479 lines
20 KiB
XML
479 lines
20 KiB
XML
<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>. 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>:
|
|
<programlisting>{ stdenv, buildPlatform, hostPlatform, fooDep, barDep, .. }: ...buildPlatform...</programlisting>
|
|
, or just off <varname>stdenv</varname>:
|
|
<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 <nixpkgs> --arg crossSystem '(import <nixpkgs/lib>).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 <nixpkgs> --arg crossSystem.config '<arch>-<os>-<vendor>-<abi>' -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
|
|
<nixpkgs> { 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>
|