<chapter xmlns="http://docbook.org/ns/docbook"
         xmlns:xlink="http://www.w3.org/1999/xlink"
         xml:id="chap-cross">
 <title>Cross-compilation</title>
 <section xml:id="sec-cross-intro">
  <title>Introduction</title>

  <para>
   "Cross-compilation" means compiling a program on one machine for another
   type of machine. For example, a typical use of cross compilation is to
   compile programs for embedded devices. These devices often don't have the
   computing power and memory to compile their own programs. One might think
   that cross-compilation is a fairly niche concern, but there are advantages
   to being rigorous about distinguishing build-time vs run-time environments
   even when one is developing and deploying on the same machine. Nixpkgs is
   increasingly adopting the opinion that packages should be written with
   cross-compilation in mind, and nixpkgs should evaluate in a similar way (by
   minimizing cross-compilation-specific special cases) whether or not one is
   cross-compiling.
  </para>

  <para>
   This chapter will be organized in three parts. First, it will describe the
   basics of how to package software in a way that supports cross-compilation.
   Second, it will describe how to use Nixpkgs when cross-compiling. Third, it
   will describe the internal infrastructure supporting cross-compilation.
  </para>
 </section>
<!--============================================================-->
 <section xml:id="sec-cross-packaging">
  <title>Packaging in a cross-friendly manner</title>

  <section xml:id="sec-cross-platform-parameters">
   <title>Platform parameters</title>

   <para>
    Nixpkgs follows the
    <link xlink:href="https://gcc.gnu.org/onlinedocs/gccint/Configure-Terms.html">common
    historical convention of GNU autoconf</link> of distinguishing between 3
    types of platform: <wordasword>build</wordasword>,
    <wordasword>host</wordasword>, and <wordasword>target</wordasword>. In
    summary, <wordasword>build</wordasword> is the platform on which a package
    is being built, <wordasword>host</wordasword> is the platform on which it
    is to run. The third attribute, <wordasword>target</wordasword>, is
    relevant only for certain specific compilers and build tools.
   </para>

   <para>
    In Nixpkgs, these three platforms are defined as attribute sets under the
    names <literal>buildPlatform</literal>, <literal>hostPlatform</literal>,
    and <literal>targetPlatform</literal>. They are always defined as
    attributes in the standard environment. That means one can access them
    like:
<programlisting>{ stdenv, fooDep, barDep, .. }: ...stdenv.buildPlatform...</programlisting>
    .
   </para>

   <variablelist>
    <varlistentry>
     <term>
      <varname>buildPlatform</varname>
     </term>
     <listitem>
      <para>
       The "build platform" is the platform on which a package is built. Once
       someone has a built package, or pre-built binary package, the build
       platform should not matter and be safe to ignore.
      </para>
     </listitem>
    </varlistentry>
    <varlistentry>
     <term>
      <varname>hostPlatform</varname>
     </term>
     <listitem>
      <para>
       The "host platform" is the platform on which a package will be run. This
       is the simplest platform to understand, but also the one with the worst
       name.
      </para>
     </listitem>
    </varlistentry>
    <varlistentry>
     <term>
      <varname>targetPlatform</varname>
     </term>
     <listitem>
      <para>
       The "target platform" attribute is, unlike the other two attributes, not
       actually fundamental to the process of building software. Instead, it is
       only relevant for compatibility with building certain specific compilers
       and build tools. It can be safely ignored for all other packages.
      </para>
      <para>
       The build process of certain compilers is written in such a way that the
       compiler resulting from a single build can itself only produce binaries
       for a single platform. The task specifying this single "target platform"
       is thus pushed to build time of the compiler. The root cause of this
       mistake is often that the compiler (which will be run on the host) and
       the the standard library/runtime (which will be run on the target) are
       built by a single build process.
      </para>
      <para>
       There is no fundamental need to think about a single target ahead of
       time like this. If the tool supports modular or pluggable backends, both
       the need to specify the target at build time and the constraint of
       having only a single target disappear. An example of such a tool is
       LLVM.
      </para>
      <para>
       Although the existence of a "target platfom" is arguably a historical
       mistake, it is a common one: examples of tools that suffer from it are
       GCC, Binutils, GHC and Autoconf. Nixpkgs tries to avoid sharing in the
       mistake where possible. Still, because the concept of a target platform
       is so ingrained, it is best to support it as is.
      </para>
     </listitem>
    </varlistentry>
   </variablelist>

   <para>
    The exact schema these fields follow is a bit ill-defined due to a long and
    convoluted evolution, but this is slowly being cleaned up. You can see
    examples of ones used in practice in
    <literal>lib.systems.examples</literal>; note how they are not all very
    consistent. For now, here are few fields can count on them containing:
   </para>

   <variablelist>
    <varlistentry>
     <term>
      <varname>system</varname>
     </term>
     <listitem>
      <para>
       This is a two-component shorthand for the platform. Examples of this
       would be "x86_64-darwin" and "i686-linux"; see
       <literal>lib.systems.doubles</literal> for more. This format isn't very
       standard, but has built-in support in Nix, such as the
       <varname>builtins.currentSystem</varname> impure string.
      </para>
     </listitem>
    </varlistentry>
    <varlistentry>
     <term>
      <varname>config</varname>
     </term>
     <listitem>
      <para>
       This is a 3- or 4- component shorthand for the platform. Examples of
       this would be "x86_64-unknown-linux-gnu" and "aarch64-apple-darwin14".
       This is a standard format called the "LLVM target triple", as they are
       pioneered by LLVM and traditionally just used for the
       <varname>targetPlatform</varname>. This format is strictly more
       informative than the "Nix host double", as the previous format could
       analogously be termed. This needs a better name than
       <varname>config</varname>!
      </para>
     </listitem>
    </varlistentry>
    <varlistentry>
     <term>
      <varname>parsed</varname>
     </term>
     <listitem>
      <para>
       This is a nix representation of a parsed LLVM target triple with
       white-listed components. This can be specified directly, or actually
       parsed from the <varname>config</varname>. [Technically, only one need
       be specified and the others can be inferred, though the precision of
       inference may not be very good.] See
       <literal>lib.systems.parse</literal> for the exact representation.
      </para>
     </listitem>
    </varlistentry>
    <varlistentry>
     <term>
      <varname>libc</varname>
     </term>
     <listitem>
      <para>
       This is a string identifying the standard C library used. Valid
       identifiers include "glibc" for GNU libc, "libSystem" for Darwin's
       Libsystem, and "uclibc" for µClibc. It should probably be refactored to
       use the module system, like <varname>parse</varname>.
      </para>
     </listitem>
    </varlistentry>
    <varlistentry>
     <term>
      <varname>is*</varname>
     </term>
     <listitem>
      <para>
       These predicates are defined in <literal>lib.systems.inspect</literal>,
       and slapped on every platform. They are superior to the ones in
       <varname>stdenv</varname> as they force the user to be explicit about
       which platform they are inspecting. Please use these instead of those.
      </para>
     </listitem>
    </varlistentry>
    <varlistentry>
     <term>
      <varname>platform</varname>
     </term>
     <listitem>
      <para>
       This is, quite frankly, a dumping ground of ad-hoc settings (it's an
       attribute set). See <literal>lib.systems.platforms</literal> for
       examples—there's hopefully one in there that will work verbatim for
       each platform that is working. Please help us triage these flags and
       give them better homes!
      </para>
     </listitem>
    </varlistentry>
   </variablelist>
  </section>

  <section xml:id="sec-cross-specifying-dependencies">
   <title>Specifying Dependencies</title>

   <para>
    In this section we explore the relationship between both runtime and
    buildtime dependencies and the 3 Autoconf platforms.
   </para>

   <para>
    A runtime dependency between 2 packages implies that between them both the
    host and target platforms match. This is directly implied by the meaning of
    "host platform" and "runtime dependency": The package dependency exists
    while both packages are running on a single host platform.
   </para>

   <para>
    A build time dependency, however, implies a shift in platforms between the
    depending package and the depended-on package. The meaning of a build time
    dependency is that to build the depending package we need to be able to run
    the depended-on's package. The depending package's build platform is
    therefore equal to the depended-on package's host platform. Analogously,
    the depending package's host platform is equal to the depended-on package's
    target platform.
   </para>

   <para>
    In this manner, given the 3 platforms for one package, we can determine the
    three platforms for all its transitive dependencies. This is the most
    important guiding principle behind cross-compilation with Nixpkgs, and will
    be called the <wordasword>sliding window principle</wordasword>.
   </para>

   <para>
    Some examples will probably make this clearer. If a package is being built
    with a <literal>(build, host, target)</literal> platform triple of
    <literal>(foo, bar, bar)</literal>, then its build-time dependencies would
    have a triple of <literal>(foo, foo, bar)</literal>, and <emphasis>those
    packages'</emphasis> build-time dependencies would have triple of
    <literal>(foo, foo, foo)</literal>. In other words, it should take two
    "rounds" of following build-time dependency edges before one reaches a
    fixed point where, by the sliding window principle, the platform triple no
    longer changes. Indeed, this happens with cross compilation, where only
    rounds of native dependencies starting with the second necessarily coincide
    with native packages.
   </para>

   <note>
    <para>
     The depending package's target platform is unconstrained by the sliding
     window principle, which makes sense in that one can in principle build
     cross compilers targeting arbitrary platforms.
    </para>
   </note>

   <para>
    How does this work in practice? Nixpkgs is now structured so that
    build-time dependencies are taken from <varname>buildPackages</varname>,
    whereas run-time dependencies are taken from the top level attribute set.
    For example, <varname>buildPackages.gcc</varname> should be used at build
    time, while <varname>gcc</varname> should be used at run time. Now, for
    most of Nixpkgs's history, there was no <varname>buildPackages</varname>,
    and most packages have not been refactored to use it explicitly. Instead,
    one can use the six (<emphasis>gasp</emphasis>) attributes used for
    specifying dependencies as documented in
    <xref linkend="ssec-stdenv-dependencies"/>. We "splice" together the
    run-time and build-time package sets with <varname>callPackage</varname>,
    and then <varname>mkDerivation</varname> for each of four attributes pulls
    the right derivation out. This splicing can be skipped when not cross
    compiling as the package sets are the same, but is a bit slow for cross
    compiling. Because of this, a best-of-both-worlds solution is in the works
    with no splicing or explicit access of <varname>buildPackages</varname>
    needed. For now, feel free to use either method.
   </para>

   <note>
    <para>
     There is also a "backlink" <varname>targetPackages</varname>, yielding a
     package set whose <varname>buildPackages</varname> is the current package
     set. This is a hack, though, to accommodate compilers with lousy build
     systems. Please do not use this unless you are absolutely sure you are
     packaging such a compiler and there is no other way.
    </para>
   </note>
  </section>

  <section xml:id="sec-cross-cookbook">
   <title>Cross packaging cookbook</title>

   <para>
    Some frequently problems when packaging for cross compilation are good to
    just spell and answer. Ideally the information above is exhaustive, so this
    section cannot provide any new information, but its ludicrous and cruel to
    expect everyone to spend effort working through the interaction of many
    features just to figure out the same answer to the same common problem.
    Feel free to add to this list!
   </para>

   <qandaset>
    <qandaentry xml:id="cross-qa-build-c-program-in-build-environment">
     <question>
      <para>
       What if my package's build system needs to build a C program to be run
       under the build environment?
      </para>
     </question>
     <answer>
      <para>
<programlisting>depsBuildBuild = [ buildPackages.stdenv.cc ];</programlisting>
       Add it to your <function>mkDerivation</function> invocation.
      </para>
     </answer>
    </qandaentry>
    <qandaentry xml:id="cross-qa-fails-to-find-ar">
     <question>
      <para>
       My package fails to find <command>ar</command>.
      </para>
     </question>
     <answer>
      <para>
       Many packages assume that an unprefixed <command>ar</command> is
       available, but Nix doesn't provide one. It only provides a prefixed one,
       just as it only does for all the other binutils programs. It may be
       necessary to patch the package to fix the build system to use a prefixed
       `ar`.
      </para>
     </answer>
    </qandaentry>
    <qandaentry xml:id="cross-testsuite-runs-host-code">
     <question>
      <para>
       My package's testsuite needs to run host platform code.
      </para>
     </question>
     <answer>
      <para>
<programlisting>doCheck = stdenv.hostPlatform != stdenv.buildPlatfrom;</programlisting>
       Add it to your <function>mkDerivation</function> invocation.
      </para>
     </answer>
    </qandaentry>
   </qandaset>
  </section>
 </section>
<!--============================================================-->
 <section xml:id="sec-cross-usage">
  <title>Cross-building packages</title>

  <note>
   <para>
    More information needs to moved from the old wiki, especially
    <link xlink:href="https://nixos.org/wiki/CrossCompiling" />, for this
    section.
   </para>
  </note>

  <para>
   Nixpkgs can be instantiated with <varname>localSystem</varname> alone, in
   which case there is no cross compiling and everything is built by and for
   that system, 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:
<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. Additionally,
   <varname>libc</varname> will be inferred from <varname>parse</varname>.
   Finally, <literal>localSystem.system</literal> is also
   <emphasis>impurely</emphasis> inferred based on the platform evaluation
   occurs. This means it is often not necessary to pass
   <varname>localSystem</varname> at all, as in the command-line example in the
   previous paragraph.
  </para>

  <note>
   <para>
    Many sources (manual, wiki, etc) probably mention passing
    <varname>system</varname>, <varname>platform</varname>, along with the
    optional <varname>crossSystem</varname> to nixpkgs: <literal>import
    &lt;nixpkgs&gt; { system = ..; platform = ..; crossSystem = ..;
    }</literal>. Passing those two instead of <varname>localSystem</varname> is
    still supported for compatibility, but is discouraged. Indeed, much of the
    inference we do for these parameters is motivated by compatibility as much
    as convenience.
   </para>
  </note>

  <para>
   One would think that <varname>localSystem</varname> and
   <varname>crossSystem</varname> overlap horribly with the three
   <varname>*Platforms</varname> (<varname>buildPlatform</varname>,
   <varname>hostPlatform,</varname> and <varname>targetPlatform</varname>; see
   <varname>stage.nix</varname> or the manual). Actually, those identifiers are
   purposefully not used here to draw a subtle but important distinction: While
   the granularity of having 3 platforms is necessary to properly *build*
   packages, it is overkill for specifying the user's *intent* when making a
   build plan or package set. A simple "build vs deploy" dichotomy is adequate:
   the sliding window principle described in the previous section shows how to
   interpolate between the these two "end points" to get the 3 platform triple
   for each bootstrapping stage. That means for any package a given package
   set, even those not bound on the top level but only reachable via
   dependencies or <varname>buildPackages</varname>, the three platforms will
   be defined as one of <varname>localSystem</varname> or
   <varname>crossSystem</varname>, with the former replacing the latter as one
   traverses build-time dependencies. A last simple difference then is
   <varname>crossSystem</varname> should be null when one doesn't want to
   cross-compile, while the <varname>*Platform</varname>s are always non-null.
   <varname>localSystem</varname> is always non-null.
  </para>
 </section>
<!--============================================================-->
 <section xml:id="sec-cross-infra">
  <title>Cross-compilation infrastructure</title>

  <para>
   To be written.
  </para>

  <note>
   <para>
    If one explores nixpkgs, they will see derivations with names like
    <literal>gccCross</literal>. Such <literal>*Cross</literal> derivations is
    a holdover from before we properly distinguished between the host and
    target platforms —the derivation with "Cross" in the name covered the
    <literal>build = host != target</literal> case, while the other covered the
    <literal>host = target</literal>, with build platform the same or not based
    on whether one was using its <literal>.nativeDrv</literal> or
    <literal>.crossDrv</literal>. This ugliness will disappear soon.
   </para>
  </note>
 </section>
</chapter>
