I wrote a simple wasmer-wasi example that reads directory entries but it always fails.
#[no_mangle]
pub fn start() {
std::fs::read_dir("/").unwrap();
}
use wasmer::{Instance, Module, Store};
use wasmer_wasi::WasiState;
fn main() -> Result<(), Box<dyn std::error::Error>> {
let wasm = std::fs::read("target/wasm32-wasi/debug/wasi_fs_example.wasm")?;
let store = Store::default();
let module = Module::new(&store, wasm)?;
let mut wasi_env = WasiState::new("wasi_fs_example")
.preopen_dir("target")?
.finalize()?;
let import_object = wasi_env.import_object(&module)?;
let instance = Instance::new(&module, &import_object)?;
let start = instance.exports.get_function("start")?;
start.call(&[])?;
Ok(())
}
This produce the error while running:
cargo build -p wasi_fs_example --target wasm32-wasi
cargo run -p runner
thread '<unnamed>' panicked at 'called `Result::unwrap()` on an `Err` value: Custom { kind: Other, error: "failed to find a pre-opened file descriptor through which \"/\" could be opened" }', wasi_fs_example/src/lib.rs:6:10
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
Full source code of the example is located here . Why doesn't it work?
Currently in Rust we can really only create WASI binaries, not libraries. WASI only works for the duration of a main function that needs to be there. Calling the wasi functions from anywhere else segfaults because libpreopen isn't initialized.
So to summarize, how to fix this currently:
lib.crate-type
at allsrc/main.rs
, not src/lib.rs
#![no_main]
at the top of the src/main.rs
RUSTFLAGS="-Z wasi-exec-model=reactor" cargo +nightly build --target wasm32-wasi
_initialize
before anything elsehttps://github.com/WebAssembly/WASI/issues/24
https://github.com/rust-lang/rust/pull/79997
In my case:
#![no_main]
#[no_mangle]
pub extern "C" fn start() {
std::fs::read_dir("/").unwrap();
}
...
let start = instance.exports.get_function("_initialize")?;
start.call(&[])?;
let start = instance.exports.get_function("start")?;
start.call(&[])?;
...
RUSTFLAGS="-Z wasi-exec-model=reactor" cargo +nightly build -p wasi_fs_example --target wasm32-wasi
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