<section xmlns="http://docbook.org/ns/docbook"
xmlns:xlink="http://www.w3.org/1999/xlink"
xmlns:xi="http://www.w3.org/2001/XInclude"
version="5.0"
xml:id="sec-advanced-attributes">
<title>Advanced Attributes</title>
<para>Derivations can declare some infrequently used optional
attributes.</para>
<variablelist>
<varlistentry><term><varname>allowedReferences</varname></term>
<listitem><para>The optional attribute
<varname>allowedReferences</varname> specifies a list of legal
references (dependencies) of the output of the builder. For
example,
<programlisting>
allowedReferences = [];
</programlisting>
enforces that the output of a derivation cannot have any runtime
dependencies on its inputs. To allow an output to have a runtime
dependency on itself, use <literal>"out"</literal> as a list item.
This is used in NixOS to check that generated files such as
initial ramdisks for booting Linux don’t have accidental
dependencies on other paths in the Nix store.</para></listitem>
</varlistentry>
<varlistentry><term><varname>exportReferencesGraph</varname></term>
<listitem><para>This attribute allows builders access to the
references graph of their inputs. The attribute is a list of
inputs in the Nix store whose references graph the builder needs
to know. The value of this attribute should be a list of pairs
<literal>[ <replaceable>name1</replaceable>
<replaceable>path1</replaceable> <replaceable>name2</replaceable>
<replaceable>path2</replaceable> <replaceable>...</replaceable>
]</literal>. The references graph of each
<replaceable>pathN</replaceable> will be stored in a text file
<replaceable>nameN</replaceable> in the temporary build directory.
The text files have the format used by <command>nix-store
--register-validity</command> (with the deriver fields left
empty). For example, when the following derivation is built:
<programlisting>
derivation {
...
exportReferencesGraph = [ "libfoo-graph" libfoo ];
};
</programlisting>
the references graph of <literal>libfoo</literal> is placed in the
file <filename>libfoo-graph</filename> in the temporary build
directory.</para>
<para><varname>exportReferencesGraph</varname> is useful for
builders that want to do something with the closure of a store
path. Examples include the builders in NixOS that generate the
initial ramdisk for booting Linux (a <command>cpio</command>
archive containing the closure of the boot script) and the
ISO-9660 image for the installation CD (which is populated with a
Nix store containing the closure of a bootable NixOS
configuration).</para></listitem>
</varlistentry>
<varlistentry xml:id="fixed-output-drvs">
<term><varname>outputHash</varname></term>
<term><varname>outputHashAlgo</varname></term>
<term><varname>outputHashMode</varname></term>
<listitem><para>These attributes declare that the derivation is a
so-called <emphasis>fixed-output derivation</emphasis>, which
means that a cryptographic hash of the output is already known in
advance. When the build of a fixed-output derivation finishes,
Nix computes the cryptographic hash of the output and compares it
to the hash declared with these attributes. If there is a
mismatch, the build fails.</para>
<para>The rationale for fixed-output derivations is derivations
such as those produced by the <function>fetchurl</function>
function. This function downloads a file from a given URL. To
ensure that the downloaded file has not been modified, the caller
must also specify a cryptographic hash of the file. For example,
<programlisting>
fetchurl {
url = http://ftp.gnu.org/pub/gnu/hello/hello-2.1.1.tar.gz;
md5 = "70c9ccf9fac07f762c24f2df2290784d";
}
</programlisting>
It sometimes happens that the URL of the file changes, e.g.,
because servers are reorganised or no longer available. We then
must update the call to <function>fetchurl</function>, e.g.,
<programlisting>
fetchurl {
url = ftp://ftp.nluug.nl/pub/gnu/hello/hello-2.1.1.tar.gz;
md5 = "70c9ccf9fac07f762c24f2df2290784d";
}
</programlisting>
If a <function>fetchurl</function> derivation was treated like a
normal derivation, the output paths of the derivation and
<emphasis>all derivations depending on it</emphasis> would change.
For instance, if we were to change the URL of the Glibc source
distribution in Nixpkgs (a package on which almost all other
packages depend) massive rebuilds would be needed. This is
unfortunate for a change which we know cannot have a real effect
as it propagates upwards through the dependency graph.</para>
<para>For fixed-output derivations, on the other hand, the name of
the output path only depends on the <varname>outputHash*</varname>
and <varname>name</varname> attributes, while all other attributes
are ignored for the purpose of computing the output path. (The
<varname>name</varname> attribute is included because it is part
of the path.)</para>
<para>As an example, here is the (simplified) Nix expression for
<varname>fetchurl</varname>:
<programlisting>
{ stdenv, curl }: # The <command>curl</command> program is used for downloading.
{ url, md5 }:
stdenv.mkDerivation {
name = baseNameOf (toString url);
builder = ./builder.sh;
buildInputs = [ curl ];
# This is a fixed-output derivation; the output must be a regular
# file with MD5 hash <varname>md5</varname>.
outputHashMode = "flat";
outputHashAlgo = "md5";
outputHash = md5;
inherit url;
}
</programlisting>
</para>
<para>The <varname>outputHashAlgo</varname> attribute specifies
the hash algorithm used to compute the hash. It can currently be
<literal>"md5"</literal>, <literal>"sha1"</literal> or
<literal>"sha256"</literal>.</para>
<para>The <varname>outputHashMode</varname> attribute determines
how the hash is computed. It must be one of the following two
values:
<variablelist>
<varlistentry><term><literal>"flat"</literal></term>
<listitem><para>The output must be a non-executable regular
file. If it isn’t, the build fails. The hash is simply
computed over the contents of that file (so it’s equal to what
Unix commands like <command>md5sum</command> or
<command>sha1sum</command> produce).</para>
<para>This is the default.</para></listitem>
</varlistentry>
<varlistentry><term><literal>"recursive"</literal></term>
<listitem><para>The hash is computed over the NAR archive dump
of the output (i.e., the result of <link
linkend="refsec-nix-store-dump"><command>nix-store
--dump</command></link>). In this case, the output can be
anything, including a directory tree.</para></listitem>
</varlistentry>
</variablelist>
</para>
<para>The <varname>outputHash</varname> attribute, finally, must
be a string containing the hash in either hexadecimal or base-32
notation. (See the <link
linkend="sec-nix-hash"><command>nix-hash</command> command</link>
for information about converting to and from base-32
notation.)</para></listitem>
</varlistentry>
<varlistentry><term><varname>impureEnvVars</varname></term>
<listitem><para>This attribute allows you to specify a list of
environment variables that should be passed from the environment
of the calling user to the builder. Usually, the environment is
cleared completely when the builder is executed, but with this
attribute you can allow specific environment variables to be
passed unmodified. For example, <function>fetchurl</function> in
Nixpkgs has the line
<programlisting>
impureEnvVars = [ "http_proxy" "https_proxy" <replaceable>...</replaceable> ];
</programlisting>
to make it use the proxy server configuration specified by the
user in the environment variables <envar>http_proxy</envar> and
friends.</para>
<para>This attribute is only allowed in <link
linkend="fixed-output-drvs">fixed-output derivations</link>, where
impurities such as these are okay since (the hash of) the output
is known in advance. It is ignored for all other
derivations.</para></listitem>
</varlistentry>
<varlistentry><term><varname>preferLocalBuild</varname></term>
<listitem><para>If this attribute is set to
<literal>true</literal>, it has two effects. First, the
derivation will always be built, not substituted, even if a
substitute is available. Second, if <link
linkend="chap-distributed-builds">distributed building is
enabled</link>, then, if possible, the derivaton will be built
locally instead of forwarded to a remote machine. This is
appropriate for trivial builders where the cost of doing a
download or remote build would exceed the cost of building
locally.</para></listitem>
</varlistentry>
</variablelist>
</section>
