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<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-generic-builder'>
<title>Generic Builder Syntax</title>
<para>Recall from <xref linkend='ex-hello-builder' /> that the builder
looked something like this:
<programlisting>
PATH=$perl/bin:$PATH
tar xvfz $src
cd hello-*
./configure --prefix=$out
make
make install</programlisting>
The builders for almost all Unix packages look like this — set up some
environment variables, unpack the sources, configure, build, and
install. For this reason the standard environment provides some Bash
functions that automate the build process. Here is what a builder
using the generic build facilities looks like:</para>
<programlisting>
buildInputs="$perl" ①
source $stdenv/setup ②
genericBuild ③</programlisting>
<para>Here is what each line means:
<orderedlist>
<listitem>
<para>The <literal>buildInputs</literal> variable tells
<filename>setup</filename> to use the indicated packages as
<quote>inputs</quote>. This means that if a package provides a
<filename>bin</filename> subdirectory, it's added to
<literal>PATH</literal>; if it has a <filename>include</filename>
subdirectory, it's added to GCC's header search path; and so
on.<footnote><para>How does it work? <filename>setup</filename>
tries to source the file
<filename><emphasis>pkg</emphasis>/nix-support/setup-hook</filename>
of all dependencies. These “setup hooks” can then set up whatever
environment variables they want; for instance, the setup hook for
Perl sets the <literal>PERL5LIB</literal> environment variable to
contain the <filename>lib/site_perl</filename> directories of all
inputs.</para></footnote>
</para>
</listitem>
<listitem arearefs='ex-hello-builder2-co-2'>
<para>The function <function>genericBuild</function> is defined in
the file <literal>$stdenv/setup</literal>.</para>
</listitem>
<listitem arearefs='ex-hello-builder2-co-3'>
<para>The final step calls the shell function
<function>genericBuild</function>, which performs the steps that
were done explicitly in <xref linkend='ex-hello-builder' />. The
generic builder is smart enough to figure out whether to unpack
the sources using <command>gzip</command>,
<command>bzip2</command>, etc. It can be customised in many ways;
see the Nixpkgs manual for details.</para>
</listitem>
</orderedlist>
</para>
<para>Discerning readers will note that the
<literal>buildInputs</literal> could just as well have been set in the Nix
expression, like this:
<programlisting>
buildInputs = [ perl ];</programlisting>
The <varname>perl</varname> attribute can then be removed, and the
builder becomes even shorter:
<programlisting>
source $stdenv/setup
genericBuild</programlisting>
In fact, <varname>mkDerivation</varname> provides a default builder
that looks exactly like that, so it is actually possible to omit the
builder for Hello entirely.</para>
</section>
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