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I have been trying to pass a string to a Rust function (compiled to Wasm), however for what I understood, right now there is no way to pass strings directy, because the "str" is not a type in the "FFI world" (at least that's what the rust compiler is saying): = help: consider using `*const u8` and a length instead

So what I did was changing the function to this form (instead of using a simple &str type):

#[no_mangle]
pub extern "C" fn greet(s: *mut u8, len: usize) {
    let s = std::str::from_utf8(unsafe { std::slice::from_raw_parts(s, len) }).unwrap();
    println!("Hello, {}!", s)
}

This means that I need a pointer and the length of the string in u8.

However, someone made me notice that WASM modules are sandboxed, so they can't use normal pointers like normal applications. Thus, I have to use a function like this one to allocate memory into the module's linear memory:

use std::alloc::{alloc, dealloc, Layout};
#[no_mangle]
pub unsafe fn my_alloc(len: usize) -> *mut u8 {
    let align = std::mem::align_of::<usize>();
    let layout = Layout::from_size_align_unchecked(size, align);
    alloc(layout)
}

This is an example of a JS function that uses an alloc function like this one:

function copyMemory(data, instance) {
  var ptr = instance.exports.alloc(data.length);
  var mem = new Uint8Array(instance.exports.memory.buffer, ptr, data.length);
  mem.set(new Uint8Array(data));
  return ptr;
}

My problem is that I don't know how to convert this function to Go, that's because I am stuck at the "var mem" line, for these reasons:

  • I can't find the equivalent of "instance.exports.memory.buffer" in Go (instance is a "*wasmtime.Instance" type).
  • I don't know how to do what Unit8Buffer does in Go.

Good read about memory in Wasm: (https://radu-matei.com/blog/practical-guide-to-wasm-memory/#exchanging-strings-between-modules-and-runtimes)

JioFrost
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    Not sure if I understand your question, but I'll say a pointer to a slice is not the same as a pointer to the first element of the slice (as it is with arrays) – Hymns For Disco Jan 18 '21 at 21:32
  • @HymnsForDisco, I am sorry If I was unclear. I will try to make it more understandable. Thank you for the answer. – JioFrost Jan 18 '21 at 22:08
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    You can use `&s[0]` to get a pointer to the first element of the slice (slices are contiguous memory so you should be safe as long as you pass the right `len`). I'm not a wasmtime user though so I don't know if this will accomplish your ultimate goal, but there's my 2 cents. – Hymns For Disco Jan 18 '21 at 22:17
  • To be honest, I am so lost, that everything helps. Thanks – JioFrost Jan 18 '21 at 22:29

1 Answers1

0

Took me a little to understand exactly how the wasmtime Go-package worked, but in the end I resolved my problem doing this:

func main() {
    dir, err := ioutil.TempDir("", "out")
    check(err)
    defer os.RemoveAll(dir)
    stdoutPath := filepath.Join(dir, "stdout")

    engine := wasmtime.NewEngine()
    store := wasmtime.NewStore(engine)

    linker := wasmtime.NewLinker(store)

    // Configure WASI imports to write stdout into a file.
    wasiConfig := wasmtime.NewWasiConfig()
    wasiConfig.SetStdoutFile(stdoutPath)


    wasi, err := wasmtime.NewWasiInstance(store, wasiConfig, "wasi_snapshot_preview1")
    check(err)

    // Link WASI
    err = linker.DefineWasi(wasi)
    check(err)

    // Create our module
    module, err := wasmtime.NewModuleFromFile(store.Engine, "wasm_file.wasm")
    check(err)
    instance, err := linker.Instantiate(module)
    check(err)


    fn := instance.GetExport("greet").Func()
    memory := instance.GetExport("memory").Memory()
    alloc := instance.GetExport("my_alloc").Func()

    // // string for alloc
    size2 := int32(len([]byte("Elvis")))

    // //Allocate memomory
    ptr2, err := alloc.Call(size2)
    pointer, _ := ptr2.(int32)

    buf := memory.UnsafeData()
    for i, v := range []byte("Elvis") {
        buf[pointer+int32(i)] = v
    }

    // Use string func
    _, err = fn.Call(pointer, size2 )
    check(err)
    
    // Print WASM stdout
    out, err := ioutil.ReadFile(stdoutPath)
    check(err)
    fmt.Print(string(out))
}
JioFrost
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