WASI hello world via preview 2

[rikhuijzer]

There is a nice "hello world" example in the docs:

(module
    ;; Import the required fd_write WASI function which will write the given io vectors to stdout
    ;; The function signature for fd_write is:
    ;; (File Descriptor, *iovs, iovs_len, *nwritten) -> Returns 0 on success, nonzero on error
    (import "wasi_snapshot_preview1" "fd_write" (func $fd_write (param i32 i32 i32 i32) (result i32)))

    (memory 1)
    (export "memory" (memory 0))

    ;; Write 'hello world\n' to memory at an offset of 8 bytes
    ;; Note the trailing newline which is required for the text to appear
    (data (i32.const 8) "hello world\n")

    (func $main (export "_start")
        ;; Creating a new io vector within linear memory
        (i32.store (i32.const 0) (i32.const 8))  ;; iov.iov_base - This is a pointer to the start of the 'hello world\n' string
        (i32.store (i32.const 4) (i32.const 12))  ;; iov.iov_len - The length of the 'hello world\n' string

        (call $fd_write
            (i32.const 1) ;; file_descriptor - 1 for stdout
            (i32.const 0) ;; *iovs - The pointer to the iov array, which is stored at memory location 0
            (i32.const 1) ;; iovs_len - We're printing 1 string stored in an iov - so one.
            (i32.const 20) ;; nwritten - A place in memory to store the number of bytes written
        )
        drop ;; Discard the number of bytes written from the top of the stack
    )
)

I've just tested this with wasmtime 28.0.0 and it works fine (prints "hello world" to stdout).

However, this example is using the "wasi_snapshot_preview1" API while preview 2 is currently stable. I think this example can probably be rewritten using output-stream write from streams.wit, but I'm a bit at a loss where to find code related to this.

Could anyone give some pointers on how to print via preview 2?

[alexcrichton]

I think you probably want this:

(module
  (import "wasi:cli/stdout@0.2.3" "get-stdout"
    (func $get-stdout (result i32)))
  (import "wasi:io/streams@0.2.3" "[method]output-stream.blocking-write-and-flush"
    (func $blocking-write-and-flush (param i32 i32 i32 i32)))
  (memory (export "memory") 1)
  (func (export "wasi:cli/run@0.2.3#run") (result i32)
    call $get-stdout
    i32.const 100
    i32.const 15
    i32.const 96
    call $blocking-write-and-flush
    i32.const 0
  )
  (data (i32.const 100) "Hello, world!\n")
)

Execute this with:

$ wasm-tools component embed ./wasi.wit foo.wat | wasm-tools component new -o component.wasm 
$ wasmtime component.wasm

This is more component-model level documentation rather than Wasmtime documentation which is why it's not currently in this repository (it'd be a good addition to https://component-model.bytecodealliance.org/ probably!)

[rikhuijzer]

Thank you Alex for the help. I've spent now an hour going through the various projects in order to figure out what should be inside "wasi.wit". I've looked at wit-deps and tried hello-wasi-http, but get errors like even when I have included all the wit files from hello-wasi-http.

error: package not found
     --> ./world.wit:4:11
      |
    4 |   include wasi:http/proxy@0.2.0;
      |           ^--------

Anyway no need to debug my attempts. How would you go about creating wasi.wit?

(it'd be a good addition to https://component-model.bytecodealliance.org/ probably!)

I'll happily add it once I've got something working 👍

[alexcrichton]

There's a number of ways to get the WIT right now but you're right in that most aren't well documented and there's not necessarily a canonical way of doing so AFAIK for this sort of CLI use case. One way though is to:

• Clone https://github.com/WebAssembly/WASI
• Checkout the tag you want (e.g. 0.2.2)
• Use the wasip2 directory as the wasi.wit input (e.g. pass in a directory, not a WIT file).

This might also require a --world argument to specify wasi:cli/run for example

[rikhuijzer]

• Clone https://github.com/WebAssembly/WASI
• Checkout the tag you want (e.g. 0.2.2)
• Use the wasip2 directory as the wasi.wit input (e.g. pass in a directory, not a WIT file).

Is the following indeed the direction I should take? hello.wat here is the code you wrote with the 0.2.3 replaced by 0.2.2:

$ wasm-tools component embed WASI/wasip2/io hello.wat # this at least doesn't crash
error: cannot print binary wasm output to a terminal, pass the `-t` flag to print the text format

$ wasm-tools component embed WASI/wasip2/io hello.wat | wasm-tools component new -o component.wasm
error: failed to encode a component from module

Caused by:
    0: failed to decode world from module
    1: module was not valid
    2: failed to resolve import `wasi:cli/stdout@0.2.2::get-stdout`
    3: module requires an import interface named `wasi:cli/stdout@0.2.2`

So I should point embed to WASI/wasip2 because io doesn't include cli:

$ wasm-tools component embed WASI/wasip2/ hello.wat
error: failed to resolve directory while parsing WIT for path [WASI/wasip2/]

Caused by:
    0: failed to parse package: WASI/wasip2/
    1: no `package` header was found in any WIT file for this package
error: expected at least one module field
     --> <stdin>:1:1
      |
    1 |
      | ^

$ wasm-tools --version
wasm-tools 1.223.0

Now I wonder whether the thing I'm asking for in this issue is actually an aim of wasmtime? Is getting this to work an aim of the wasmtime project or am I trying to do the wrong thing here? If yes? How what problems should I use wasmtime for?