102 lines
2.9 KiB
Markdown
102 lines
2.9 KiB
Markdown
# Embedding Wasmtime in Rust
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This document shows how to embed Wasmtime using the Rust API, and run a simple
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wasm program.
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# Create some wasm
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Let's create a simple WebAssembly file with a single exported function that returns an integer:
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```wat
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(;; wat2wasm hello.wat -o $WASM_FILES/hello.wasm ;;)
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(module
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(func (export "answer") (result i32)
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i32.const 42
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)
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)
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```
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# Create rust project
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```
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$ cargo new --bin wasmtime_hello
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$ cd wasmtime_hello
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$ cp $WASM_FILES/hello.wasm .
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```
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We will be using the wasmtime engine/API to run the wasm file, so we will add the dependency to `Cargo.toml`:
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```
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[dependencies]
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wasmtime = "<current version>"
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```
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where "<current version>" is the current version number of the `wasmtime` crate.
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It is time to add code to the `src/main.rs`. First, the engine and storage need to be activated:
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```rust
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use wasmtime_api::*;
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let engine = HostRef::new(Engine::default());
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let store = HostRef::new(Store::new(&engine));
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```
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The `HostRef` will be used a lot -- it is a "convenience" object to store and refer an object between the host and
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the embedded environments.
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The `hello.wasm` can be read from the file system and provided to the `Module` object constructor as `&[u8]`:
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```rust
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use std::fs::read;
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let hello_wasm = read("hello.wasm").expect("wasm file");
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let module = HostRef::new(Module::new(&store, &hello_wasm).expect("wasm module"));
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```
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The module instance can now be created. Normally, you would provide exports, but in this case, there is none required:
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```rust
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let instance = Instance::new(&store, &module, &[]).expect("wasm instance");
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```
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Everything is set. If a WebAssembly module has a start function -- it was run.
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The instance's exports can be used at this point. This wasm file has only one export, so we can index it directly:
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```rust
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let answer_fn = instance.exports()[0].func().expect("answer function");
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```
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The exported function can be called using the `call` method. Remember that in most of the cases,
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a `HostRef<_>` object will be returned, so `borrow()` or `borrow_mut()` method has to be used to refer the
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specific object. The exported "answer" function accepts no parameters and returns a single `i32` value.
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```rust
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let result = answer_fn.borrow().call(&[]).expect("success");
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println!("Answer: {}", result[0].i32());
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```
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The names of the WebAssembly module's imports and exports can be discovered by means of module's corresponding methods.
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# src/main.rs
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```rust
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use std::fs::read;
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use wasmtime_api::*;
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fn main() {
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let engine = HostRef::new(Engine::default());
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let store = HostRef::new(Store::new(&engine));
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let wasm = read("hello.wasm").expect("wasm file");
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let module = HostRef::new(Module::new(&store, &wasm).expect("wasm module"));
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let instance = Instance::new(&store, &module, &[]).expect("wasm instance");
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let answer_fn = instance.exports()[0].func().expect("answer function");
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let result = answer_fn.borrow().call(&[]).expect("success");
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println!("Answer: {}", result[0].i32());
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}
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```
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