1 //! Small example of how to instantiate a wasm module that imports one function, 2 //! showing how you can fill in host functionality for a wasm module. 3 4 // You can execute this example with `cargo run --example hello` 5 6 use wasmtime::*; 7 8 struct MyState { 9 name: String, 10 count: usize, 11 } 12 13 fn main() -> Result<()> { 14 // First the wasm module needs to be compiled. This is done with a global 15 // "compilation environment" within an `Engine`. Note that engines can be 16 // further configured through `Config` if desired instead of using the 17 // default like this is here. 18 println!("Compiling module..."); 19 let engine = Engine::default(); 20 let module = Module::from_file(&engine, "examples/hello.wat")?; 21 22 // After a module is compiled we create a `Store` which will contain 23 // instantiated modules and other items like host functions. A Store 24 // contains an arbitrary piece of host information, and we use `MyState` 25 // here. 26 println!("Initializing..."); 27 let mut store = Store::new( 28 &engine, 29 MyState { 30 name: "hello, world!".to_string(), 31 count: 0, 32 }, 33 ); 34 35 // Our wasm module we'll be instantiating requires one imported function. 36 // the function takes no parameters and returns no results. We create a host 37 // implementation of that function here, and the `caller` parameter here is 38 // used to get access to our original `MyState` value. 39 println!("Creating callback..."); 40 let hello_func = Func::wrap(&mut store, |mut caller: Caller<'_, MyState>| { 41 println!("Calling back..."); 42 println!("> {}", caller.data().name); 43 caller.data_mut().count += 1; 44 }); 45 46 // Once we've got that all set up we can then move to the instantiation 47 // phase, pairing together a compiled module as well as a set of imports. 48 // Note that this is where the wasm `start` function, if any, would run. 49 println!("Instantiating module..."); 50 let imports = [hello_func.into()]; 51 let instance = Instance::new(&mut store, &module, &imports)?; 52 53 // Next we poke around a bit to extract the `run` function from the module. 54 println!("Extracting export..."); 55 let run = instance.get_typed_func::<(), ()>(&mut store, "run")?; 56 57 // And last but not least we can call it! 58 println!("Calling export..."); 59 run.call(&mut store, ())?; 60 61 println!("Done."); 62 Ok(()) 63 } 64