"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.
Nixpkgs follows the <linkxlink: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.
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>:
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.
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.
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.
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.
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.]
This is the most important guiding principle behind cross-compilation with Nixpkgs, and will be called the <wordasword>sliding window principle</wordasword>.
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.
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.
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.
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.
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.
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 like <command>nix-build <nixpkgs> --arg crossSystem '(import <nixpkgs/lib>).systems.examples.fooBarBaz'</command>.
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:
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.
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>.
