212 lines
9 KiB
XML
212 lines
9 KiB
XML
<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="ssec-derivation">
|
||
|
||
<title>Derivations</title>
|
||
|
||
<para>The most important built-in function is
|
||
<function>derivation</function>, which is used to describe a single
|
||
derivation (a build action). It takes as input a set, the attributes
|
||
of which specify the inputs of the build.</para>
|
||
|
||
<itemizedlist>
|
||
|
||
<listitem xml:id="attr-system"><para>There must be an attribute named
|
||
<varname>system</varname> whose value must be a string specifying a
|
||
Nix platform identifier, such as <literal>"i686-linux"</literal> or
|
||
<literal>"x86_64-darwin"</literal><footnote><para>To figure out
|
||
your platform identifier, look at the line <quote>Checking for the
|
||
canonical Nix system name</quote> in the output of Nix's
|
||
<filename>configure</filename> script.</para></footnote> The build
|
||
can only be performed on a machine and operating system matching the
|
||
platform identifier. (Nix can automatically forward builds for
|
||
other platforms by forwarding them to other machines; see <xref
|
||
linkend='chap-distributed-builds' />.)</para></listitem>
|
||
|
||
<listitem><para>There must be an attribute named
|
||
<varname>name</varname> whose value must be a string. This is used
|
||
as a symbolic name for the package by <command>nix-env</command>,
|
||
and it is appended to the output paths of the
|
||
derivation.</para></listitem>
|
||
|
||
<listitem><para>There must be an attribute named
|
||
<varname>builder</varname> that identifies the program that is
|
||
executed to perform the build. It can be either a derivation or a
|
||
source (a local file reference, e.g.,
|
||
<filename>./builder.sh</filename>).</para></listitem>
|
||
|
||
<listitem><para>Every attribute is passed as an environment variable
|
||
to the builder. Attribute values are translated to environment
|
||
variables as follows:
|
||
|
||
<itemizedlist>
|
||
|
||
<listitem><para>Strings and numbers are just passed
|
||
verbatim.</para></listitem>
|
||
|
||
<listitem><para>A <emphasis>path</emphasis> (e.g.,
|
||
<filename>../foo/sources.tar</filename>) causes the referenced
|
||
file to be copied to the store; its location in the store is put
|
||
in the environment variable. The idea is that all sources
|
||
should reside in the Nix store, since all inputs to a derivation
|
||
should reside in the Nix store.</para></listitem>
|
||
|
||
<listitem><para>A <emphasis>derivation</emphasis> causes that
|
||
derivation to be built prior to the present derivation; its
|
||
default output path is put in the environment
|
||
variable.</para></listitem>
|
||
|
||
<listitem><para>Lists of the previous types are also allowed.
|
||
They are simply concatenated, separated by
|
||
spaces.</para></listitem>
|
||
|
||
<listitem><para><literal>true</literal> is passed as the string
|
||
<literal>1</literal>, <literal>false</literal> and
|
||
<literal>null</literal> are passed as an empty string.
|
||
</para></listitem>
|
||
</itemizedlist>
|
||
|
||
</para></listitem>
|
||
|
||
<listitem><para>The optional attribute <varname>args</varname>
|
||
specifies command-line arguments to be passed to the builder. It
|
||
should be a list.</para></listitem>
|
||
|
||
<listitem><para>The optional attribute <varname>outputs</varname>
|
||
specifies a list of symbolic outputs of the derivation. By default,
|
||
a derivation produces a single output path, denoted as
|
||
<literal>out</literal>. However, derivations can produce multiple
|
||
output paths. This is useful because it allows outputs to be
|
||
downloaded or garbage-collected separately. For instance, imagine a
|
||
library package that provides a dynamic library, header files, and
|
||
documentation. A program that links against the library doesn’t
|
||
need the header files and documentation at runtime, and it doesn’t
|
||
need the documentation at build time. Thus, the library package
|
||
could specify:
|
||
<programlisting>
|
||
outputs = [ "lib" "headers" "doc" ];
|
||
</programlisting>
|
||
This will cause Nix to pass environment variables
|
||
<literal>lib</literal>, <literal>headers</literal> and
|
||
<literal>doc</literal> to the builder containing the intended store
|
||
paths of each output. The builder would typically do something like
|
||
<programlisting>
|
||
./configure --libdir=$lib/lib --includedir=$headers/include --docdir=$doc/share/doc
|
||
</programlisting>
|
||
for an Autoconf-style package. You can refer to each output of a
|
||
derivation by selecting it as an attribute, e.g.
|
||
<programlisting>
|
||
buildInputs = [ pkg.lib pkg.headers ];
|
||
</programlisting>
|
||
The first element of <varname>output</varname> determines the
|
||
<emphasis>default output</emphasis>. Thus, you could also write
|
||
<programlisting>
|
||
buildInputs = [ pkg pkg.headers ];
|
||
</programlisting>
|
||
since <literal>pkg</literal> is equivalent to
|
||
<literal>pkg.lib</literal>.</para></listitem>
|
||
|
||
</itemizedlist>
|
||
|
||
<para>The function <function>mkDerivation</function> in the Nixpkgs
|
||
standard environment is a wrapper around
|
||
<function>derivation</function> that adds a default value for
|
||
<varname>system</varname> and always uses Bash as the builder, to
|
||
which the supplied builder is passed as a command-line argument. See
|
||
the Nixpkgs manual for details.</para>
|
||
|
||
<para>The builder is executed as follows:
|
||
|
||
<itemizedlist>
|
||
|
||
<listitem><para>A temporary directory is created under the directory
|
||
specified by <envar>TMPDIR</envar> (default
|
||
<filename>/tmp</filename>) where the build will take place. The
|
||
current directory is changed to this directory.</para></listitem>
|
||
|
||
<listitem><para>The environment is cleared and set to the derivation
|
||
attributes, as specified above.</para></listitem>
|
||
|
||
<listitem><para>In addition, the following variables are set:
|
||
|
||
<itemizedlist>
|
||
|
||
<listitem><para><envar>NIX_BUILD_TOP</envar> contains the path of
|
||
the temporary directory for this build.</para></listitem>
|
||
|
||
<listitem><para>Also, <envar>TMPDIR</envar>,
|
||
<envar>TEMPDIR</envar>, <envar>TMP</envar>, <envar>TEMP</envar>
|
||
are set to point to the temporary directory. This is to prevent
|
||
the builder from accidentally writing temporary files anywhere
|
||
else. Doing so might cause interference by other
|
||
processes.</para></listitem>
|
||
|
||
<listitem><para><envar>PATH</envar> is set to
|
||
<filename>/path-not-set</filename> to prevent shells from
|
||
initialising it to their built-in default value.</para></listitem>
|
||
|
||
<listitem><para><envar>HOME</envar> is set to
|
||
<filename>/homeless-shelter</filename> to prevent programs from
|
||
using <filename>/etc/passwd</filename> or the like to find the
|
||
user's home directory, which could cause impurity. Usually, when
|
||
<envar>HOME</envar> is set, it is used as the location of the home
|
||
directory, even if it points to a non-existent
|
||
path.</para></listitem>
|
||
|
||
<listitem><para><envar>NIX_STORE</envar> is set to the path of the
|
||
top-level Nix store directory (typically,
|
||
<filename>/nix/store</filename>).</para></listitem>
|
||
|
||
<listitem><para>For each output declared in
|
||
<varname>outputs</varname>, the corresponding environment variable
|
||
is set to point to the intended path in the Nix store for that
|
||
output. Each output path is a concatenation of the cryptographic
|
||
hash of all build inputs, the <varname>name</varname> attribute
|
||
and the output name. (The output name is omitted if it’s
|
||
<literal>out</literal>.)</para></listitem>
|
||
|
||
</itemizedlist>
|
||
|
||
</para></listitem>
|
||
|
||
<listitem><para>If an output path already exists, it is removed.
|
||
Also, locks are acquired to prevent multiple Nix instances from
|
||
performing the same build at the same time.</para></listitem>
|
||
|
||
<listitem><para>A log of the combined standard output and error is
|
||
written to <filename>/nix/var/log/nix</filename>.</para></listitem>
|
||
|
||
<listitem><para>The builder is executed with the arguments specified
|
||
by the attribute <varname>args</varname>. If it exits with exit
|
||
code 0, it is considered to have succeeded.</para></listitem>
|
||
|
||
<listitem><para>The temporary directory is removed (unless the
|
||
<option>-K</option> option was specified).</para></listitem>
|
||
|
||
<listitem><para>If the build was successful, Nix scans each output
|
||
path for references to input paths by looking for the hash parts of
|
||
the input paths. Since these are potential runtime dependencies,
|
||
Nix registers them as dependencies of the output
|
||
paths.</para></listitem>
|
||
|
||
<listitem><para>After the build, Nix sets the last-modified
|
||
timestamp on all files in the build result to 1 (00:00:01 1/1/1970
|
||
UTC), sets the group to the default group, and sets the mode of the
|
||
file to 0444 or 0555 (i.e., read-only, with execute permission
|
||
enabled if the file was originally executable). Note that possible
|
||
<literal>setuid</literal> and <literal>setgid</literal> bits are
|
||
cleared. Setuid and setgid programs are not currently supported by
|
||
Nix. This is because the Nix archives used in deployment have no
|
||
concept of ownership information, and because it makes the build
|
||
result dependent on the user performing the build.</para></listitem>
|
||
|
||
</itemizedlist>
|
||
|
||
</para>
|
||
|
||
<xi:include href="advanced-attributes.xml" />
|
||
|
||
</section>
|