Add example of compiling wat and running with wasmtime

README.md

@@ -28,8 +28,8 @@ as with "cargo build --release". For information on installing clang, see
 [rust-bindgen's documentation](https://github.com/rust-lang/rust-bindgen/blob/master/book/src/requirements.md).
 
 There are Rust, C, and C++ toolchains that can compile programs with WASI. See
-[here][WASI intro] for more information, and [here][WASI tutorial] for a
-tutorial on compiling and running programs using WASI and wasmtime, as
+the [WASI intro][WASI intro] for more information, and the [WASI tutorial][WASI tutorial]
+for a tutorial on compiling and running programs using WASI and wasmtime, as
 well as an overview of the filesystem sandboxing system.
 
 Wasmtime does not yet implement Spectre mitigations, such as those being

docs/WASI-tutorial.md

@@ -1,16 +1,21 @@
 # WASI tutorial
 We'll split the tutorial into two parts: in the first part we'll walk through
-compiling C and Rust programs to WASI, and in the second part how to execute
-the compiled WebAssembly module using `wasmtime` runtime.
+compiling C and Rust programs to WASI and executing the compiled WebAssembly module
+using `wasmtime` runtime. In the second part we will discuss the compilation of a
+simpler WebAssembly program written using the WebAssembly text format, and executing
+this using the `wasmtime` runtime.
 
 - [WASI tutorial](#wasi-tutorial)
+  - [Running common languages with WASI](#running-common-languages-with-wasi)
     - [Compiling to WASI](#compiling-to-wasi)
         - [From C](#from-c)
         - [From Rust](#from-rust)
     - [Executing in `wasmtime` runtime](#executing-in-wasmtime-runtime)
+  - [Web assembly text example](#web-assembly-text-example)
 
+## Running common languages with WASI
 ## Compiling to WASI
-### From C
+#### From C
 Let's start with a simple C program which performs a file copy, which will
 show to compile and run programs, as well as perform simple sandbox
 configuration. The C code here uses standard POSIX APIs, and doesn't have
@@ -87,7 +92,7 @@ demo.wasm: WebAssembly (wasm) binary module version 0x1 (MVP)
 ```
 
 
-### From Rust
+#### From Rust
 The same effect can be achieved with Rust. Firstly, go ahead and create a new
 binary crate:
 
@@ -244,3 +249,68 @@ The capability model is very powerful, and what's shown here is just the beginni
 In the future, we'll be exposing much more functionality, including finer-grained
 capabilities, capabilities for network ports, and the ability for applications to
 explicitly request capabilities.
+
+## Web assembly text example
+
+In this example we will look at compiling the WebAssembly text format into wasm, and
+running the compiled WebAssembly module using the `wasmtime` runtime. This example
+makes use of WASI's `fd_write` implementation to write `hello world` to stdout.
+
+First, create a new `demo.wat` file:
+
+```wat
+(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 number of bytes written
+    (import "wasi_unstable" "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 $fd_write
+            (i32.const 1) ;; file_descriptor - 1 for stdout
+            (i32.const 0) ;; *iovs - The pointer to the iov array, which is stored at memory location 0
+            (i32.const 1) ;; iovs_len - We're printing 1 string stored in an iov - so one.
+            (i32.const 20) ;; nwritten - A place in memory to store the number of bytes writen
+        )
+        drop ;; Discard the number of bytes written from the top the stack
+    )
+)
+```
+
+`wasmtime` can directly execute `.wat` files:
+
+```
+$ wasmtime demo.wat
+hello world
+```
+
+Or, you can compile the `.wat` WebAssembly text format into the wasm binary format
+yourself using the [wabt] command line tools:
+
+```
+$ wat2wasm demo.wat
+```
+
+The created `.wasm` file can now be executed with `wasmtime` directly like so:
+
+```
+$ wasmtime demo.wasm
+hello world
+```
+
+To run this example within the browser, simply upload the compiled `.wasm` file to
+the [WASI browser polyfill].
+
+[wabt]: https://github.com/WebAssembly/wabt
+[WASI browser polyfill]: https://wasi.dev/polyfill/