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README.md

WASI example

This example shows how to use I/O in your WebAssembly modules using WASI (WebAssembly System Interface).

$ go run cat.go /test.txt
greet filesystem

If you do not set the environment variable TOOLCHAIN, main defaults to use Wasm built with "tinygo". Here are the included examples:

  • cargo-wasi - Built via cargo wasi build --release; mv ./target/wasm32-wasi/release/cat.wasm .
  • tinygo - Built via tinygo build -o cat.wasm -scheduler=none --no-debug -target=wasi cat.go
  • zig - Built via zig build-exe cat.zig -target=wasm32-wasi -OReleaseSmall
  • zig-cc - Built via zig cc cat.c -o cat.wasm --target=wasm32-wasi -O3

To run the same example with zig-cc:

$ TOOLCHAIN=zig-cc go run cat.go /test.txt
greet filesystem

Toolchain notes

Examples here check in the resulting binary, as doing so removes a potentially expensive toolchain setup. This means we have to be careful how wasm is built, so that the repository doesn't bloat (ex more bandwidth for git clone).

While WASI attempts to be portable, there are no specification tests and some compilers partially implement features. Notes about portability follow.

cargo-wasi (Rustlang)

The Rustlang source uses cargo-wasi because the normal release target wasm32-wasi results in almost 2MB, which is too large to check into our source tree.

Concretely, if you use cargo build --target wasm32-wasi --release, the binary ./target/wasm32-wasi/release/cat.wasm is over 1.9MB. One explanation for this bloat is wasm32-wasi target is not pure rust. As that issue is over two years old, it is unlikely to change. This means the only way to create smaller wasm is via optimization.

The cargo-wasi crate includes many optimizations in its release target, including wasm-opt, a part of binaryen. cargo wasi build --release generates 82KB of wasm, which is small enough to check in.

emscripten

Emscripten is not included as we cannot create a cat program without using custom filesystem code. Emscripten only supports WASI I/O for stdin/stdout/stderr and suggest using wasi-libc instead. This is used in the zig-cc example.