259 lines
17 KiB
XML
259 lines
17 KiB
XML
<chapter xmlns="http://docbook.org/ns/docbook"
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xmlns:xlink="http://www.w3.org/1999/xlink"
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xml:id="chap-cross">
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<title>Cross-compilation</title>
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<section xml:id="sec-cross-intro">
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<title>Introduction</title>
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<para>
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"Cross-compilation" means compiling a program on one machine for another type of machine.
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For example, a typical use of cross compilation is to compile programs for embedded devices.
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These devices often don't have the computing power and memory to compile their own programs.
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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.
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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.
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</para>
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<para>
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This chapter will be organized in three parts.
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First, it will describe the basics of how to package software in a way that supports cross-compilation.
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Second, it will describe how to use Nixpkgs when cross-compiling.
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Third, it will describe the internal infrastructure supporting cross-compilation.
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</para>
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</section>
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<!--============================================================-->
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<section xml:id="sec-cross-packaging">
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<title>Packaging in a cross-friendly manner</title>
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<section>
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<title>Platform parameters</title>
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<para>
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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>.
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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.
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</para>
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<para>
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In Nixpkgs, these three platforms are defined as attribute sets under the names <literal>buildPlatform</literal>, <literal>hostPlatform</literal>, and <literal>targetPlatform</literal>.
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All three are always defined as attributes in the standard environment, and at the top level. That means one can get at them just like a dependency in a function that is imported with <literal>callPackage</literal>:
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<programlisting>{ stdenv, buildPlatform, hostPlatform, fooDep, barDep, .. }: ...buildPlatform...</programlisting>, or just off <varname>stdenv</varname>:
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<programlisting>{ stdenv, fooDep, barDep, .. }: ...stdenv.buildPlatform...</programlisting>.
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</para>
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<variablelist>
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<varlistentry>
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<term><varname>buildPlatform</varname></term>
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<listitem><para>
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The "build platform" is the platform on which a package is built.
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Once someone has a built package, or pre-built binary package, the build platform should not matter and be safe to ignore.
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</para></listitem>
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</varlistentry>
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<varlistentry>
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<term><varname>hostPlatform</varname></term>
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<listitem><para>
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The "host platform" is the platform on which a package will be run.
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This is the simplest platform to understand, but also the one with the worst name.
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</para></listitem>
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</varlistentry>
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<varlistentry>
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<term><varname>targetPlatform</varname></term>
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<listitem>
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<para>
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The "target platform" attribute is, unlike the other two attributes, not actually fundamental to the process of building software.
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Instead, it is only relevant for compatability with building certain specific compilers and build tools.
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It can be safely ignored for all other packages.
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</para>
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<para>
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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.
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The task specifying this single "target platform" is thus pushed to build time of the compiler.
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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.
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</para>
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<para>
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There is no fundamental need to think about a single target ahead of time like this.
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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.
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An example of such a tool is LLVM.
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</para>
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<para>
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Although the existance 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.
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Nixpkgs tries to avoid sharing in the mistake where possible.
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Still, because the concept of a target platform is so ingrained, it is best to support it as is.
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</para>
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</listitem>
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</varlistentry>
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</variablelist>
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<para>
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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.
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You can see examples of ones used in practice in <literal>lib.systems.examples</literal>; note how they are not all very consistent.
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For now, here are few fields can count on them containing:
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</para>
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<variablelist>
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<varlistentry>
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<term><varname>system</varname></term>
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<listitem>
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<para>
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This is a two-component shorthand for the platform.
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Examples of this would be "x86_64-darwin" and "i686-linux"; see <literal>lib.systems.doubles</literal> for more.
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This format isn't very standard, but has built-in support in Nix, such as the <varname>builtins.currentSystem</varname> impure string.
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term><varname>config</varname></term>
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<listitem>
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<para>
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This is a 3- or 4- component shorthand for the platform.
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Examples of this would be "x86_64-unknown-linux-gnu" and "aarch64-apple-darwin14".
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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>.
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This format is strictly more informative than the "Nix host double", as the previous format could analogously be termed.
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This needs a better name than <varname>config</varname>!
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term><varname>parsed</varname></term>
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<listitem>
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<para>
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This is a nix representation of a parsed LLVM target triple with white-listed components.
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This can be specified directly, or actually parsed from the <varname>config</varname>.
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[Technically, only one need be specified and the others can be inferred, though the precision of inference may not be very good.]
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See <literal>lib.systems.parse</literal> for the exact representation.
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term><varname>libc</varname></term>
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<listitem>
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<para>
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This is a string identifying the standard C library used.
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Valid identifiers include "glibc" for GNU libc, "libSystem" for Darwin's Libsystem, and "uclibc" for µClibc.
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It should probably be refactored to use the module system, like <varname>parse</varname>.
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term><varname>is*</varname></term>
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<listitem>
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<para>
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These predicates are defined in <literal>lib.systems.inspect</literal>, and slapped on every platform.
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They are superior to the ones in <varname>stdenv</varname> as they force the user to be explicit about which platform they are inspecting.
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Please use these instead of those.
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term><varname>platform</varname></term>
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<listitem>
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<para>
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This is, quite frankly, a dumping ground of ad-hoc settings (it's an attribute set).
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See <literal>lib.systems.platforms</literal> for examples—there's hopefully one in there that will work verbatim for each platform that is working.
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Please help us triage these flags and give them better homes!
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</para>
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</listitem>
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</varlistentry>
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</variablelist>
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</section>
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<section>
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<title>Specifying Dependencies</title>
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<para>
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In this section we explore the relationship between both runtime and buildtime dependencies and the 3 Autoconf platforms.
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</para>
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<para>
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A runtime dependency between 2 packages implies that between them both the host and target platforms match.
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This is directly implied by the meaning of "host platform" and "runtime dependency":
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The package dependency exists while both packages are runnign on a single host platform.
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</para>
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<para>
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A build time dependency, however, implies a shift in platforms between the depending package and the depended-on package.
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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.
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The depending package's build platform is therefore equal to the depended-on package's host platform.
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Analogously, the depending package's host platform is equal to the depended-on package's target platform.
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</para>
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<para>
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In this manner, given the 3 platforms for one package, we can determine the three platforms for all its transitive dependencies.
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This is the most important guiding principle behind cross-compilation with Nixpkgs, and will be called the <wordasword>sliding window principle</wordasword>.
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</para>
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<para>
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Some examples will probably make this clearer.
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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>.
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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.
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Indeed, this happens with cross compilation, where only rounds of native dependencies starting with the second necessarily coincide with native packages.
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</para>
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<note><para>
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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.
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</para></note>
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<para>
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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.
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For example, <varname>buildPackages.gcc</varname> should be used at build time, while <varname>gcc</varname> should be used at run time.
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Now, for most of Nixpkgs's history, there was no <varname>buildPackages</varname>, and most packages have not been refactored to use it explicitly.
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Instead, one can use the four attributes used for specifying dependencies as documented in <xref linkend="ssec-stdenv-attributes"/>.
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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.
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This splicing can be skipped when not cross compiling as the package sets are the same, but is a bit slow for cross compiling.
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Because of this, a best-of-both-worlds solution is in the works with no splicing or explicit access of <varname>buildPackages</varname> needed.
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For now, feel free to use either method.
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</para>
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<note><para>
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There is also a "backlink" <varname>targetPackages</varname>, yielding a package set whose <varname>buildPackages</varname> is the current package set.
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This is a hack, though, to accommodate compilers with lousy build systems.
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Please do not use this unless you are absolutely sure you are packaging such a compiler and there is no other way.
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</para></note>
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</section>
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</section>
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<!--============================================================-->
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<section xml:id="sec-cross-usage">
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<title>Cross-building packages</title>
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<note><para>
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More information needs to moved from the old wiki, especially <link xlink:href="https://nixos.org/wiki/CrossCompiling" />, for this section.
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</para></note>
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<para>
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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,
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or also with <varname>crossSystem</varname>, in which case packages run on the latter, but all building happens on the former.
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Both parameters take the same schema as the 3 (build, host, and target) platforms defined in the previous section.
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As mentioned above, <literal>lib.systems.examples</literal> has some platforms which are used as arguments for these parameters in practice.
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You can use them programmatically, or on the command line like <command>nix-build <nixpkgs> --arg crossSystem '(import <nixpkgs/lib>).systems.examples.fooBarBaz'</command>.
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</para>
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<para>
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While one is free to pass both parameters in full, there's a lot of logic to fill in missing fields.
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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.
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Additionally, <varname>libc</varname> will be inferred from <varname>parse</varname>.
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Finally, <literal>localSystem.system</literal> is also <emphasis>impurely</emphasis> inferred based on the platform evaluation occurs.
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This means it is often not necessary to pass <varname>localSystem</varname> at all, as in the command-line example in the previous paragraph.
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</para>
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<note>
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<para>
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Many sources (manual, wiki, etc) probably mention passing <varname>system</varname>, <varname>platform</varname>, along with the optional <varname>crossSystem</varname> to nixpkgs:
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<literal>import <nixpkgs> { system = ..; platform = ..; crossSystem = ..; }</literal>.
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Passing those two instead of <varname>localSystem</varname> is still supported for compatibility, but is discouraged.
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Indeed, much of the inference we do for these parameters is motivated by compatibility as much as convenience.
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</para>
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</note>
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<para>
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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).
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Actually, those identifiers are purposefully not used here to draw a subtle but important distinction:
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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.
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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.
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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.
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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.
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<varname>localSystem</varname> is always non-null.
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</para>
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</section>
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<!--============================================================-->
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<section xml:id="sec-cross-infra">
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<title>Cross-compilation infrastructure</title>
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<para>To be written.</para>
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<note><para>
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If one explores nixpkgs, they will see derivations with names like <literal>gccCross</literal>.
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Such <literal>*Cross</literal> derivations is a holdover from before we properly distinguished between the host and target platforms
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—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>.
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This ugliness will disappear soon.
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</para></note>
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</section>
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</chapter>
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