How to import WASI and write a simple console log function in WASM WAT

Question

I want to understand the WebAssembly text format for educational purposes and for writing a POC compiler that compiles straight to WASM.

I want to understand how to properly import the necessary WASI functions to create a console log function.

I'd love to see an example written in somewhat readable WAT of a module that imports WASI and implements a simple console log/printf/echo function that can run outside the browser.

e.g.,

(module
  (func $wasi_snapshot_preview1.fd_close (;0;) (import "wasi_snapshot_preview1" "fd_close") (param i32) (result i32))
  (func $wasi_snapshot_preview1.fd_seek (;1;) (import "wasi_snapshot_preview1" "fd_seek") (param i32 i64 i32 i32) (result i32))
  (func $wasi_snapshot_preview1.fd_write (;2;) (import "wasi_snapshot_preview1" "fd_write") (param i32 i32 i32 i32) (result i32))
  (func $wasi_snapshot_preview1.proc_exit (;3;) (import "wasi_snapshot_preview1" "proc_exit") (param i32))
  (data (; ... ;) )
  (memory (; ... ;) )
  (func $log (; ... ;))
)

I'm not sure if it is of any help, but i know that rust has a console-log crate specifically for WASM ( crates.io/crates/console_log ) ; you may benefit either from the rust sourcecode of that project, or by rust--build-->wasm-->wasm_textmode. — Raxi
– Raxi, Commented Dec 18, 2021 at 1:55
I've tried compiling a rust hello world using the wasm32-wasi target, but the println! macro generates a huge amount of overhead along with the standard library. I'm not looking to use the browser console.log, but instead write to the stdout file descriptor in a desktop/embedded wasi environment. — José Mancharo
– José Mancharo, Commented Dec 18, 2021 at 4:33
Ahyea, shame. I hadn't played with it myself yet tbh, but just figured it may be relevant to your project. — Raxi
– Raxi, Commented Dec 18, 2021 at 4:45
The Wasmtime docs include an example of using the WebAssembly text format (.wat) to write a file. It's not console.log, but it might prove a useful starting point: github.com/bytecodealliance/wasmtime/blob/main/docs/… — Jordan Eldredge
– Jordan Eldredge, Commented Dec 23, 2022 at 5:50

shluvme · Accepted Answer · 2023-01-18 01:53:53Z

0

This code imports the log function from JS and uses it to print a number:

(module
  (func $log (import "imports" "log") (param i32))
  (func (export "logNumber")
    i32.const 13
    call $log)
)

answered Jan 18, 2023 at 1:53

shluvme

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