478 lines
20 KiB
XML
478 lines
20 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
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type of machine. For example, a typical use of cross compilation is to
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compile programs for embedded devices. These devices often don't have the
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computing power and memory to compile their own programs. One might think
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that cross-compilation is a fairly niche concern, but there are advantages
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to being rigorous about distinguishing build-time vs run-time environments
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even when one is developing and deploying on the same machine. Nixpkgs is
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increasingly adopting the opinion that packages should be written with
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cross-compilation in mind, and nixpkgs should evaluate in a similar way (by
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minimizing cross-compilation-specific special cases) whether or not one is
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cross-compiling.
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</para>
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<para>
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This chapter will be organized in three parts. First, it will describe the
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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. Third, it
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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
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<link xlink:href="https://gcc.gnu.org/onlinedocs/gccint/Configure-Terms.html">common
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historical convention of GNU autoconf</link> of distinguishing between 3
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types of platform: <wordasword>build</wordasword>,
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<wordasword>host</wordasword>, and <wordasword>target</wordasword>. In
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summary, <wordasword>build</wordasword> is the platform on which a package
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is being built, <wordasword>host</wordasword> is the platform on which it
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is to run. The third attribute, <wordasword>target</wordasword>, is
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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
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names <literal>buildPlatform</literal>, <literal>hostPlatform</literal>,
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and <literal>targetPlatform</literal>. All three are always defined as
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attributes in the standard environment, and at the top level. That means
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one can get at them just like a dependency in a function that is imported
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with <literal>callPackage</literal>:
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<programlisting>{ stdenv, buildPlatform, hostPlatform, fooDep, barDep, .. }: ...buildPlatform...</programlisting>
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, or just off <varname>stdenv</varname>:
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<programlisting>{ stdenv, fooDep, barDep, .. }: ...stdenv.buildPlatform...</programlisting>
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.
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</para>
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<variablelist>
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<varlistentry>
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<term>
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<varname>buildPlatform</varname>
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</term>
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<listitem>
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<para>
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The "build platform" is the platform on which a package is built. Once
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someone has a built package, or pre-built binary package, the build
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platform should not matter and be safe to ignore.
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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>
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<varname>hostPlatform</varname>
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</term>
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<listitem>
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<para>
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The "host platform" is the platform on which a package will be run. This
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is the simplest platform to understand, but also the one with the worst
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name.
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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>
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<varname>targetPlatform</varname>
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</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
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actually fundamental to the process of building software. Instead, it is
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only relevant for compatibility with building certain specific compilers
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and build tools. 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
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compiler resulting from a single build can itself only produce binaries
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for a single platform. The task specifying this single "target platform"
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is thus pushed to build time of the compiler. The root cause of this
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mistake is often that the compiler (which will be run on the host) and
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the the standard library/runtime (which will be run on the target) are
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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
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time like this. If the tool supports modular or pluggable backends, both
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the need to specify the target at build time and the constraint of
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having only a single target disappear. An example of such a tool is
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LLVM.
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</para>
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<para>
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Although the existence of a "target platfom" is arguably a historical
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mistake, it is a common one: examples of tools that suffer from it are
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GCC, Binutils, GHC and Autoconf. Nixpkgs tries to avoid sharing in the
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mistake where possible. Still, because the concept of a target platform
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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
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convoluted evolution, but this is slowly being cleaned up. You can see
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examples of ones used in practice in
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<literal>lib.systems.examples</literal>; note how they are not all very
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consistent. 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>
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<varname>system</varname>
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</term>
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<listitem>
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<para>
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This is a two-component shorthand for the platform. Examples of this
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would be "x86_64-darwin" and "i686-linux"; see
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<literal>lib.systems.doubles</literal> for more. This format isn't very
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standard, but has built-in support in Nix, such as the
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<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>
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<varname>config</varname>
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</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. Examples of
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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
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pioneered by LLVM and traditionally just used for the
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<varname>targetPlatform</varname>. This format is strictly more
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informative than the "Nix host double", as the previous format could
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analogously be termed. This needs a better name than
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<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>
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<varname>parsed</varname>
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</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
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white-listed components. This can be specified directly, or actually
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parsed from the <varname>config</varname>. [Technically, only one need
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be specified and the others can be inferred, though the precision of
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inference may not be very good.] See
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<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>
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<varname>libc</varname>
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</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. Valid
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identifiers include "glibc" for GNU libc, "libSystem" for Darwin's
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Libsystem, and "uclibc" for µClibc. It should probably be refactored to
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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>
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<varname>is*</varname>
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</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>,
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and slapped on every platform. They are superior to the ones in
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<varname>stdenv</varname> as they force the user to be explicit about
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which platform they are inspecting. 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>
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<varname>platform</varname>
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</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
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attribute set). See <literal>lib.systems.platforms</literal> for
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examples—there's hopefully one in there that will work verbatim for
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each platform that is working. Please help us triage these flags and
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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
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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
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host and target platforms match. This is directly implied by the meaning of
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"host platform" and "runtime dependency": The package dependency exists
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while both packages are running 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
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depending package and the depended-on package. The meaning of a build time
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dependency is that to build the depending package we need to be able to run
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the depended-on's package. The depending package's build platform is
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therefore equal to the depended-on package's host platform. Analogously,
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the depending package's host platform is equal to the depended-on package's
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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
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three platforms for all its transitive dependencies. This is the most
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important guiding principle behind cross-compilation with Nixpkgs, and will
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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. If a package is being built
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with a <literal>(build, host, target)</literal> platform triple of
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<literal>(foo, bar, bar)</literal>, then its build-time dependencies would
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have a triple of <literal>(foo, foo, bar)</literal>, and <emphasis>those
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packages'</emphasis> build-time dependencies would have triple of
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<literal>(foo, foo, foo)</literal>. In other words, it should take two
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"rounds" of following build-time dependency edges before one reaches a
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fixed point where, by the sliding window principle, the platform triple no
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longer changes. Indeed, this happens with cross compilation, where only
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rounds of native dependencies starting with the second necessarily coincide
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with native packages.
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</para>
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<note>
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<para>
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The depending package's target platform is unconstrained by the sliding
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window principle, which makes sense in that one can in principle build
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cross compilers targeting arbitrary platforms.
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</para>
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</note>
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<para>
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How does this work in practice? Nixpkgs is now structured so that
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build-time dependencies are taken from <varname>buildPackages</varname>,
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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
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time, while <varname>gcc</varname> should be used at run time. Now, for
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most of Nixpkgs's history, there was no <varname>buildPackages</varname>,
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and most packages have not been refactored to use it explicitly. Instead,
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one can use the six (<emphasis>gasp</emphasis>) attributes used for
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specifying dependencies as documented in
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<xref linkend="ssec-stdenv-dependencies"/>. We "splice" together the
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run-time and build-time package sets with <varname>callPackage</varname>,
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and then <varname>mkDerivation</varname> for each of four attributes pulls
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the right derivation out. This splicing can be skipped when not cross
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compiling as the package sets are the same, but is a bit slow for cross
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compiling. Because of this, a best-of-both-worlds solution is in the works
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with no splicing or explicit access of <varname>buildPackages</varname>
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needed. For now, feel free to use either method.
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</para>
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<note>
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<para>
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There is also a "backlink" <varname>targetPackages</varname>, yielding a
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package set whose <varname>buildPackages</varname> is the current package
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set. This is a hack, though, to accommodate compilers with lousy build
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systems. Please do not use this unless you are absolutely sure you are
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packaging such a compiler and there is no other way.
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</para>
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</note>
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</section>
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<section>
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<title>Cross packaging cookbook</title>
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<para>
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Some frequently problems when packaging for cross compilation are good to
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just spell and answer. Ideally the information above is exhaustive, so this
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section cannot provide any new information, but its ludicrous and cruel to
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expect everyone to spend effort working through the interaction of many
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features just to figure out the same answer to the same common problem.
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Feel free to add to this list!
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</para>
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<qandaset>
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<qandaentry>
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<question>
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<para>
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What if my package's build system needs to build a C program to be run
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under the build environment?
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</para>
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</question>
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<answer>
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<para>
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<programlisting>depsBuildBuild = [ buildPackages.stdenv.cc ];</programlisting>
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Add it to your <function>mkDerivation</function> invocation.
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</para>
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</answer>
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</qandaentry>
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<qandaentry>
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<question>
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<para>
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My package fails to find <command>ar</command>.
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</para>
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</question>
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<answer>
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<para>
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Many packages assume that an unprefixed <command>ar</command> is
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available, but Nix doesn't provide one. It only provides a prefixed one,
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just as it only does for all the other binutils programs. It may be
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necessary to patch the package to fix the build system to use a prefixed
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`ar`.
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</para>
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</answer>
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</qandaentry>
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<qandaentry>
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<question>
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<para>
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My package's testsuite needs to run host platform code.
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</para>
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</question>
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<answer>
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<para>
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<programlisting>doCheck = stdenv.hostPlatform != stdenv.buildPlatfrom;</programlisting>
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Add it to your <function>mkDerivation</function> invocation.
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</para>
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</answer>
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</qandaentry>
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</qandaset>
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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>
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<para>
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More information needs to moved from the old wiki, especially
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<link xlink:href="https://nixos.org/wiki/CrossCompiling" />, for this
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section.
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</para>
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</note>
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<para>
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Nixpkgs can be instantiated with <varname>localSystem</varname> alone, in
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which case there is no cross compiling and everything is built by and for
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that system, or also with <varname>crossSystem</varname>, in which case
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packages run on the latter, but all building happens on the former. Both
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parameters take the same schema as the 3 (build, host, and target) platforms
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defined in the previous section. As mentioned above,
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<literal>lib.systems.examples</literal> has some platforms which are used as
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arguments for these parameters in practice. You can use them
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programmatically, or on the command line:
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<programlisting>
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nix-build <nixpkgs> --arg crossSystem '(import <nixpkgs/lib>).systems.examples.fooBarBaz' -A whatever</programlisting>
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</para>
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<note>
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<para>
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Eventually we would like to make these platform examples an unnecessary
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convenience so that
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<programlisting>
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nix-build <nixpkgs> --arg crossSystem.config '<arch>-<os>-<vendor>-<abi>' -A whatever</programlisting>
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works in the vast majority of cases. The problem today is dependencies on
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other sorts of configuration which aren't given proper defaults. We rely on
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the examples to crudely to set those configuration parameters in some
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vaguely sane manner on the users behalf. Issue
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<link xlink:href="https://github.com/NixOS/nixpkgs/issues/34274">#34274</link>
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tracks this inconvenience along with its root cause in crufty configuration
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options.
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</para>
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</note>
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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
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fill in missing fields. As discussed in the previous section, only one of
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<varname>system</varname>, <varname>config</varname>, and
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<varname>parsed</varname> is needed to infer the other two. Additionally,
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<varname>libc</varname> will be inferred from <varname>parse</varname>.
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Finally, <literal>localSystem.system</literal> is also
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<emphasis>impurely</emphasis> inferred based on the platform evaluation
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occurs. This means it is often not necessary to pass
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<varname>localSystem</varname> at all, as in the command-line example in the
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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
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<varname>system</varname>, <varname>platform</varname>, along with the
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optional <varname>crossSystem</varname> to nixpkgs: <literal>import
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<nixpkgs> { system = ..; platform = ..; crossSystem = ..;
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}</literal>. Passing those two instead of <varname>localSystem</varname> is
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still supported for compatibility, but is discouraged. Indeed, much of the
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inference we do for these parameters is motivated by compatibility as much
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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
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<varname>crossSystem</varname> overlap horribly with the three
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<varname>*Platforms</varname> (<varname>buildPlatform</varname>,
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<varname>hostPlatform,</varname> and <varname>targetPlatform</varname>; see
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<varname>stage.nix</varname> or the manual). Actually, those identifiers are
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purposefully not used here to draw a subtle but important distinction: While
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the granularity of having 3 platforms is necessary to properly *build*
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packages, it is overkill for specifying the user's *intent* when making a
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build plan or package set. A simple "build vs deploy" dichotomy is adequate:
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the sliding window principle described in the previous section shows how to
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interpolate between the these two "end points" to get the 3 platform triple
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for each bootstrapping stage. That means for any package a given package
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set, even those not bound on the top level but only reachable via
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dependencies or <varname>buildPackages</varname>, the three platforms will
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be defined as one of <varname>localSystem</varname> or
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<varname>crossSystem</varname>, with the former replacing the latter as one
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traverses build-time dependencies. A last simple difference then is
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<varname>crossSystem</varname> should be null when one doesn't want to
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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>
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To be written.
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</para>
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<note>
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<para>
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If one explores nixpkgs, they will see derivations with names like
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<literal>gccCross</literal>. Such <literal>*Cross</literal> derivations is
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a holdover from before we properly distinguished between the host and
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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>
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|
</chapter>
|