use eyre::Result;
use tinywasm::{types::FuncRef, Extern, FuncContext, Imports, Module, Store};
use wat;
fn main() -> Result<()> {
by_func_ref_passed()?;
by_func_ref_returned()?;
Ok(())
}
/// example of passing wasm functions (as funcref) to imported host function
/// and imported host function calling them
fn by_func_ref_passed() -> Result<()> {
// a module with:
// imported function "host.call_this" that accepts a callback
// exported wasm function "tell_host_to_call" that calls "host.call_this" with wasm functions $add and $sub
// wasm functions $add and $sub and imported function $mul used as callbacks
// (just to show that imported functions can be referenced too)
// exported wasm function "call_binop_by_ref" is a proxy used by host to call func-references of type (i32, i32)->i32
const WASM: &str = r#"
(module
(import "host" "call_this" (func $host_callback_caller (param funcref)))
(import "host" "mul" (func $host_mul (param $x i32) (param $y i32) (result i32)))
(func $tell_host_to_call (export "tell_host_to_call")
(call $host_callback_caller (ref.func $add))
(call $host_callback_caller (ref.func $sub))
(call $host_callback_caller (ref.func $host_mul))
)
(type $binop (func (param i32 i32) (result i32)))
(table 3 funcref)
(elem (i32.const 0) $add $sub $host_mul) ;; function can only be taken reference of if it's added to a table
(func $add (param $x i32) (param $y i32) (result i32)
local.get $x
local.get $y
i32.add
)
(func $sub (param $x i32) (param $y i32) (result i32)
local.get $x
local.get $y
i32.sub
)
(table $callback_register 1 funcref)
(func (export "call_binop_by_ref") (param funcref i32 i32) (result i32)
(table.set $callback_register (i32.const 0) (local.get 0))
(call_indirect $callback_register (type $binop) (local.get 1)(local.get 2)(i32.const 0))
)
)
"#;
let wasm = wat::parse_str(WASM).expect("failed to parse wat");
let module = Module::parse_bytes(&wasm)?;
let mut store = Store::default();
let mut imports = Imports::new();
// import host function that takes callbacks and calls them
imports.define(
"host",
"call_this",
Extern::typed_func(|mut ctx: FuncContext<'_>, fn_ref: FuncRef| -> tinywasm::Result<()> {
let proxy_caller =
ctx.module().exported_func::<(FuncRef, i32, i32), i32>(ctx.store(), "call_binop_by_ref")?;
// call callback we got as argument using call_binop_by_ref
let res = proxy_caller.call(ctx.store_mut(), (fn_ref, 5, 3))?;
println!("(funcref {fn_ref:?})(5,3) results in {res}");
Ok(())
}),
)?;
// import host.mul function (one of the functions whose references are taken)
imports.define(
"host",
"mul",
Extern::typed_func(|_, args: (i32, i32)| -> tinywasm::Result<i32> { Ok(args.0 * args.1) }),
)?;
let instance = module.instantiate(&mut store, Some(imports))?;
let caller = instance.exported_func::<(), ()>(&mut store, "tell_host_to_call")?;
// call the tell_host_to_call
caller.call(&mut store, ())?;
// interesting detail is that neither $add $sub $mul were exported,
// but with a little help from proxy "call_binop_by_ref" references to them host was able to call them
Ok(())
}
/// example of returning wasm function as callback to host function
/// and host function calling it
fn by_func_ref_returned() -> Result<()> {
// a module with:
// an exported function "what_should_host_call" that returns 3 funcrefs
// wasm functions $add and $sub and imported function $mul used as callbacks
// (just to show that imported functions can be referenced too)
// exported wasm function "call_binop_by_ref" is a proxy used by host to call func-references of type (i32, i32)->i32
const WASM: &str = r#"
(module
(import "host" "mul" (func $host_mul (param $x i32) (param $y i32) (result i32)))
(type $binop (func (param i32 i32) (result i32)))
(table 3 funcref)
(elem (i32.const 0) $add $sub $host_mul)
(func $add (param $x i32) (param $y i32) (result i32)
local.get $x
local.get $y
i32.add
)
(func $sub (param $x i32) (param $y i32) (result i32)
local.get $x
local.get $y
i32.sub
)
(func $ref_to_funcs (export "what_should_host_call") (result funcref funcref funcref)
(ref.func $add)
(ref.func $sub)
(ref.func $host_mul)
)
(table $callback_register 1 funcref)
(func $call (export "call_binop_by_ref") (param funcref i32 i32) (result i32)
(table.set $callback_register (i32.const 0) (local.get 0))
(call_indirect $callback_register (type $binop) (local.get 1)(local.get 2)(i32.const 0))
)
)
"#;
let wasm = wat::parse_str(WASM).expect("failed to parse wat");
let module = Module::parse_bytes(&wasm)?;
let mut store = Store::default();
let mut imports = Imports::new();
// import host.mul function (one of the possible operations)
imports.define(
"host",
"mul",
Extern::typed_func(|_, args: (i32, i32)| -> tinywasm::Result<i32> { Ok(args.0 * args.1) }),
)?;
let instance = module.instantiate(&mut store, Some(imports))?;
{
// ask module what should we call
let funcrefs = {
let address_getter =
instance.exported_func::<(), (FuncRef, FuncRef, FuncRef)>(&mut store, "what_should_host_call")?;
address_getter.call(&mut store, ())?
};
let proxy_caller = instance.exported_func::<(FuncRef, i32, i32), i32>(&mut store, "call_binop_by_ref")?;
for (idx, func_ref) in [funcrefs.0, funcrefs.1, funcrefs.2].iter().enumerate() {
// call those funcrefs via "call_binop_by_ref"
let res = proxy_caller.call(&mut store, (*func_ref, 5, 3))?;
println!("at idx: {idx} funcref {func_ref:?}(5,3) results in {res}");
}
}
Ok(())
}
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