Stream: git-wasmtime

Topic: wasmtime / Issue #2373 Can access stdin from a WASI progr...

Wasmtime GitHub notifications bot (Nov 06 2020 at 18:38):

flavio opened Issue #2373:

I hope I'm opening this bug on the right GitHub repo, if not please forgive me :sweat_smile:

I've written a simple "echo" program using AssemblyScript and as-wasi. The program reads the user input from STDIN and writes it back to STDOUT.

Unfortunately it looks like I can never get back the input I enter.

• What are the steps to reproduce the issue?

This the source code of the AssemblyScript program I'm running:

import "wasi"

import {Console} from "as-wasi"

Console.log("type something");

let input: string | null;
let msg : string = "nothing";

input = Console.readAll();
if (input != null) {
  msg = input!
}

Console.log('I got: ' + msg);

The program can be compiled to a WASM binary by doing:

$ asc echo.ts -b echo.wasm

And it can be run in this way:

$ wasmtime run echo.wasm

The program will start and I'll be able to enter my text. The program will keep reading from STDIN until I send the EOF symbol (I'm on Linux -> CTRL-D).

Unfortunately the input variable is always null.

• What do you expect to happen? What does actually happen? Does it panic, and
  if so, with which assertion?

I would expect the WASM program to be able to read data from stdin. The input object should hold the text I entered on my terminal.

• Which Wasmtime version / commit hash / branch are you using?

This is my stack:

* wasmtime: 0.21.0
* AssemblyScript: 0.17.1
* as-wasi: 0.4.0

• If relevant, can you include some extra information about your environment?
  (Rust version, operating system, architecture...)

I'm running on a x86_64 Linux box that has openSUSE Tumbleweed

More information...

The as-wasi handles STDIN/STDOUT/STDERR by using instances of the Descriptor class. The library simply opens the file descriptor 0 for STDIN, 1 for STDOUT and 2 for STDERR. In my tests it looks like opening the file descriptor 0 always returns a null instance of Descriptor; this doesn't happen with STDOUT and STDERR.

The same issue happens also when running the program through a custom made Go program I wrote leveraging wasmtime-go.
In that case I even tried to start the WASM binary not by inheriting the parent STDIN, but instead by using a text file I previously created. Also in this case the WASM binary got a null object when reading from the STDIN.

Relevant: a similar WASM binary, generated by translating Rust -> WASM, just works as expected.

Thanks for any kind of help you can give me. I gotta say, WebAssembly and wasmtime are really cool :)

Wasmtime GitHub notifications bot (Nov 06 2020 at 18:38):

flavio labeled Issue #2373:

I hope I'm opening this bug on the right GitHub repo, if not please forgive me :sweat_smile:

I've written a simple "echo" program using AssemblyScript and as-wasi. The program reads the user input from STDIN and writes it back to STDOUT.

Unfortunately it looks like I can never get back the input I enter.

• What are the steps to reproduce the issue?

This the source code of the AssemblyScript program I'm running:

import "wasi"

import {Console} from "as-wasi"

Console.log("type something");

let input: string | null;
let msg : string = "nothing";

input = Console.readAll();
if (input != null) {
  msg = input!
}

Console.log('I got: ' + msg);

The program can be compiled to a WASM binary by doing:

$ asc echo.ts -b echo.wasm

And it can be run in this way:

$ wasmtime run echo.wasm

The program will start and I'll be able to enter my text. The program will keep reading from STDIN until I send the EOF symbol (I'm on Linux -> CTRL-D).

Unfortunately the input variable is always null.

• What do you expect to happen? What does actually happen? Does it panic, and
  if so, with which assertion?

I would expect the WASM program to be able to read data from stdin. The input object should hold the text I entered on my terminal.

• Which Wasmtime version / commit hash / branch are you using?

This is my stack:

* wasmtime: 0.21.0
* AssemblyScript: 0.17.1
* as-wasi: 0.4.0

• If relevant, can you include some extra information about your environment?
  (Rust version, operating system, architecture...)

I'm running on a x86_64 Linux box that has openSUSE Tumbleweed

More information...

The as-wasi handles STDIN/STDOUT/STDERR by using instances of the Descriptor class. The library simply opens the file descriptor 0 for STDIN, 1 for STDOUT and 2 for STDERR. In my tests it looks like opening the file descriptor 0 always returns a null instance of Descriptor; this doesn't happen with STDOUT and STDERR.

The same issue happens also when running the program through a custom made Go program I wrote leveraging wasmtime-go.
In that case I even tried to start the WASM binary not by inheriting the parent STDIN, but instead by using a text file I previously created. Also in this case the WASM binary got a null object when reading from the STDIN.

Relevant: a similar WASM binary, generated by translating Rust -> WASM, just works as expected.

Thanks for any kind of help you can give me. I gotta say, WebAssembly and wasmtime are really cool :)

Last updated: Dec 13 2025 at 19:03 UTC