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Expand cross compilation docs and add wasi example

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Also refactored current examples.
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oxalica
2021-11-22 05:19:00 +08:00
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docs/cross_compilation.md
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# Cross compilation with `rust-overlay`
There an example for cross compilation to `aarch64-linux` in
[`example/cross-aarch64`](../examples/cross-aarch64).
There are examples for cross compilation in [`example` directory](../examples).
To try examples,
1. `cd` into `example/cross-aarch64` (or other directory).
2. `nix-shell` to enter the development environment.
3. `make run` to build and run the program in an emulater.
The structure of `shell.nix` should like this,
```nix
# Import from global `<nixpkgs>`. Can also be from flake input if you like.
(import <nixpkgs> {
# Target triple in nixpkgs format, can be abbreviated.
# Some special targets, like `wasm32-wasi`, need special configurations here, check
# `examples/cross-wasi/shell.nix` for details.
crossSystem = "aarch64-linux";
# Apply `rust-overlay`.
overlays = [ (import ../..) ];
# Workaround: we need `callPackage` to enable auto package-splicing, thus we don't need to manually prefix
# everything in `nativeBuildInputs` with `buildPackages.`.
# See: https://github.com/NixOS/nixpkgs/issues/49526
}).callPackage (
{ mkShell, stdenv, rust-bin, pkg-config, openssl, qemu }:
mkShell {
# Build-time dependencies. build = host = your-machine, target = aarch64
# Typically contains,
# - Configure-related: cmake, pkg-config
# - Compiler-related: gcc, rustc, binutils
# - Code generators run at build time: yacc, bision
nativeBuildInputs = [
# `minimal` is enough for basic building and running.
# Can also be `default` to bring other components like `rustfmt`, `clippy`, and etc.
rust-bin.stable.latest.minimal
pkg-config
];
# Build-time tools which are target agnostic. build = host = target = your-machine.
# Emulaters should essentially also go `nativeBuildInputs`. But with some packaging issue,
# currently it would cause some rebuild.
# We put them here just for a workaround.
# See: https://github.com/NixOS/nixpkgs/pull/146583
depsBuildBuild = [ qemu ];
# Run-time dependencies. build = your-matchine, host = target = aarch64
# Usually are libraries to be linked.
buildInputs = [ openssl ];
# Tell cargo about the linker and an optional emulater. So they can be used in `cargo build`
# and `cargo run`.
# Environment variables are in format `CARGO_TARGET_<UPPERCASE_UNDERSCORE_RUST_TRIPLE>_LINKER`.
# They are also be set in `.cargo/config.toml` instead.
# See: https://doc.rust-lang.org/cargo/reference/config.html#target
CARGO_TARGET_AARCH64_UNKNOWN_LINUX_GNU_LINKER = "${stdenv.cc.targetPrefix}cc";
CARGO_TARGET_AARCH64_UNKNOWN_LINUX_GNU_RUNNER = "qemu-aarch64";
}) {}
```
For more details about these different kinds of dependencies,
see also [Nix Wiki - Cross Compiling](https://nixos.wiki/wiki/Cross_Compiling#How_to_specify_dependencies).