Stream: git-wasmtime

Topic: wasmtime / issue #5801 How should I call a function point...

Wasmtime GitHub notifications bot (Feb 16 2023 at 14:55):

Officeyutong opened issue #5801:

#[link(wasm_import_module = "env")]
extern "C" {
    // We are using `wasm32-wasi`, so pointers are 32bit
    fn register_callback(ptr: u32);
    fn run_callback();
}

fn callback_func() {
    println!("Called!");
}

fn main() {
    unsafe {
        register_callback(callback_func as u32);
        run_callback();
    }
}

register_callback and run_callback are functions that the host-side provides.

I expect that the host side will call the function pointer (casted to u32) when run_callback is called, what should I do to implement this?

Wasmtime GitHub notifications bot (Feb 16 2023 at 15:24):

alexcrichton commented on issue #5801:

Function pointers in WebAssembly are represented as integers which are indexes into a table in the module. The callback_func given to the host should be used to index a Table which is exported from the module. After loading a Func from that table then you can cast it to the appropriate type and then call it.

Wasmtime GitHub notifications bot (Feb 16 2023 at 15:24):

alexcrichton closed issue #5801:

#[link(wasm_import_module = "env")]
extern "C" {
    // We are using `wasm32-wasi`, so pointers are 32bit
    fn register_callback(ptr: u32);
    fn run_callback();
}

fn callback_func() {
    println!("Called!");
}

fn main() {
    unsafe {
        register_callback(callback_func as u32);
        run_callback();
    }
}

register_callback and run_callback are functions that the host-side provides.

I expect that the host side will call the function pointer (casted to u32) when run_callback is called, what should I do to implement this?

Last updated: Dec 06 2025 at 06:05 UTC