decide to apply a multiplicative factor to this value. When filling this value in, try to keep it approximately consistent with other values already present in `nixpkgs`.
### `hydraPlatforms` {#var-meta-hydraPlatforms}
The list of Nix platform types for which the Hydra instance at `hydra.nixos.org` will build the package. (Hydra is the Nix-based continuous build system.) It defaults to the value of `meta.platforms`. Thus, the only reason to set `meta.hydraPlatforms` is if you want `hydra.nixos.org` to build the package on a subset of `meta.platforms`, or not at all, e.g.
```nix
meta.platforms = lib.platforms.linux;
meta.hydraPlatforms = [];
```
### `broken` {#var-meta-broken}
If set to `true`, the package is marked as "broken", meaning that it won’t show up in `nix-env -qa`, and cannot be built or installed. Such packages should be removed from Nixpkgs eventually unless they are fixed.
### `updateWalker` {#var-meta-updateWalker}
If set to `true`, the package is tested to be updated correctly by the `update-walker.sh` script without additional settings. Such packages have `meta.version` set and their homepage (or the page specified by `meta.downloadPage`) contains a direct link to the package tarball.
## Licenses {#sec-meta-license}
The `meta.license` attribute should preferably contain a value from `lib.licenses` defined in [`nixpkgs/lib/licenses.nix`](https://github.com/NixOS/nixpkgs/blob/master/lib/licenses.nix), or in-place license description of the same format if the license is unlikely to be useful in another expression.
Although it’s typically better to indicate the specific license, a few generic options are available:
### `lib.licenses.free`, `"free"`
Catch-all for free software licenses not listed above.
### `lib.licenses.unfreeRedistributable`, `"unfree-redistributable"`
Unfree package that can be redistributed in binary form. That is, it’s legal to redistribute the *output* of the derivation. This means that the package can be included in the Nixpkgs channel.
Sometimes proprietary software can only be redistributed unmodified. Make sure the builder doesn’t actually modify the original binaries; otherwise we’re breaking the license. For instance, the NVIDIA X11 drivers can be redistributed unmodified, but our builder applies `patchelf` to make them work. Thus, its license is `"unfree"` and it cannot be included in the Nixpkgs channel.
### `lib.licenses.unfree`, `"unfree"`
Unfree package that cannot be redistributed. You can build it yourself, but you cannot redistribute the output of the derivation. Thus it cannot be included in the Nixpkgs channel.
### `lib.licenses.unfreeRedistributableFirmware`, `"unfree-redistributable-firmware"`
This package supplies unfree, redistributable firmware. This is a separate value from `unfree-redistributable` because not everybody cares whether firmware is free.

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<chapter xmlns="http://docbook.org/ns/docbook"
xmlns:xlink="http://www.w3.org/1999/xlink"
xml:id="chap-meta">
<title>Meta-attributes</title>
<para>
Nix packages can declare <emphasis>meta-attributes</emphasis> that contain information about a package such as a description, its homepage, its license, and so on. For instance, the GNU Hello package has a <varname>meta</varname> declaration like this:
<programlisting>
meta = with lib; {
description = "A program that produces a familiar, friendly greeting";
longDescription = ''
GNU Hello is a program that prints "Hello, world!" when you run it.
It is fully customizable.
'';
homepage = "https://www.gnu.org/software/hello/manual/";
license = licenses.gpl3Plus;
maintainers = [ maintainers.eelco ];
platforms = platforms.all;
};
</programlisting>
</para>
<para>
Meta-attributes are not passed to the builder of the package. Thus, a change to a meta-attribute doesn’t trigger a recompilation of the package. The value of a meta-attribute must be a string.
</para>
<para>
The meta-attributes of a package can be queried from the command-line using <command>nix-env</command>:
<screen>
<prompt>$ </prompt>nix-env -qa hello --json
{
"hello": {
"meta": {
"description": "A program that produces a familiar, friendly greeting",
"homepage": "https://www.gnu.org/software/hello/manual/",
"license": {
"fullName": "GNU General Public License version 3 or later",
"shortName": "GPLv3+",
"url": "http://www.fsf.org/licensing/licenses/gpl.html"
},
"longDescription": "GNU Hello is a program that prints \"Hello, world!\" when you run it.\nIt is fully customizable.\n",
"maintainers": [
"Ludovic Court\u00e8s &lt;ludo@gnu.org>"
],
"platforms": [
"i686-linux",
"x86_64-linux",
"armv5tel-linux",
"armv7l-linux",
"mips32-linux",
"x86_64-darwin",
"i686-cygwin",
"i686-freebsd",
"x86_64-freebsd",
"i686-openbsd",
"x86_64-openbsd"
],
"position": "/home/user/dev/nixpkgs/pkgs/applications/misc/hello/default.nix:14"
},
"name": "hello-2.9",
"system": "x86_64-linux"
}
}
</screen>
<command>nix-env</command> knows about the <varname>description</varname> field specifically:
<screen>
<prompt>$ </prompt>nix-env -qa hello --description
hello-2.3 A program that produces a familiar, friendly greeting
</screen>
</para>
<section xml:id="sec-standard-meta-attributes">
<title>Standard meta-attributes</title>
<para>
It is expected that each meta-attribute is one of the following:
</para>
<variablelist>
<varlistentry xml:id="var-meta-description">
<term>
<varname>description</varname>
</term>
<listitem>
<para>
A short (one-line) description of the package. This is shown by <command>nix-env -q --description</command> and also on the Nixpkgs release pages.
</para>
<para>
Don’t include a period at the end. Don’t include newline characters. Capitalise the first character. For brevity, don’t repeat the name of package — just describe what it does.
</para>
<para>
Wrong: <literal>"libpng is a library that allows you to decode PNG images."</literal>
</para>
<para>
Right: <literal>"A library for decoding PNG images"</literal>
</para>
</listitem>
</varlistentry>
<varlistentry xml:id="var-meta-longDescription">
<term>
<varname>longDescription</varname>
</term>
<listitem>
<para>
An arbitrarily long description of the package.
</para>
</listitem>
</varlistentry>
<varlistentry xml:id="var-meta-branch">
<term>
<varname>branch</varname>
</term>
<listitem>
<para>
Release branch. Used to specify that a package is not going to receive updates that are not in this branch; for example, Linux kernel 3.0 is supposed to be updated to 3.0.X, not 3.1.
</para>
</listitem>
</varlistentry>
<varlistentry xml:id="var-meta-homepage">
<term>
<varname>homepage</varname>
</term>
<listitem>
<para>
The package’s homepage. Example: <literal>https://www.gnu.org/software/hello/manual/</literal>
</para>
</listitem>
</varlistentry>
<varlistentry xml:id="var-meta-downloadPage">
<term>
<varname>downloadPage</varname>
</term>
<listitem>
<para>
The page where a link to the current version can be found. Example: <literal>https://ftp.gnu.org/gnu/hello/</literal>
</para>
</listitem>
</varlistentry>
<varlistentry xml:id="var-meta-changelog">
<term>
<varname>changelog</varname>
</term>
<listitem>
<para>
A link or a list of links to the location of Changelog for a package. A link may use expansion to refer to the correct changelog version. Example: <literal>"https://git.savannah.gnu.org/cgit/hello.git/plain/NEWS?h=v${version}"</literal>
</para>
</listitem>
</varlistentry>
<varlistentry xml:id="var-meta-license">
<term>
<varname>license</varname>
</term>
<listitem>
<para>
The license, or licenses, for the package. One from the attribute set defined in <link
xlink:href="https://github.com/NixOS/nixpkgs/blob/master/lib/licenses.nix"> <filename>nixpkgs/lib/licenses.nix</filename></link>. At this moment using both a list of licenses and a single license is valid. If the license field is in the form of a list representation, then it means that parts of the package are licensed differently. Each license should preferably be referenced by their attribute. The non-list attribute value can also be a space delimited string representation of the contained attribute shortNames or spdxIds. The following are all valid examples:
<itemizedlist>
<listitem>
<para>
Single license referenced by attribute (preferred) <literal>lib.licenses.gpl3Only</literal>.
</para>
</listitem>
<listitem>
<para>
Single license referenced by its attribute shortName (frowned upon) <literal>"gpl3Only"</literal>.
</para>
</listitem>
<listitem>
<para>
Single license referenced by its attribute spdxId (frowned upon) <literal>"GPL-3.0-only"</literal>.
</para>
</listitem>
<listitem>
<para>
Multiple licenses referenced by attribute (preferred) <literal>with lib.licenses; [ asl20 free ofl ]</literal>.
</para>
</listitem>
<listitem>
<para>
Multiple licenses referenced as a space delimited string of attribute shortNames (frowned upon) <literal>"asl20 free ofl"</literal>.
</para>
</listitem>
</itemizedlist>
For details, see <xref linkend='sec-meta-license'/>.
</para>
</listitem>
</varlistentry>
<varlistentry xml:id="var-meta-maintainers">
<term>
<varname>maintainers</varname>
</term>
<listitem>
<para>
A list of the maintainers of this Nix expression. Maintainers are defined in <link xlink:href="https://github.com/NixOS/nixpkgs/blob/master/maintainers/maintainer-list.nix"><filename>nixpkgs/maintainers/maintainer-list.nix</filename></link>. There is no restriction to becoming a maintainer, just add yourself to that list in a separate commit titled 'maintainers: add alice', and reference maintainers with <literal>maintainers = with lib.maintainers; [ alice bob ]</literal>.
</para>
</listitem>
</varlistentry>
<varlistentry xml:id="var-meta-priority">
<term>
<varname>priority</varname>
</term>
<listitem>
<para>
The <emphasis>priority</emphasis> of the package, used by <command>nix-env</command> to resolve file name conflicts between packages. See the Nix manual page for <command>nix-env</command> for details. Example: <literal>"10"</literal> (a low-priority package).
</para>
</listitem>
</varlistentry>
<varlistentry xml:id="var-meta-platforms">
<term>
<varname>platforms</varname>
</term>
<listitem>
<para>
The list of Nix platform types on which the package is supported. Hydra builds packages according to the platform specified. If no platform is specified, the package does not have prebuilt binaries. An example is:
<programlisting>
meta.platforms = lib.platforms.linux;
</programlisting>
Attribute Set <varname>lib.platforms</varname> defines <link xlink:href="https://github.com/NixOS/nixpkgs/blob/master/lib/systems/doubles.nix"> various common lists</link> of platforms types.
</para>
</listitem>
</varlistentry>
<varlistentry xml:id="var-meta-tests">
<term>
<varname>tests</varname>
</term>
<listitem>
<warning>
<para>
This attribute is special in that it is not actually under the <literal>meta</literal> attribute set but rather under the <literal>passthru</literal> attribute set. This is due to how <literal>meta</literal> attributes work, and the fact that they are supposed to contain only metadata, not derivations.
</para>
</warning>
<para>
An attribute set with as values tests. A test is a derivation, which builds successfully when the test passes, and fails to build otherwise. A derivation that is a test needs to have <literal>meta.timeout</literal> defined.
</para>
<para>
The NixOS tests are available as <literal>nixosTests</literal> in parameters of derivations. For instance, the OpenSMTPD derivation includes lines similar to:
<programlisting>
{ /* ... */, nixosTests }:
{
# ...
passthru.tests = {
basic-functionality-and-dovecot-integration = nixosTests.opensmtpd;
};
}
</programlisting>
</para>
</listitem>
</varlistentry>
<varlistentry xml:id="var-meta-timeout">
<term>
<varname>timeout</varname>
</term>
<listitem>
<para>
A timeout (in seconds) for building the derivation. If the derivation takes longer than this time to build, it can fail due to breaking the timeout. However, all computers do not have the same computing power, hence some builders may decide to apply a multiplicative factor to this value. When filling this value in, try to keep it approximately consistent with other values already present in <literal>nixpkgs</literal>.
</para>
</listitem>
</varlistentry>
<varlistentry xml:id="var-meta-hydraPlatforms">
<term>
<varname>hydraPlatforms</varname>
</term>
<listitem>
<para>
The list of Nix platform types for which the Hydra instance at <literal>hydra.nixos.org</literal> will build the package. (Hydra is the Nix-based continuous build system.) It defaults to the value of <varname>meta.platforms</varname>. Thus, the only reason to set <varname>meta.hydraPlatforms</varname> is if you want <literal>hydra.nixos.org</literal> to build the package on a subset of <varname>meta.platforms</varname>, or not at all, e.g.
<programlisting>
meta.platforms = lib.platforms.linux;
meta.hydraPlatforms = [];
</programlisting>
</para>
</listitem>
</varlistentry>
<varlistentry xml:id="var-meta-broken">
<term>
<varname>broken</varname>
</term>
<listitem>
<para>
If set to <literal>true</literal>, the package is marked as “broken”, meaning that it won’t show up in <literal>nix-env -qa</literal>, and cannot be built or installed. Such packages should be removed from Nixpkgs eventually unless they are fixed.
</para>
</listitem>
</varlistentry>
<varlistentry xml:id="var-meta-updateWalker">
<term>
<varname>updateWalker</varname>
</term>
<listitem>
<para>
If set to <literal>true</literal>, the package is tested to be updated correctly by the <literal>update-walker.sh</literal> script without additional settings. Such packages have <varname>meta.version</varname> set and their homepage (or the page specified by <varname>meta.downloadPage</varname>) contains a direct link to the package tarball.
</para>
</listitem>
</varlistentry>
</variablelist>
</section>
<section xml:id="sec-meta-license">
<title>Licenses</title>
<para>
The <varname>meta.license</varname> attribute should preferrably contain a value from <varname>lib.licenses</varname> defined in <link xlink:href="https://github.com/NixOS/nixpkgs/blob/master/lib/licenses.nix"> <filename>nixpkgs/lib/licenses.nix</filename></link>, or in-place license description of the same format if the license is unlikely to be useful in another expression.
</para>
<para>
Although it's typically better to indicate the specific license, a few generic options are available:
<variablelist>
<varlistentry>
<term>
<varname>lib.licenses.free</varname>, <varname>"free"</varname>
</term>
<listitem>
<para>
Catch-all for free software licenses not listed above.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<varname>lib.licenses.unfreeRedistributable</varname>, <varname>"unfree-redistributable"</varname>
</term>
<listitem>
<para>
Unfree package that can be redistributed in binary form. That is, it’s legal to redistribute the <emphasis>output</emphasis> of the derivation. This means that the package can be included in the Nixpkgs channel.
</para>
<para>
Sometimes proprietary software can only be redistributed unmodified. Make sure the builder doesn’t actually modify the original binaries; otherwise we’re breaking the license. For instance, the NVIDIA X11 drivers can be redistributed unmodified, but our builder applies <command>patchelf</command> to make them work. Thus, its license is <varname>"unfree"</varname> and it cannot be included in the Nixpkgs channel.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<varname>lib.licenses.unfree</varname>, <varname>"unfree"</varname>
</term>
<listitem>
<para>
Unfree package that cannot be redistributed. You can build it yourself, but you cannot redistribute the output of the derivation. Thus it cannot be included in the Nixpkgs channel.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<varname>lib.licenses.unfreeRedistributableFirmware</varname>, <varname>"unfree-redistributable-firmware"</varname>
</term>
<listitem>
<para>
This package supplies unfree, redistributable firmware. This is a separate value from <varname>unfree-redistributable</varname> because not everybody cares whether firmware is free.
</para>
</listitem>
</varlistentry>
</variablelist>
</para>
</section>
</chapter>

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# Multiple-output packages {#chap-multiple-output}
## Introduction {#sec-multiple-outputs-introduction}
The Nix language allows a derivation to produce multiple outputs, which is similar to what is utilized by other Linux distribution packaging systems. The outputs reside in separate Nix store paths, so they can be mostly handled independently of each other, including passing to build inputs, garbage collection or binary substitution. The exception is that building from source always produces all the outputs.
The main motivation is to save disk space by reducing runtime closure sizes; consequently also sizes of substituted binaries get reduced. Splitting can be used to have more granular runtime dependencies, for example the typical reduction is to split away development-only files, as those are typically not needed during runtime. As a result, closure sizes of many packages can get reduced to a half or even much less.
::: note
The reduction effects could be instead achieved by building the parts in completely separate derivations. That would often additionally reduce build-time closures, but it tends to be much harder to write such derivations, as build systems typically assume all parts are being built at once. This compromise approach of single source package producing multiple binary packages is also utilized often by rpm and deb.
:::
A number of attributes can be used to work with a derivation with multiple outputs. The attribute `outputs` is a list of strings, which are the names of the outputs. For each of these names, an identically named attribute is created, corresponding to that output. The attribute `meta.outputsToInstall` is used to determine the default set of outputs to install when using the derivation name unqualified.
## Installing a split package {#sec-multiple-outputs-installing}
When installing a package with multiple outputs, the package’s `meta.outputsToInstall` attribute determines which outputs are actually installed. `meta.outputsToInstall` is a list whose [default installs binaries and the associated man pages](https://github.com/NixOS/nixpkgs/blob/f1680774340d5443a1409c3421ced84ac1163ba9/pkgs/stdenv/generic/make-derivation.nix#L310-L320). The following sections describe ways to install different outputs.
### Selecting outputs to install via NixOS {#sec-multiple-outputs-installing-nixos}
NixOS provides two ways to select the outputs to install for packages listed in `environment.systemPackages`:
- The configuration option `environment.extraOutputsToInstall` is appended to each package’s `meta.outputsToInstall` attribute to determine the outputs to install. It can for example be used to install `info` documentation or debug symbols for all packages.
- The outputs can be listed as packages in `environment.systemPackages`. For example, the `"out"` and `"info"` outputs for the `coreutils` package can be installed by including `coreutils` and `coreutils.info` in `environment.systemPackages`.
### Selecting outputs to install via `nix-env` {#sec-multiple-outputs-installing-nix-env}
`nix-env` lacks an easy way to select the outputs to install. When installing a package, `nix-env` always installs the outputs listed in `meta.outputsToInstall`, even when the user explicitly selects an output.
::: warning
`nix-env` silenty disregards the outputs selected by the user, and instead installs the outputs from `meta.outputsToInstall`. For example,
```ShellSession
$ nix-env -iA nixpkgs.coreutils.info
```
installs the `"out"` output (`coreutils.meta.outputsToInstall` is `[ "out" ]`) instead of the requested `"info"`.
:::
The only recourse to select an output with `nix-env` is to override the package’s `meta.outputsToInstall`, using the functions described in <xref linkend="chap-overrides" />. For example, the following overlay adds the `"info"` output for the `coreutils` package:
```nix
self: super:
{
coreutils = super.coreutils.overrideAttrs (oldAttrs: {
meta = oldAttrs.meta // { outputsToInstall = oldAttrs.meta.outputsToInstall or [ "out" ] ++ [ "info" ]; };
});
}
```
## Using a split package {#sec-multiple-outputs-using-split-packages}
In the Nix language the individual outputs can be reached explicitly as attributes, e.g. `coreutils.info`, but the typical case is just using packages as build inputs.
When a multiple-output derivation gets into a build input of another derivation, the `dev` output is added if it exists, otherwise the first output is added. In addition to that, `propagatedBuildOutputs` of that package which by default contain `$outputBin` and `$outputLib` are also added. (See <xref linkend="multiple-output-file-type-groups" />.)
In some cases it may be desirable to combine different outputs under a single store path. A function `symlinkJoin` can be used to do this. (Note that it may negate some closure size benefits of using a multiple-output package.)
## Writing a split derivation {#sec-multiple-outputs-}
Here you find how to write a derivation that produces multiple outputs.
In nixpkgs there is a framework supporting multiple-output derivations. It tries to cover most cases by default behavior. You can find the source separated in `<nixpkgs/pkgs/build-support/setup-hooks/multiple-outputs.sh>`; it’s relatively well-readable. The whole machinery is triggered by defining the `outputs` attribute to contain the list of desired output names (strings).
```nix
outputs = [ "bin" "dev" "out" "doc" ];
```
Often such a single line is enough. For each output an equally named environment variable is passed to the builder and contains the path in nix store for that output. Typically you also want to have the main `out` output, as it catches any files that didn’t get elsewhere.
::: note
There is a special handling of the `debug` output, described at <xref linkend="stdenv-separateDebugInfo" />.
:::
### “Binaries first” {#multiple-output-file-binaries-first-convention}
A commonly adopted convention in `nixpkgs` is that executables provided by the package are contained within its first output. This convention allows the dependent packages to reference the executables provided by packages in a uniform manner. For instance, provided with the knowledge that the `perl` package contains a `perl` executable it can be referenced as `${pkgs.perl}/bin/perl` within a Nix derivation that needs to execute a Perl script.
The `glibc` package is a deliberate single exception to the “binaries first” convention. The `glibc` has `libs` as its first output allowing the libraries provided by `glibc` to be referenced directly (e.g. `${stdenv.glibc}/lib/ld-linux-x86-64.so.2`). The executables provided by `glibc` can be accessed via its `bin` attribute (e.g. `${stdenv.glibc.bin}/bin/ldd`).
The reason for why `glibc` deviates from the convention is because referencing a library provided by `glibc` is a very common operation among Nix packages. For instance, third-party executables packaged by Nix are typically patched and relinked with the relevant version of `glibc` libraries from Nix packages (please see the documentation on [patchelf](https://github.com/NixOS/patchelf/blob/master/README) for more details).
### File type groups {#multiple-output-file-type-groups}
The support code currently recognizes some particular kinds of outputs and either instructs the build system of the package to put files into their desired outputs or it moves the files during the fixup phase. Each group of file types has an `outputFoo` variable specifying the output name where they should go. If that variable isn’t defined by the derivation writer, it is guessed – a default output name is defined, falling back to other possibilities if the output isn’t defined.
#### ` $outputDev`
is for development-only files. These include C(++) headers (`include/`), pkg-config (`lib/pkgconfig/`), cmake (`lib/cmake/`) and aclocal files (`share/aclocal/`). They go to `dev` or `out` by default.
#### ` $outputBin`
is meant for user-facing binaries, typically residing in `bin/`. They go to `bin` or `out` by default.
#### ` $outputLib`
is meant for libraries, typically residing in `lib/` and `libexec/`. They go to `lib` or `out` by default.
#### ` $outputDoc`
is for user documentation, typically residing in `share/doc/`. It goes to `doc` or `out` by default.
#### ` $outputDevdoc`
is for _developer_ documentation. Currently we count gtk-doc and devhelp books, typically residing in `share/gtk-doc/` and `share/devhelp/`, in there. It goes to `devdoc` or is removed (!) by default. This is because e.g. gtk-doc tends to be rather large and completely unused by nixpkgs users.
#### ` $outputMan`
is for man pages (except for section 3), typically residing in `share/man/man[0-9]/`. They go to `man` or `$outputBin` by default.
#### ` $outputDevman`
is for section 3 man pages, typically residing in `share/man/man[0-9]/`. They go to `devman` or `$outputMan` by default.
#### ` $outputInfo`
is for info pages, typically residing in `share/info/`. They go to `info` or `$outputBin` by default.
### Common caveats {#sec-multiple-outputs-caveats}
- Some configure scripts don’t like some of the parameters passed by default by the framework, e.g. `--docdir=/foo/bar`. You can disable this by setting `setOutputFlags = false;`.
- The outputs of a single derivation can retain references to each other, but note that circular references are not allowed. (And each strongly-connected component would act as a single output anyway.)
- Most of split packages contain their core functionality in libraries. These libraries tend to refer to various kind of data that typically gets into `out`, e.g. locale strings, so there is often no advantage in separating the libraries into `lib`, as keeping them in `out` is easier.
- Some packages have hidden assumptions on install paths, which complicates splitting.

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View File

@ -1,261 +0,0 @@
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE chapter [
<!ENTITY ndash "&#x2013;"> <!-- @vcunat likes to use this one ;-) -->
]>
<chapter xmlns="http://docbook.org/ns/docbook"
xmlns:xlink="http://www.w3.org/1999/xlink"
xml:id="chap-multiple-output">
<title>Multiple-output packages</title>
<section xml:id="sec-multiple-outputs-introduction">
<title>Introduction</title>
<para>
The Nix language allows a derivation to produce multiple outputs, which is similar to what is utilized by other Linux distribution packaging systems. The outputs reside in separate Nix store paths, so they can be mostly handled independently of each other, including passing to build inputs, garbage collection or binary substitution. The exception is that building from source always produces all the outputs.
</para>
<para>
The main motivation is to save disk space by reducing runtime closure sizes; consequently also sizes of substituted binaries get reduced. Splitting can be used to have more granular runtime dependencies, for example the typical reduction is to split away development-only files, as those are typically not needed during runtime. As a result, closure sizes of many packages can get reduced to a half or even much less.
</para>
<note>
<para>
The reduction effects could be instead achieved by building the parts in completely separate derivations. That would often additionally reduce build-time closures, but it tends to be much harder to write such derivations, as build systems typically assume all parts are being built at once. This compromise approach of single source package producing multiple binary packages is also utilized often by rpm and deb.
</para>
</note>
<para>
A number of attributes can be used to work with a derivation with multiple outputs. The attribute <varname>outputs</varname> is a list of strings, which are the names of the outputs. For each of these names, an identically named attribute is created, corresponding to that output. The attribute <varname>meta.outputsToInstall</varname> is used to determine the default set of outputs to install when using the derivation name unqualified.
</para>
</section>
<section xml:id="sec-multiple-outputs-installing">
<title>Installing a split package</title>
<para>
When installing a package with multiple outputs, the package's <varname>meta.outputsToInstall</varname> attribute determines which outputs are actually installed. <varname>meta.outputsToInstall</varname> is a list whose <link xlink:href="https://github.com/NixOS/nixpkgs/blob/f1680774340d5443a1409c3421ced84ac1163ba9/pkgs/stdenv/generic/make-derivation.nix#L310-L320">default installs binaries and the associated man pages</link>. The following sections describe ways to install different outputs.
</para>
<section xml:id="sec-multiple-outputs-installing-nixos">
<title>Selecting outputs to install via NixOS</title>
<para>
NixOS provides two ways to select the outputs to install for packages listed in <varname>environment.systemPackages</varname>:
</para>
<itemizedlist>
<listitem>
<para>
The configuration option <varname>environment.extraOutputsToInstall</varname> is appended to each package's <varname>meta.outputsToInstall</varname> attribute to determine the outputs to install. It can for example be used to install <literal>info</literal> documentation or debug symbols for all packages.
</para>
</listitem>
<listitem>
<para>
The outputs can be listed as packages in <varname>environment.systemPackages</varname>. For example, the <literal>"out"</literal> and <literal>"info"</literal> outputs for the <varname>coreutils</varname> package can be installed by including <varname>coreutils</varname> and <varname>coreutils.info</varname> in <varname>environment.systemPackages</varname>.
</para>
</listitem>
</itemizedlist>
</section>
<section xml:id="sec-multiple-outputs-installing-nix-env">
<title>Selecting outputs to install via <command>nix-env</command></title>
<para>
<command>nix-env</command> lacks an easy way to select the outputs to install. When installing a package, <command>nix-env</command> always installs the outputs listed in <varname>meta.outputsToInstall</varname>, even when the user explicitly selects an output.
</para>
<warning>
<para>
<command>nix-env</command> silenty disregards the outputs selected by the user, and instead installs the outputs from <varname>meta.outputsToInstall</varname>. For example,
</para>
<screen><prompt>$ </prompt>nix-env -iA nixpkgs.coreutils.info</screen>
<para>
installs the <literal>"out"</literal> output (<varname>coreutils.meta.outputsToInstall</varname> is <literal>[ "out" ]</literal>) instead of the requested <literal>"info"</literal>.
</para>
</warning>
<para>
The only recourse to select an output with <command>nix-env</command> is to override the package's <varname>meta.outputsToInstall</varname>, using the functions described in <xref linkend="chap-overrides" />. For example, the following overlay adds the <literal>"info"</literal> output for the <varname>coreutils</varname> package:
</para>
<programlisting>self: super:
{
coreutils = super.coreutils.overrideAttrs (oldAttrs: {
meta = oldAttrs.meta // { outputsToInstall = oldAttrs.meta.outputsToInstall or [ "out" ] ++ [ "info" ]; };
});
}
</programlisting>
</section>
</section>
<section xml:id="sec-multiple-outputs-using-split-packages">
<title>Using a split package</title>
<para>
In the Nix language the individual outputs can be reached explicitly as attributes, e.g. <varname>coreutils.info</varname>, but the typical case is just using packages as build inputs.
</para>
<para>
When a multiple-output derivation gets into a build input of another derivation, the <varname>dev</varname> output is added if it exists, otherwise the first output is added. In addition to that, <varname>propagatedBuildOutputs</varname> of that package which by default contain <varname>$outputBin</varname> and <varname>$outputLib</varname> are also added. (See <xref linkend="multiple-output-file-type-groups" />.)
</para>
<para>
In some cases it may be desirable to combine different outputs under a single store path. A function <literal>symlinkJoin</literal> can be used to do this. (Note that it may negate some closure size benefits of using a multiple-output package.)
</para>
</section>
<section xml:id="sec-multiple-outputs-">
<title>Writing a split derivation</title>
<para>
Here you find how to write a derivation that produces multiple outputs.
</para>
<para>
In nixpkgs there is a framework supporting multiple-output derivations. It tries to cover most cases by default behavior. You can find the source separated in &lt;<filename>nixpkgs/pkgs/build-support/setup-hooks/multiple-outputs.sh</filename>&gt;; it's relatively well-readable. The whole machinery is triggered by defining the <varname>outputs</varname> attribute to contain the list of desired output names (strings).
</para>
<programlisting>outputs = [ "bin" "dev" "out" "doc" ];</programlisting>
<para>
Often such a single line is enough. For each output an equally named environment variable is passed to the builder and contains the path in nix store for that output. Typically you also want to have the main <varname>out</varname> output, as it catches any files that didn't get elsewhere.
</para>
<note>
<para>
There is a special handling of the <varname>debug</varname> output, described at <xref linkend="stdenv-separateDebugInfo" />.
</para>
</note>
<section xml:id="multiple-output-file-binaries-first-convention">
<title><quote>Binaries first</quote></title>
<para>
A commonly adopted convention in <literal>nixpkgs</literal> is that executables provided by the package are contained within its first output. This convention allows the dependent packages to reference the executables provided by packages in a uniform manner. For instance, provided with the knowledge that the <literal>perl</literal> package contains a <literal>perl</literal> executable it can be referenced as <literal>${pkgs.perl}/bin/perl</literal> within a Nix derivation that needs to execute a Perl script.
</para>
<para>
The <literal>glibc</literal> package is a deliberate single exception to the <quote>binaries first</quote> convention. The <literal>glibc</literal> has <literal>libs</literal> as its first output allowing the libraries provided by <literal>glibc</literal> to be referenced directly (e.g. <literal>${stdenv.glibc}/lib/ld-linux-x86-64.so.2</literal>). The executables provided by <literal>glibc</literal> can be accessed via its <literal>bin</literal> attribute (e.g. <literal>${stdenv.glibc.bin}/bin/ldd</literal>).
</para>
<para>
The reason for why <literal>glibc</literal> deviates from the convention is because referencing a library provided by <literal>glibc</literal> is a very common operation among Nix packages. For instance, third-party executables packaged by Nix are typically patched and relinked with the relevant version of <literal>glibc</literal> libraries from Nix packages (please see the documentation on <link xlink:href="https://github.com/NixOS/patchelf/blob/master/README">patchelf</link> for more details).
</para>
</section>
<section xml:id="multiple-output-file-type-groups">
<title>File type groups</title>
<para>
The support code currently recognizes some particular kinds of outputs and either instructs the build system of the package to put files into their desired outputs or it moves the files during the fixup phase. Each group of file types has an <varname>outputFoo</varname> variable specifying the output name where they should go. If that variable isn't defined by the derivation writer, it is guessed &ndash; a default output name is defined, falling back to other possibilities if the output isn't defined.
</para>
<variablelist>
<varlistentry>
<term>
<varname> $outputDev</varname>
</term>
<listitem>
<para>
is for development-only files. These include C(++) headers (<filename>include/</filename>), pkg-config (<filename>lib/pkgconfig/</filename>), cmake (<filename>lib/cmake/</filename>) and aclocal files (<varname>share/aclocal/</varname>). They go to <varname>dev</varname> or <varname>out</varname> by default.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<varname> $outputBin</varname>
</term>
<listitem>
<para>
is meant for user-facing binaries, typically residing in <filename>bin/</filename>. They go to <varname>bin</varname> or <varname>out</varname> by default.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<varname> $outputLib</varname>
</term>
<listitem>
<para>
is meant for libraries, typically residing in <filename>lib/</filename> and <filename>libexec/</filename>. They go to <varname>lib</varname> or <varname>out</varname> by default.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<varname> $outputDoc</varname>
</term>
<listitem>
<para>
is for user documentation, typically residing in <filename>share/doc/</filename>. It goes to <varname>doc</varname> or <varname>out</varname> by default.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<varname> $outputDevdoc</varname>
</term>
<listitem>
<para>
is for <emphasis>developer</emphasis> documentation. Currently we count gtk-doc and devhelp books, typically residing in <filename>share/gtk-doc/</filename> and <filename>share/devhelp/</filename>, in there. It goes to <varname>devdoc</varname> or is removed (!) by default. This is because e.g. gtk-doc tends to be rather large and completely unused by nixpkgs users.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<varname> $outputMan</varname>
</term>
<listitem>
<para>
is for man pages (except for section 3), typically residing in <filename>share/man/man[0-9]/</filename>. They go to <varname>man</varname> or <varname>$outputBin</varname> by default.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<varname> $outputDevman</varname>
</term>
<listitem>
<para>
is for section 3 man pages, typically residing in <filename>share/man/man3/</filename>. They go to <varname>devman</varname> or <varname>$outputMan</varname> by default.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<varname> $outputInfo</varname>
</term>
<listitem>
<para>
is for info pages, typically residing in <filename>share/info/</filename>. They go to <varname>info</varname> or <varname>$outputBin</varname> by default.
</para>
</listitem>
</varlistentry>
</variablelist>
</section>
<section xml:id="sec-multiple-outputs-caveats">
<title>Common caveats</title>
<itemizedlist>
<listitem>
<para>
Some configure scripts don't like some of the parameters passed by default by the framework, e.g. <literal>--docdir=/foo/bar</literal>. You can disable this by setting <literal>setOutputFlags = false;</literal>.
</para>
</listitem>
<listitem>
<para>
The outputs of a single derivation can retain references to each other, but note that circular references are not allowed. (And each strongly-connected component would act as a single output anyway.)
</para>
</listitem>
<listitem>
<para>
Most of split packages contain their core functionality in libraries. These libraries tend to refer to various kind of data that typically gets into <varname>out</varname>, e.g. locale strings, so there is often no advantage in separating the libraries into <varname>lib</varname>, as keeping them in <varname>out</varname> is easier.
</para>
</listitem>
<listitem>
<para>
Some packages have hidden assumptions on install paths, which complicates splitting.
</para>
</listitem>
</itemizedlist>
</section>
</section>
<!--Writing a split derivation-->
</chapter>

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nixos/modules/misc/documentation.nix
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@ -98,7 +98,7 @@ in
See "Multiple-output packages" chapter in the nixpkgs manual for more info.
'';
# which is at ../../../doc/multiple-output.xml
# which is at ../../../doc/multiple-output.chapter.md
};
man.enable = mkOption {

4
nixos/modules/misc/meta.nix
View File

@ -47,9 +47,9 @@ in
doc = mkOption {
type = docFile;
internal = true;
example = "./meta.xml";
example = "./meta.chapter.xml";
description = ''
Documentation prologe for the set of options of each module. This
Documentation prologue for the set of options of each module. This
option should be defined at most once per module.
'';
};