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Merge pull request #34302 from obsidiansystems/cross-doc

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doc: Two cleanups to the cross compilation chapter
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John Ericson 2018-01-26 13:18:49 -05:00 committed by GitHub
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doc/cross-compilation.xml
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@ -253,8 +253,19 @@
or also with <varname>crossSystem</varname>, in which case packages run on the latter, but all building happens on the former.
Both parameters take the same schema as the 3 (build, host, and target) platforms defined in the previous section.
As mentioned above, <literal>lib.systems.examples</literal> has some platforms which are used as arguments for these parameters in practice.
You can use them programmatically, or on the command line like <command>nix-build &lt;nixpkgs&gt; --arg crossSystem '(import &lt;nixpkgs/lib&gt;).systems.examples.fooBarBaz'</command>.
You can use them programmatically, or on the command line: <programlisting>
nix-build &lt;nixpkgs&gt; --arg crossSystem '(import &lt;nixpkgs/lib&gt;).systems.examples.fooBarBaz' -A whatever</programlisting>
</para>
<note>
<para>
Eventually we would like to make these platform examples an unnecessary convenience so that <programlisting>
nix-build &lt;nixpkgs&gt; --arg crossSystem.config '&lt;arch&gt;-&lt;os&gt;-&lt;vendor&gt;-&lt;abi&gt;' -A whatever</programlisting>
works in the vast majority of cases.
The problem today is dependencies on other sorts of configuration which aren't given proper defaults.
We rely on the examples to crudely to set those configuration parameters in some vaguely sane manner on the users behalf.
Issue <link xlink:href="https://github.com/NixOS/nixpkgs/issues/34274">#34274</link> tracks this inconvenience along with its root cause in crufty configuration options.
</para>
</note>
<para>
While one is free to pass both parameters in full, there's a lot of logic to fill in missing fields.
As discussed in the previous section, only one of <varname>system</varname>, <varname>config</varname>, and <varname>parsed</varname> is needed to infer the other two.