Crosscompile a Rust project with C dependencies into Web Assembly (WASM and WASI) using Zig

Easy way using rust-bindgen and cargo-zigbuild CLI tools.

Generate Rust bindings for the C code

We just need the C/C++ header files wit the definitions and rust-bindgen will generate the Rust FFI bindings.

bindgen some-c-code/gcd.h -o src/bindings.rs # generate Rust FFI bindings for gcd.h

Quick WASI try out

Using zigbuild we can cross compile to WASI

rustup target add wasm32-wasip1 # make sure wasm32-wasi target is installed 
cargo zigbuild --target=wasm32-wasip1 --release # cross compile to WASI, release flag is optional

Warning

Previously the target wasm32-wasip1 was wasm32-wasi but it is now being deprecated, still you might want to use it even if you get some warnings.

we can try it with wasm3 engine

wasm3 target/wasm32-wasip1/release/rust-ffi-playground.wasm # try it out, requires wasm3 

or wasmi

wasmi_cli target/wasm32-wasip1/release/rust-ffi-playground.wasm # run it with wasmi runtime

Cross compile for web (WASM)

Generate code for WASM with zigbuild, and then use wasm-bindgen to generate the js/ts bindings to the WASM code.

cargo zigbuild --target=wasm32-unknown-unknown --release # cross compile to WASM, release flag is optional
wasm-bindgen target/wasm32-unknown-unknown/release/rust-ffi-playground.wasm --out-dir ./dist --target web # generate JS and TS FFI bindings into WASM code

To try it, manually include the script tag to load and initialize the wasm module

<script type="module">
  import init from "./bin/rust-ffi-playground.js";
  init().then(() => console.log("WASM Loaded"));
</script>

or use a WASM plugin like vite-plugin-wasm or use Trunk

Note: As in this github issue it can be compiled to wasm32-unknown-emscripten.

Same example but using wasm-pack, hence wasm-bindgen instead

Going further

Not a trivial project, eg. building a native library, link to a system library...

Replace CLI commands with a simple Makefile or (even better) a builder.rs file, eg. use cargo_zigbuild and bindgen directly in build.rs by replacing cc with zigbuild

use cargo_zigbuild::Zig::Cc;
use std::{env, error::Error};

fn main() -> Result<(), Box<dyn Error>> {
    cc::Build::new().file("some-c-code/def.c").compile("def");

    let out_dir = env::var("OUT_DIR").unwrap();

    let cc = Cc {
        args: vec![
            format!("some-c-code/def.c"),
            "-c".to_string(),
            "-o".to_string(),
            format!("{}/def.o", out_dir),
        ],
    };

    cc.execute().expect("Failed to compile def.c");

    let ar = cargo_zigbuild::Zig::Ar {
        args: vec![
            "crus".to_string(),
            format!("{}/libdef.a", out_dir),
            format!("{}/def.o", out_dir),
        ],
    };

    ar.execute().expect("Failed to create def.a");

    println!("cargo:rustc-link-search=native={}", out_dir);
    println!("cargo:rustc-link-lib=static=def");

    println!("cargo:rerun-if-changed=some-c-code");
    println!("cargo:rerun-if-changed=build.rs");
    Ok(())
}

References

• The cc-rs project
• Official cargo reference
• Zig cross compilation
• Bindgen tutorial
• The embedded Rust book
• Shrinking .wasm code size

Issues

wasm-bindgen targets wasm32-unknown-unknown and wasi-unknown do not (fully) support C-ABI, only older targets like wasm32-unknown-emscripten.

See comment, comment and documentation PR

There is an experimental flag --Z wasm_c_abi=spec that circumvents this limitation

Other tools

• c2rust - C to Rust translator produces unsafe Rust code from C99-compilant C code. It does not support cross compilation, but maybe it can with the help of Zig.

From their website:

C source code is parsed and typechecked using clang before being translated by our tool.

would it be possible to use it with Zig as a drop-in replacement for clang?

From their README:

I translated code on platform X, but it didn't work correctly on platform Y. We run the C preprocessor before translation to Rust. This specializes the code to the host platform. For this reason, we do not support cross compiling translated code at the moment. What platforms can C2Rust be run on? The translator and refactoring tool support both macOS and Linux. Other features, such as cross checking the functionality between C and Rust code, are currently limited to Linux hosts.

Utils

• https://wasm-feature-detect.surma.technology/ source

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Remarks

In general, a lib<name>.so or lib<name>.a should be referenced in the build file by <name>.