#assemblyscript #wasmer #assembly-script


Helpers to read and write memory in AssemblyScript wasm modules

11 releases

0.2.7 Sep 21, 2022
0.2.6 Sep 15, 2022
0.2.5 Jun 29, 2022
0.2.3 Feb 14, 2022
0.1.1 Dec 24, 2021

#93 in Memory management

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105 downloads per month
Used in massa-sc-runtime


520 lines

AssemblyScript bindings cargo version

Currently, this binding library is compatible with Wasmer, and we are studying on a compatibility with WasmTime. With this helper are able to read, write and use the allocation in your WebAssembly modules compiled from AssemblyScript.


We provide two helpers, StringPtr and BufferPtr. The string correspond to a string in AssemblyScript and the BufferPtr to ArrayBuffer. If you want to read the return of a native function, it's really easy:

use as_ffi_bindings::{Read, StringPtr};
let instance = Instance::new(&module, &import_object)?;
let memory = instance.exports.get_memory("memory").expect("get memory");
// Instanciation of your wasmer things

let get_string = instance
    .get_native_function::<(), StringPtr>("getString")?;
// Get native function in your module

let str_ptr: StringPtr = get_string.call()?;
// Get directly a string pointer

let string: String = str_ptr.read(memory)?;

Note: If you choose to use as_ffi_bindings::Env you also have access to the memory when you declare your native function with env (using Wasmer) so the memory of the instance is always available:

let str_ptr: BufferPtr = get_buffer.call()?;
// Get directly a buffer pointer

let buffer: Vec<u8> = buffer_ptr.read(memory)?;


use as_ffi_bindings::{Write};

The Write traits allow you to free (in development), write and allocate StringPtr and BufferPtr. This allows you to send a value to your AssemblyScript module from Rust:

let input: Vec<u8> = vec![0x03, 0x02, 0x00, 0x01];
// instanciate a vector
let buffer_ptr = BufferPtr::alloc(&input, &env)?;
// Allocate a new buffer on the defined environment
sort_buffer.call(buffer_ptr.offset() as i32)?;
// Sort your buffer in webassembly

Everything remains accessible in the rust side. You can modify your variable in the rust side with the .write() method and obviously in the WebAssembly module. So it's better to consider this as an unsafe action, pay attention 🥲.

no_thread feature

The feature 'no_thread' can be enabled to efficiently copy a buffer into a BufferPtr. To avoid data races, you should only use 1 thread to deal with the memory of your Wasmer instance.

Env instantiation

You need to init your environment to allocate and write, it's because you need to use exported function as __new, __pin, accordingly to the beautiful AssemblyScript memory documentation 📚. This is automatically initialized when Wasmer call a function in the ImportObject with an environment (examples coming soon).

But when Wasmer isn't behind you, you have to use your own hands! Look:

let mut env = Env::default();

Not hard, right?

More usage example

There are more subtle things to initialize, as the abort function in the ImportObject. Full examples for using features are in the test_features.rs file, and we tried to use simple examples.


Licensed under either of Apache License, Version 2.0 or MIT license at your option.

Unless you explicitly state otherwise, any contribution intentionally submitted for inclusion in this crate by you, as defined in the Apache-2.0 license, shall be dual licensed as above, without any additional terms or conditions.


You can have a look at the original @onsails wasmer-as experiment, on which this repository is a fork.

Minimal Rust version: 1.56.1


~149K SLoC