Introduction
Wasmtime is a Bytecode Alliance project that is a standalone wasm-only optimizing runtime for WebAssembly and WASI. It runs WebAssembly code outside of the Web, and can be used both as a command-line utility or as a library embedded in a larger application.
Wasmtime strives to be a highly configurable and embeddable runtime to run on any scale of application. Many features are still under development so if you have a question don't hesitate to file an issue.
This guide is intended to serve a number of purposes and within you'll find:
- How to create simple wasm modules
- How to use Wasmtime from a number of languages
- How to install and use the
wasmtimeCLI - Information about stability and security in Wasmtime.
... and more! The source for this guide lives on GitHub and contributions are welcome!
Tutorial
This tutorial walks through creating a simple Hello World WebAssembly program and then running it.
Creating hello-world.wasm
There are a number of ways to create .wasm files but for the purposes of this
tutorial, we'll be using the Rust toolchain. You can find more information on
creating .wasm files from other languages in the
Writing WebAssembly section.
To build WebAssembly binaries with Rust, you'll need the standard Rust toolchain.
Follow these instructions to install rustc, rustup and cargo
Next, you should add WebAssembly as a build target for cargo like so:
$ rustup target add wasm32-wasi
Finally, create a new Rust project called 'hello-world'. You can do this by running:
$ cargo new hello-world
After that, the hello-world folder should look like this.
hello-world/
├── Cargo.lock
├── Cargo.toml
└── src
└── main.rs
And the main.rs file inside the src folder should contain the following rust code.
fn main() { println!("Hello, world!"); }
Now, we can tell cargo to build a WebAssembly file:
$ cargo build --target wasm32-wasi
Now, in the target folder, there's a hello-world.wasm file. You can find it here:
hello-world/
├── Cargo.lock
├── Cargo.toml
├── src
└── target
└── ...
└── wasm32-wasi
└── debug
└── ...
└── hello-world.wasm
Running hello-world.wasm with Wasmtime
Installing Wasmtime
The Wasmtime CLI can be installed on Linux and macOS with a small install script:
$ curl https://wasmtime.dev/install.sh -sSf | bash
You can find more information about installing the Wasmtime CLI in the CLI Installation section
Running hello-world.wasm
There are a number of ways to run a .wasm file with Wasmtime. In this
tutorial, we'll be using the CLI, Wasmtime can also be embedded in your
applications. More information on this can be found in the
Embedding Wasmtime section.
If you've built the hello-world.wasm file (the instructions for doing so are in the
previous section),
you can run it with Wasmtime from the command line like so:
$ wasmtime target/wasm32-wasi/debug/hello-world.wasm
Examples
The examples contained in this section explain how to use Wasmtime in several common scenarios.
Markdown Parser
... more coming soon
Debugging WebAssembly
The following steps describe a common way to debug a WebAssembly module in Wasmtime:
-
Compile your WebAssembly with debug info enabled, usually
-g; for example:clang foo.c -g -o foo.wasm -
Run Wasmtime with the debug info enabled; this is
-gfrom the CLI andConfig::debug_info(true)in an embedding (e.g. see debugging in a Rust embedding) -
Use a supported debugger:
lldb -- wasmtime run -g foo.wasmgdb --args wasmtime run -g foo.wasm
If you run into trouble, the following discussions might help:
- On MacOS with LLDB you may need to run:
settings set plugin.jit-loader.gdb.enable on(#1953) - With LLDB, call
__vmctx.set()to set the current context before calling any dereference operators (#1482):(lldb) p __vmctx->set() (lldb) p *foo
Profiling WebAssembly
One of WebAssembly's major goals is to be quite close to native code in terms of performance, so typically when executing wasm you'll be quite interested in how well your wasm module is performing! From time to time you might want to dive a bit deeper into the performance of your wasm, and this is where profiling comes into the picture.
Profiling support in Wasmtime is still under development, but if you're using either perf or Vtune the examples in these sections are targeted at helping you get some information about the performance of your wasm modules.
Using perf on Linux
One profiler supported by Wasmtime is the perf
profiler for Linux. This is
an extremely powerful profiler with lots of documentation on the web, but for
the rest of this section we'll assume you're running on Linux and already have
perf installed.
Profiling support with perf uses the "jitdump" support in the perf CLI. This
requires runtime support from Wasmtime itself, so you will need to manually
change a few things to enable profiling support in your application. First
you'll want to make sure that Wasmtime is compiled with the jitdump Cargo
feature (which is enabled by default). Otherwise enabling runtime support
depends on how you're using Wasmtime:
-
Rust API - you'll want to call the [
Config::profiler] method withProfilingStrategy::JitDumpto enable profiling of your wasm modules. -
C API - you'll want to call the
wasmtime_config_profiler_setAPI with aWASMTIME_PROFILING_STRATEGY_JITDUMPvalue. -
Command Line - you'll want to pass the
--jitdumpflag on the command line.
Once jitdump support is enabled, you'll use perf record like usual to record
your application's performance. You'll need to also be sure to pass the
--clockid mono or -k mono flag to perf record.
For example if you're using the CLI, you'll execute:
$ perf record -k mono wasmtime --jitdump foo.wasm
This will create a perf.data file as per usual, but it will also create a
jit-XXXX.dump file. This extra *.dump file is the jitdump file which is
specified by perf and Wasmtime generates at runtime.
The next thing you need to do is to merge the *.dump file into the
perf.data file, which you can do with the perf inject command:
$ perf inject --jit --input perf.data --output perf.jit.data
This will read perf.data, automatically pick up the *.dump file that's
correct, and then create perf.jit.data which merges all the JIT information
together. This should also create a lot of jitted-XXXX-N.so files in the
current directory which are ELF images for all the JIT functions that were
created by Wasmtime.
After that you can explore the perf.jit.data profile as you usually would,
for example with:
$ perf report --input perf.jit.data
You should be able to annotate wasm functions and see their raw assembly. You should also see entries for wasm functions show up as one function and the name of each function matches the debug name section in the wasm file.
Note that support for jitdump is still relatively new in Wasmtime, so if you have any problems, please don't hesitate to file an issue!
perf and DWARF information
If the jitdump profile doesn't give you enough information by default, you can
also enable dwarf debug information to be generated for JIT code which should
give the perf profiler more information about what's being profiled. This can
include information like more desriptive function names, filenames, and line
numbers.
Enabling dwarf debug information for JIT code depends on how you're using Wasmtime:
-
Rust API - you'll want to call the
Config::debug_infomethod. -
C API - you'll want to call the
wasmtime_config_debug_info_setAPI. -
Command Line - you'll want to pass the
-gflag on the command line.
You shouldn't need to do anything else to get this information into perf. The
perf collection data should automatically pick up all this dwarf debug
information.
perf example
Let's run through a quick example with perf to get the feel for things. First
let's take a look at some wasm:
fn main() { let n = 42; println!("fib({}) = {}", n, fib(n)); } fn fib(n: u32) -> u32 { if n <= 2 { 1 } else { fib(n - 1) + fib(n - 2) } }
To collect perf information for this wasm module we'll execute:
$ rustc --target wasm32-wasi fib.rs -O
$ perf record -k mono wasmtime --jitdump fib.wasm
fib(42) = 267914296
[ perf record: Woken up 1 times to write data ]
[ perf record: Captured and wrote 0.147 MB perf.data (3435 samples) ]
$ perf inject --jit --input perf.data --output perf.jit.data
And we should have all out information now! We can execute perf report for
example to see that 99% of our runtime (as expected) is spent in our fib
function. Note that the symbol has been demangled to fib::fib which is what
the Rust symbol is:
$ perf report --input perf.jit.data
Alternatively we could also use perf annotate to take a look at the
disassembly of the fib function, seeing what the JIT generated:
$ perf annotate --input perf.jit.data
Using VTune on Linux
VTune Profiler is a popular performance profiling tool that targets both 32-bit and 64-bit x86 architectures. The tool collects profiling data during runtime and then either through command line or gui, provides a variety of options for viewing and doing anaysis on that data. VTune Profiler is available in both commerical and free options. The free download version backed by a community forum for support, is available here. This version is appropriate for detailed analysis of your WASM program. Note for jit support, Wasmtime only supports VTune profiling on linux platforms but other platforms are expected to be enabled in the future.
VTune support in wasmtime is provided through the jit profiling APIs at https://github.com/intel/ittapi. These APIs are provided for code generators (or the runtimes that use them) to report jit activities. These APIs are implemented in a shared library (built from the same ittapi project) which wasmtime pulls in and links to when vtune support is specified through the vtune cargo feature flag. This feature is not enabled by default. When the VTune collector is run, it links to this same shared library to handle profiling request related to the reported jit activities. Specifically, Wasmtime pulls in the ittapi-rs system crate which provides the shared library and Rust interface to the jit profiling APIs.
For jit profiling with VTune Profiler, first you want to make sure the vtune feature is enabled. After that, enabling runtime support is based on how you are using Wasmtime:
-
Rust API - you'll want to call the [
Config::profiler] method withProfilingStrategy::VTuneto enable profiling of your wasm modules. -
C API - you'll want to call the
wasmtime_config_profiler_setAPI with aWASMTIME_PROFILING_STRATEGY_VTUNEvalue. -
Command Line - you'll want to pass the
--vtuneflag on the command line.
After profiling is complete, a results folder will hold profiling data that can then be read and analyzed with VTune.
Also note, VTune is capable of profiling a single process or system wide. As such, and like perf, VTune is plenty capable of profiling the wasmtime runtime itself without any added support. However, APIs here also support an interface for marking the start and stop of code regions for easy isolatation in the VTune Profiler. Support for these APIs are expected to be added in the future.
Take the following example: with VTune properly installed, if you're using the CLI you'll execute with:
$ cargo build --features=vtune
$ amplxe-cl -run-pass-thru=--no-altstack -collect hotspots target/debug/wasmtime --vtune foo.wasm
This command tells the VTune collector (amplxe-cl) to collect hotspot profiling data on wasmtime that is executing foo.wasm. The --vtune flag enables VTune support in wasmtime so that the collector is also alerted to jit events that take place during runtime. The first time this is run, the result of the command is a results diretory r000hs/ which contains hotspot profiling data for wasmtime and the execution of foo.wasm. This data can then be read and displayed via the command line or via the VTune gui by importing the result.
VTune example
Running through a familiar algorithm, first we'll start with the following wasm:
fn main() { let n = 45; println!("fib({}) = {}", n, fib(n)); } fn fib(n: u32) -> u32 { if n <= 2 { 1 } else { fib(n - 1) + fib(n - 2) } }
Profiling data using vtune can be collected a number of ways and profiling data can be collected to focus on certain types of analysis. Below we show a command line executable option using amplxe-cl, which is installed and in our path, to help find hotspots in our wasm module. To collect profiling information then, we'll simply execute:
$ rustc --target wasm32-wasi fib.rs -C opt-level=z -C lto=yes
$ amplxe-cl -run-pass-thru=--no-altstack -v -collect hotspots target/debug/wasmtime --vtune fib.wasm
fib(45) = 1134903170
amplxe: Collection stopped.
amplxe: Using result path /home/jlb6740/wasmtime/r000hs
amplxe: Executing actions 7 % Clearing the database
amplxe: The database has been cleared, elapsed time is 0.239 seconds.
amplxe: Executing actions 14 % Updating precomputed scalar metrics
amplxe: Raw data has been loaded to the database, elapsed time is 0.792 seconds.
amplxe: Executing actions 19 % Processing profile metrics and debug information
...
...
Top Hotspots
Function Module CPU Time
-------------------------------------------------------------------------------------------- -------------- --------
h2bacf53cb3845acf [Dynamic code] 3.480s
__memmove_avx_unaligned_erms libc.so.6 0.222s
cranelift_codegen::ir::instructions::InstructionData::opcode::hee6f5b6a72fc684e wasmtime 0.122s
core::ptr::slice_from_raw_parts::hc5cb6f1b39a0e7a1 wasmtime 0.066s
_$LT$usize$u20$as$u20$core..slice..SliceIndex$LT$$u5b$T$u5d$$GT$$GT$::get::h70c7f142eeeee8bd wasmtime 0.066s
Note again, wasmtime must be built with the vtune feature flag enabled. From here you there are several options for further analysis. Below is an example view of the collected as seen in VTune's gui with it's many options.
For more information on VTune and the analysis tools it provides see the docs here.
Embedding in Rust
This section is intended to showcase the Rust embedding API for Wasmtime. This
is done through the wasmtime crate. In
addition to browsing the following examples you can also browse the full API
documentation.
Hello, world!
You can also browse this source code online and clone the wasmtime repository to run the example locally.
This example shows off how to instantiate a simple wasm module and interact with
it. For more information about the types used here be sure to review the core
concepts of the wasmtime
API as well as the general
API documentation.
hello.wat
(module
(func $hello (import "" "hello"))
(func (export "run") (call $hello))
)
hello.rs
//! Small example of how to instantiate a wasm module that imports one function,
//! showing how you can fill in host functionality for a wasm module.
// You can execute this example with `cargo run --example hello`
use anyhow::Result;
use wasmtime::*;
struct MyState {
name: String,
count: usize,
}
fn main() -> Result<()> {
// First the wasm module needs to be compiled. This is done with a global
// "compilation environment" within an `Engine`. Note that engines can be
// further configured through `Config` if desired instead of using the
// default like this is here.
println!("Compiling module...");
let engine = Engine::default();
let module = Module::from_file(&engine, "examples/hello.wat")?;
// After a module is compiled we create a `Store` which will contain
// instantiated modules and other items like host functions. A Store
// contains an arbitrary piece of host information, and we use `MyState`
// here.
println!("Initializing...");
let mut store = Store::new(
&engine,
MyState {
name: "hello, world!".to_string(),
count: 0,
},
);
// Our wasm module we'll be instantiating requires one imported function.
// the function takes no parameters and returns no results. We create a host
// implementation of that function here, and the `caller` parameter here is
// used to get access to our original `MyState` value.
println!("Creating callback...");
let hello_func = Func::wrap(&mut store, |mut caller: Caller<'_, MyState>| {
println!("Calling back...");
println!("> {}", caller.data().name);
caller.data_mut().count += 1;
});
// Once we've got that all set up we can then move to the instantiation
// phase, pairing together a compiled module as well as a set of imports.
// Note that this is where the wasm `start` function, if any, would run.
println!("Instantiating module...");
let imports = [hello_func.into()];
let instance = Instance::new(&mut store, &module, &imports)?;
// Next we poke around a bit to extract the `run` function from the module.
println!("Extracting export...");
let run = instance.get_typed_func::<(), (), _>(&mut store, "run")?;
// And last but not least we can call it!
println!("Calling export...");
run.call(&mut store, ())?;
println!("Done.");
Ok(())
}
Calculating the GCD
You can also browse this source code online and clone the wasmtime repository to run the example locally.
This example shows off how run a wasm program which calculates the GCD of two numbers.
gcd.wat
(module
(func $gcd (param i32 i32) (result i32)
(local i32)
block ;; label = @1
block ;; label = @2
local.get 0
br_if 0 (;@2;)
local.get 1
local.set 2
br 1 (;@1;)
end
loop ;; label = @2
local.get 1
local.get 0
local.tee 2
i32.rem_u
local.set 0
local.get 2
local.set 1
local.get 0
br_if 0 (;@2;)
end
end
local.get 2
)
(export "gcd" (func $gcd))
)
gcd.rs
//! Example of instantiating of the WebAssembly module and invoking its exported
//! function.
// You can execute this example with `cargo run --example gcd`
use anyhow::Result;
use wasmtime::*;
fn main() -> Result<()> {
// Load our WebAssembly (parsed WAT in our case), and then load it into a
// `Module` which is attached to a `Store` cache. After we've got that we
// can instantiate it.
let mut store = Store::<()>::default();
let module = Module::from_file(store.engine(), "examples/gcd.wat")?;
let instance = Instance::new(&mut store, &module, &[])?;
// Invoke `gcd` export
let gcd = instance.get_typed_func::<(i32, i32), i32, _>(&mut store, "gcd")?;
println!("gcd(6, 27) = {}", gcd.call(&mut store, (6, 27))?);
Ok(())
}
Using linear memory
You can also browse this source code online and clone the wasmtime repository to run the example locally.
This example shows off how to interact with wasm memory in a module. Be sure to
read the documentation for Memory as well.
memory.wat
(module
(memory (export "memory") 2 3)
(func (export "size") (result i32) (memory.size))
(func (export "load") (param i32) (result i32)
(i32.load8_s (local.get 0))
)
(func (export "store") (param i32 i32)
(i32.store8 (local.get 0) (local.get 1))
)
(data (i32.const 0x1000) "\01\02\03\04")
)
memory.rs
//! An example of how to interact with wasm memory.
//!
//! Here a small wasm module is used to show how memory is initialized, how to
//! read and write memory through the `Memory` object, and how wasm functions
//! can trap when dealing with out-of-bounds addresses.
// You can execute this example with `cargo run --example example`
use anyhow::Result;
use wasmtime::*;
fn main() -> Result<()> {
// Create our `store_fn` context and then compile a module and create an
// instance from the compiled module all in one go.
let mut store: Store<()> = Store::default();
let module = Module::from_file(store.engine(), "examples/memory.wat")?;
let instance = Instance::new(&mut store, &module, &[])?;
// load_fn up our exports from the instance
let memory = instance
.get_memory(&mut store, "memory")
.ok_or(anyhow::format_err!("failed to find `memory` export"))?;
let size = instance.get_typed_func::<(), i32, _>(&mut store, "size")?;
let load_fn = instance.get_typed_func::<i32, i32, _>(&mut store, "load")?;
let store_fn = instance.get_typed_func::<(i32, i32), (), _>(&mut store, "store")?;
println!("Checking memory...");
assert_eq!(memory.size(&store), 2);
assert_eq!(memory.data_size(&store), 0x20000);
assert_eq!(memory.data_mut(&mut store)[0], 0);
assert_eq!(memory.data_mut(&mut store)[0x1000], 1);
assert_eq!(memory.data_mut(&mut store)[0x1003], 4);
assert_eq!(size.call(&mut store, ())?, 2);
assert_eq!(load_fn.call(&mut store, 0)?, 0);
assert_eq!(load_fn.call(&mut store, 0x1000)?, 1);
assert_eq!(load_fn.call(&mut store, 0x1003)?, 4);
assert_eq!(load_fn.call(&mut store, 0x1ffff)?, 0);
assert!(load_fn.call(&mut store, 0x20000).is_err()); // out of bounds trap
println!("Mutating memory...");
memory.data_mut(&mut store)[0x1003] = 5;
store_fn.call(&mut store, (0x1002, 6))?;
assert!(store_fn.call(&mut store, (0x20000, 0)).is_err()); // out of bounds trap
assert_eq!(memory.data(&store)[0x1002], 6);
assert_eq!(memory.data(&store)[0x1003], 5);
assert_eq!(load_fn.call(&mut store, 0x1002)?, 6);
assert_eq!(load_fn.call(&mut store, 0x1003)?, 5);
// Grow memory.
println!("Growing memory...");
memory.grow(&mut store, 1)?;
assert_eq!(memory.size(&store), 3);
assert_eq!(memory.data_size(&store), 0x30000);
assert_eq!(load_fn.call(&mut store, 0x20000)?, 0);
store_fn.call(&mut store, (0x20000, 0))?;
assert!(load_fn.call(&mut store, 0x30000).is_err());
assert!(store_fn.call(&mut store, (0x30000, 0)).is_err());
assert!(memory.grow(&mut store, 1).is_err());
assert!(memory.grow(&mut store, 0).is_ok());
println!("Creating stand-alone memory...");
let memorytype = MemoryType::new(Limits::new(5, Some(5)));
let memory2 = Memory::new(&mut store, memorytype)?;
assert_eq!(memory2.size(&store), 5);
assert!(memory2.grow(&mut store, 1).is_err());
assert!(memory2.grow(&mut store, 0).is_ok());
Ok(())
}
WASI
You can also browse this source code online and clone the wasmtime repository to run the example locally.
This example shows how to use the wasmtime-wasi crate to define WASI
functions within a Linker which can then be used to instantiate a
WebAssembly module.
Wasm Source code
fn main() { println!("Hello, world!"); }
wasi.rs
//! Example of instantiating of instantiating a wasm module which uses WASI
//! imports.
// You can execute this example with `cargo run --example wasi`
use anyhow::Result;
use wasmtime::*;
use wasmtime_wasi::sync::WasiCtxBuilder;
fn main() -> Result<()> {
// Define the WASI functions globally on the `Config`.
let engine = Engine::default();
let mut linker = Linker::new(&engine);
wasmtime_wasi::add_to_linker(&mut linker, |s| s)?;
// Create a WASI context and put it in a Store; all instances in the store
// share this context. `WasiCtxBuilder` provides a number of ways to
// configure what the target program will have access to.
let wasi = WasiCtxBuilder::new()
.inherit_stdio()
.inherit_args()?
.build();
let mut store = Store::new(&engine, wasi);
// Instantiate our module with the imports we've created, and run it.
let module = Module::from_file(&engine, "target/wasm32-wasi/debug/wasi.wasm")?;
linker.module(&mut store, "", &module)?;
linker
.get_default(&mut store, "")?
.typed::<(), (), _>(&store)?
.call(&mut store, ())?;
Ok(())
}
WASI state with other custom host state
The add_to_linker takes a second argument which is a closure to access &mut WasiCtx from within the T stored in the Store<T> itself. In the above
example this is trivial because the T in Store<T> is WasiCtx itself, but
you can also store other state in Store like so:
extern crate wasmtime; extern crate wasmtime_wasi; extern crate anyhow; use anyhow::Result; use std::borrow::{Borrow, BorrowMut}; use wasmtime::*; use wasmtime_wasi::{WasiCtx, sync::WasiCtxBuilder}; struct MyState { message: String, wasi: WasiCtx, } fn main() -> Result<()> { let engine = Engine::default(); let mut linker = Linker::new(&engine); wasmtime_wasi::add_to_linker(&mut linker, |state: &mut MyState| &mut state.wasi)?; let wasi = WasiCtxBuilder::new() .inherit_stdio() .inherit_args()? .build(); let mut store = Store::new(&engine, MyState { message: format!("hello!"), wasi, }); // ... let _linker: Linker<MyState> = linker; Ok(()) }
Linking modules
You can also browse this source code online and clone the wasmtime repository to run the example locally.
This example shows off how to compile and instantiate modules which link
together. Be sure to read the API documentation for Linker as well.
linking1.wat
(module
(import "linking2" "double" (func $double (param i32) (result i32)))
(import "linking2" "log" (func $log (param i32 i32)))
(import "linking2" "memory" (memory 1))
(import "linking2" "memory_offset" (global $offset i32))
(func (export "run")
;; Call into the other module to double our number, and we could print it
;; here but for now we just drop it
i32.const 2
call $double
drop
;; Our `data` segment initialized our imported memory, so let's print the
;; string there now.
global.get $offset
i32.const 14
call $log
)
(data (global.get $offset) "Hello, world!\n")
)
linking2.wat
(module
(type $fd_write_ty (func (param i32 i32 i32 i32) (result i32)))
(import "wasi_snapshot_preview1" "fd_write" (func $fd_write (type $fd_write_ty)))
(func (export "double") (param i32) (result i32)
local.get 0
i32.const 2
i32.mul
)
(func (export "log") (param i32 i32)
;; store the pointer in the first iovec field
i32.const 4
local.get 0
i32.store
;; store the length in the first iovec field
i32.const 4
local.get 1
i32.store offset=4
;; call the `fd_write` import
i32.const 1 ;; stdout fd
i32.const 4 ;; iovs start
i32.const 1 ;; number of iovs
i32.const 0 ;; where to write nwritten bytes
call $fd_write
drop
)
(memory (export "memory") 2)
(global (export "memory_offset") i32 (i32.const 65536))
)
linking.rs
//! Example of instantiating two modules which link to each other.
// You can execute this example with `cargo run --example linking`
use anyhow::Result;
use wasmtime::*;
use wasmtime_wasi::sync::WasiCtxBuilder;
fn main() -> Result<()> {
let engine = Engine::default();
// First set up our linker which is going to be linking modules together. We
// want our linker to have wasi available, so we set that up here as well.
let mut linker = Linker::new(&engine);
wasmtime_wasi::add_to_linker(&mut linker, |s| s)?;
// Load and compile our two modules
let linking1 = Module::from_file(&engine, "examples/linking1.wat")?;
let linking2 = Module::from_file(&engine, "examples/linking2.wat")?;
// Configure WASI and insert it into a `Store`
let wasi = WasiCtxBuilder::new()
.inherit_stdio()
.inherit_args()?
.build();
let mut store = Store::new(&engine, wasi);
// Instantiate our first module which only uses WASI, then register that
// instance with the linker since the next linking will use it.
let linking2 = linker.instantiate(&mut store, &linking2)?;
linker.instance(&mut store, "linking2", linking2)?;
// And with that we can perform the final link and the execute the module.
let linking1 = linker.instantiate(&mut store, &linking1)?;
let run = linking1.get_typed_func::<(), (), _>(&mut store, "run")?;
run.call(&mut store, ())?;
Ok(())
}
Debugging
You can also browse this source code online and clone the wasmtime repository to run the example locally.
This example shows off how to set up a module for dynamic runtime debugging via a native debugger like GDB or LLDB.
main.rs
//! Example of enabling debuginfo for wasm code which allows interactive
//! debugging of the wasm code. When using recent versions of LLDB
//! you can debug this executable and set breakpoints in wasm code and look at
//! the rust source code as input.
// To execute this example you'll need to run two commands:
//
// cargo build -p example-fib-debug-wasm --target wasm32-unknown-unknown
// cargo run --example fib-debug
use anyhow::Result;
use wasmtime::*;
fn main() -> Result<()> {
// Load our previously compiled wasm file (built previously with Cargo) and
// also ensure that we generate debuginfo so this executable can be
// debugged in GDB.
let engine = Engine::new(Config::new().debug_info(true))?;
let mut store = Store::new(&engine, ());
let module = Module::from_file(&engine, "target/wasm32-unknown-unknown/debug/fib.wasm")?;
let instance = Instance::new(&mut store, &module, &[])?;
// Invoke `fib` export
let fib = instance.get_typed_func::<i32, i32, _>(&mut store, "fib")?;
println!("fib(6) = {}", fib.call(&mut store, 6)?);
Ok(())
}
Using multi-value
You can also browse this source code online and clone the wasmtime repository to run the example locally.
This example shows off how to interact with a wasm module that uses multi-value exports and imports.
multi.wat
(module
(func $f (import "" "f") (param i32 i64) (result i64 i32))
(func $g (export "g") (param i32 i64) (result i64 i32)
(call $f (local.get 0) (local.get 1))
)
(func $round_trip_many
(export "round_trip_many")
(param i64 i64 i64 i64 i64 i64 i64 i64 i64 i64)
(result i64 i64 i64 i64 i64 i64 i64 i64 i64 i64)
local.get 0
local.get 1
local.get 2
local.get 3
local.get 4
local.get 5
local.get 6
local.get 7
local.get 8
local.get 9)
)
multi.rs
//! This is an example of working with multi-value modules and dealing with
//! multi-value functions.
//!
//! Note that the `Func::wrap*` interfaces cannot be used to return multiple
//! values just yet, so we need to use the more dynamic `Func::new` and
//! `Func::call` methods.
// You can execute this example with `cargo run --example multi`
use anyhow::Result;
#[cfg(not(feature = "old-x86-backend"))]
fn main() -> Result<()> {
use wasmtime::*;
println!("Initializing...");
let engine = Engine::default();
let mut store = Store::new(&engine, ());
// Compile.
println!("Compiling module...");
let module = Module::from_file(&engine, "examples/multi.wat")?;
// Create a host function which takes multiple parameters and returns
// multiple results.
println!("Creating callback...");
let callback_func = Func::wrap(&mut store, |a: i32, b: i64| -> (i64, i32) {
(b + 1, a + 1)
});
// Instantiate.
println!("Instantiating module...");
let instance = Instance::new(&mut store, &module, &[callback_func.into()])?;
// Extract exports.
println!("Extracting export...");
let g = instance.get_typed_func::<(i32, i64), (i64, i32), _>(&mut store, "g")?;
// Call `$g`.
println!("Calling export \"g\"...");
let (a, b) = g.call(&mut store, (1, 3))?;
println!("Printing result...");
println!("> {} {}", a, b);
assert_eq!(a, 4);
assert_eq!(b, 2);
// Call `$round_trip_many`.
println!("Calling export \"round_trip_many\"...");
let round_trip_many = instance
.get_typed_func::<
(i64, i64, i64, i64, i64, i64, i64, i64, i64, i64),
(i64, i64, i64, i64, i64, i64, i64, i64, i64, i64),
_,
>
(&mut store, "round_trip_many")?;
let results = round_trip_many.call(&mut store, (0, 1, 2, 3, 4, 5, 6, 7, 8, 9))?;
println!("Printing result...");
println!("> {:?}", results);
assert_eq!(results, (0, 1, 2, 3, 4, 5, 6, 7, 8, 9));
Ok(())
}
// Note that this example is not supported in the off-by-default feature of the
// old x86 compiler backend for Cranelift. Wasmtime's default configuration
// supports this example, however.
#[cfg(feature = "old-x86-backend")]
fn main() -> Result<()> {
Ok(())
}
Embedding in C
This section is intended to showcase the C embedding API for Wasmtime. Full reference documentation for the C API can be found online
Hello, world!
You can also browse this source code online and clone the wasmtime repository to run the example locally.
This example shows off how to instantiate a simple wasm module and interact with it.
hello.wat
(module
(func $hello (import "" "hello"))
(func (export "run") (call $hello))
)
hello.c
/*
Example of instantiating of the WebAssembly module and invoking its exported
function.
You can compile and run this example on Linux with:
cargo build --release -p wasmtime-c-api
cc examples/hello.c \
-I crates/c-api/include \
-I crates/c-api/wasm-c-api/include \
target/release/libwasmtime.a \
-lpthread -ldl -lm \
-o hello
./hello
Note that on Windows and macOS the command will be similar, but you'll need
to tweak the `-lpthread` and such annotations as well as the name of the
`libwasmtime.a` file on Windows.
*/
#include <assert.h>
#include <stdio.h>
#include <stdlib.h>
#include <wasm.h>
#include <wasmtime.h>
static void exit_with_error(const char *message, wasmtime_error_t *error, wasm_trap_t *trap);
static wasm_trap_t* hello_callback(
void *env,
wasmtime_caller_t *caller,
const wasmtime_val_t *args,
size_t nargs,
wasmtime_val_t *results,
size_t nresults
) {
printf("Calling back...\n");
printf("> Hello World!\n");
return NULL;
}
int main() {
int ret = 0;
// Set up our compilation context. Note that we could also work with a
// `wasm_config_t` here to configure what feature are enabled and various
// compilation settings.
printf("Initializing...\n");
wasm_engine_t *engine = wasm_engine_new();
assert(engine != NULL);
// With an engine we can create a *store* which is a long-lived group of wasm
// modules. Note that we allocate some custom data here to live in the store,
// but here we skip that and specify NULL.
wasmtime_store_t *store = wasmtime_store_new(engine, NULL, NULL);
assert(store != NULL);
wasmtime_context_t *context = wasmtime_store_context(store);
// Read our input file, which in this case is a wasm text file.
FILE* file = fopen("examples/hello.wat", "r");
assert(file != NULL);
fseek(file, 0L, SEEK_END);
size_t file_size = ftell(file);
fseek(file, 0L, SEEK_SET);
wasm_byte_vec_t wat;
wasm_byte_vec_new_uninitialized(&wat, file_size);
assert(fread(wat.data, file_size, 1, file) == 1);
fclose(file);
// Parse the wat into the binary wasm format
wasm_byte_vec_t wasm;
wasmtime_error_t *error = wasmtime_wat2wasm(wat.data, wat.size, &wasm);
if (error != NULL)
exit_with_error("failed to parse wat", error, NULL);
wasm_byte_vec_delete(&wat);
// Now that we've got our binary webassembly we can compile our module.
printf("Compiling module...\n");
wasmtime_module_t *module = NULL;
error = wasmtime_module_new(engine, (uint8_t*) wasm.data, wasm.size, &module);
wasm_byte_vec_delete(&wasm);
if (error != NULL)
exit_with_error("failed to compile module", error, NULL);
// Next up we need to create the function that the wasm module imports. Here
// we'll be hooking up a thunk function to the `hello_callback` native
// function above. Note that we can assign custom data, but we just use NULL
// for now).
printf("Creating callback...\n");
wasm_functype_t *hello_ty = wasm_functype_new_0_0();
wasmtime_func_t hello;
wasmtime_func_new(context, hello_ty, hello_callback, NULL, NULL, &hello);
// With our callback function we can now instantiate the compiled module,
// giving us an instance we can then execute exports from. Note that
// instantiation can trap due to execution of the `start` function, so we need
// to handle that here too.
printf("Instantiating module...\n");
wasm_trap_t *trap = NULL;
wasmtime_instance_t instance;
wasmtime_extern_t import;
import.kind = WASMTIME_EXTERN_FUNC;
import.of.func = hello;
error = wasmtime_instance_new(context, module, &import, 1, &instance, &trap);
if (error != NULL || trap != NULL)
exit_with_error("failed to instantiate", error, trap);
// Lookup our `run` export function
printf("Extracting export...\n");
wasmtime_extern_t run;
bool ok = wasmtime_instance_export_get(context, &instance, "run", 3, &run);
assert(ok);
assert(run.kind == WASMTIME_EXTERN_FUNC);
// And call it!
printf("Calling export...\n");
error = wasmtime_func_call(context, &run.of.func, NULL, 0, NULL, 0, &trap);
if (error != NULL || trap != NULL)
exit_with_error("failed to call function", error, trap);
// Clean up after ourselves at this point
printf("All finished!\n");
ret = 0;
wasmtime_module_delete(module);
wasmtime_store_delete(store);
wasm_engine_delete(engine);
return ret;
}
static void exit_with_error(const char *message, wasmtime_error_t *error, wasm_trap_t *trap) {
fprintf(stderr, "error: %s\n", message);
wasm_byte_vec_t error_message;
if (error != NULL) {
wasmtime_error_message(error, &error_message);
wasmtime_error_delete(error);
} else {
wasm_trap_message(trap, &error_message);
wasm_trap_delete(trap);
}
fprintf(stderr, "%.*s\n", (int) error_message.size, error_message.data);
wasm_byte_vec_delete(&error_message);
exit(1);
}
Calculating the GCD
You can also browse this source code online and clone the wasmtime repository to run the example locally.
This example shows off how run a wasm program which calculates the GCD of two numbers.
gcd.wat
(module
(func $gcd (param i32 i32) (result i32)
(local i32)
block ;; label = @1
block ;; label = @2
local.get 0
br_if 0 (;@2;)
local.get 1
local.set 2
br 1 (;@1;)
end
loop ;; label = @2
local.get 1
local.get 0
local.tee 2
i32.rem_u
local.set 0
local.get 2
local.set 1
local.get 0
br_if 0 (;@2;)
end
end
local.get 2
)
(export "gcd" (func $gcd))
)
gcd.c
/*
Example of instantiating of the WebAssembly module and invoking its exported
function.
You can compile and run this example on Linux with:
cargo build --release -p wasmtime-c-api
cc examples/gcd.c \
-I crates/c-api/include \
-I crates/c-api/wasm-c-api/include \
target/release/libwasmtime.a \
-lpthread -ldl -lm \
-o gcd
./gcd
Note that on Windows and macOS the command will be similar, but you'll need
to tweak the `-lpthread` and such annotations.
*/
#include <assert.h>
#include <stdio.h>
#include <stdlib.h>
#include <wasm.h>
#include <wasmtime.h>
static void exit_with_error(const char *message, wasmtime_error_t *error, wasm_trap_t *trap);
int main() {
int ret = 0;
// Set up our context
wasm_engine_t *engine = wasm_engine_new();
assert(engine != NULL);
wasmtime_store_t *store = wasmtime_store_new(engine, NULL, NULL);
assert(store != NULL);
wasmtime_context_t *context = wasmtime_store_context(store);
// Load our input file to parse it next
FILE* file = fopen("examples/gcd.wat", "r");
if (!file) {
printf("> Error loading file!\n");
return 1;
}
fseek(file, 0L, SEEK_END);
size_t file_size = ftell(file);
fseek(file, 0L, SEEK_SET);
wasm_byte_vec_t wat;
wasm_byte_vec_new_uninitialized(&wat, file_size);
if (fread(wat.data, file_size, 1, file) != 1) {
printf("> Error loading module!\n");
return 1;
}
fclose(file);
// Parse the wat into the binary wasm format
wasm_byte_vec_t wasm;
wasmtime_error_t *error = wasmtime_wat2wasm(wat.data, wat.size, &wasm);
if (error != NULL)
exit_with_error("failed to parse wat", error, NULL);
wasm_byte_vec_delete(&wat);
// Compile and instantiate our module
wasmtime_module_t *module = NULL;
error = wasmtime_module_new(engine, (uint8_t*) wasm.data, wasm.size, &module);
if (module == NULL)
exit_with_error("failed to compile module", error, NULL);
wasm_byte_vec_delete(&wasm);
wasm_trap_t *trap = NULL;
wasmtime_instance_t instance;
error = wasmtime_instance_new(context, module, NULL, 0, &instance, &trap);
if (error != NULL || trap != NULL)
exit_with_error("failed to instantiate", error, trap);
// Lookup our `gcd` export function
wasmtime_extern_t gcd;
bool ok = wasmtime_instance_export_get(context, &instance, "gcd", 3, &gcd);
assert(ok);
assert(gcd.kind == WASMTIME_EXTERN_FUNC);
// And call it!
int a = 6;
int b = 27;
wasmtime_val_t params[2];
params[0].kind = WASMTIME_I32;
params[0].of.i32 = a;
params[1].kind = WASMTIME_I32;
params[1].of.i32 = b;
wasmtime_val_t results[1];
error = wasmtime_func_call(context, &gcd.of.func, params, 2, results, 1, &trap);
if (error != NULL || trap != NULL)
exit_with_error("failed to call gcd", error, trap);
assert(results[0].kind == WASMTIME_I32);
printf("gcd(%d, %d) = %d\n", a, b, results[0].of.i32);
// Clean up after ourselves at this point
ret = 0;
wasmtime_module_delete(module);
wasmtime_store_delete(store);
wasm_engine_delete(engine);
return ret;
}
static void exit_with_error(const char *message, wasmtime_error_t *error, wasm_trap_t *trap) {
fprintf(stderr, "error: %s\n", message);
wasm_byte_vec_t error_message;
if (error != NULL) {
wasmtime_error_message(error, &error_message);
} else {
wasm_trap_message(trap, &error_message);
}
fprintf(stderr, "%.*s\n", (int) error_message.size, error_message.data);
wasm_byte_vec_delete(&error_message);
exit(1);
}
Using linear memory
You can also browse this source code online and clone the wasmtime repository to run the example locally.
This example shows off how to interact with wasm memory in a module. Be sure to
read the documentation for Memory as well.
memory.wat
(module
(memory (export "memory") 2 3)
(func (export "size") (result i32) (memory.size))
(func (export "load") (param i32) (result i32)
(i32.load8_s (local.get 0))
)
(func (export "store") (param i32 i32)
(i32.store8 (local.get 0) (local.get 1))
)
(data (i32.const 0x1000) "\01\02\03\04")
)
memory.c
/*
Example of instantiating of the WebAssembly module and invoking its exported
function.
You can compile and run this example on Linux with:
cargo build --release -p wasmtime-c-api
cc examples/memory.c \
-I crates/c-api/include \
-I crates/c-api/wasm-c-api/include \
target/release/libwasmtime.a \
-lpthread -ldl -lm \
-o memory
./memory
Note that on Windows and macOS the command will be similar, but you'll need
to tweak the `-lpthread` and such annotations.
Also note that this example was taken from
https://github.com/WebAssembly/wasm-c-api/blob/master/example/memory.c
originally
*/
#include <inttypes.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <wasm.h>
#include <wasmtime.h>
static void exit_with_error(const char *message, wasmtime_error_t *error, wasm_trap_t *trap);
void check(bool success) {
if (!success) {
printf("> Error, expected success\n");
exit(1);
}
}
void check_call(wasmtime_context_t *store,
wasmtime_func_t *func,
const wasmtime_val_t* args,
size_t nargs,
int32_t expected) {
wasmtime_val_t results[1];
wasm_trap_t *trap = NULL;
wasmtime_error_t *error = wasmtime_func_call(
store, func, args, nargs, results, 1, &trap
);
if (error != NULL || trap != NULL)
exit_with_error("failed to call function", error, trap);
if (results[0].of.i32 != expected) {
printf("> Error on result\n");
exit(1);
}
}
void check_call0(wasmtime_context_t *store, wasmtime_func_t *func, int32_t expected) {
check_call(store, func, NULL, 0, expected);
}
void check_call1(wasmtime_context_t *store, wasmtime_func_t *func, int32_t arg, int32_t expected) {
wasmtime_val_t args[1];
args[0].kind = WASMTIME_I32;
args[0].of.i32 = arg;
check_call(store, func, args, 1, expected);
}
void check_call2(wasmtime_context_t *store, wasmtime_func_t *func, int32_t arg1, int32_t arg2, int32_t expected) {
wasmtime_val_t args[2];
args[0].kind = WASMTIME_I32;
args[0].of.i32 = arg1;
args[1].kind = WASMTIME_I32;
args[1].of.i32 = arg2;
check_call(store, func, args, 2, expected);
}
void check_ok(wasmtime_context_t *store, wasmtime_func_t *func, const wasmtime_val_t* args, size_t nargs) {
wasm_trap_t *trap = NULL;
wasmtime_error_t *error = wasmtime_func_call(store, func, args, nargs, NULL, 0, &trap);
if (error != NULL || trap != NULL)
exit_with_error("failed to call function", error, trap);
}
void check_ok2(wasmtime_context_t *store, wasmtime_func_t *func, int32_t arg1, int32_t arg2) {
wasmtime_val_t args[2];
args[0].kind = WASMTIME_I32;
args[0].of.i32 = arg1;
args[1].kind = WASMTIME_I32;
args[1].of.i32 = arg2;
check_ok(store, func, args, 2);
}
void check_trap(wasmtime_context_t *store,
wasmtime_func_t *func,
const wasmtime_val_t *args,
size_t nargs,
size_t num_results) {
assert(num_results <= 1);
wasmtime_val_t results[1];
wasm_trap_t *trap = NULL;
wasmtime_error_t *error = wasmtime_func_call(store, func, args, nargs, results, num_results, &trap);
if (error != NULL)
exit_with_error("failed to call function", error, NULL);
if (trap == NULL) {
printf("> Error on result, expected trap\n");
exit(1);
}
wasm_trap_delete(trap);
}
void check_trap1(wasmtime_context_t *store, wasmtime_func_t *func, int32_t arg) {
wasmtime_val_t args[1];
args[0].kind = WASMTIME_I32;
args[0].of.i32 = arg;
check_trap(store, func, args, 1, 1);
}
void check_trap2(wasmtime_context_t *store, wasmtime_func_t *func, int32_t arg1, int32_t arg2) {
wasmtime_val_t args[2];
args[0].kind = WASMTIME_I32;
args[0].of.i32 = arg1;
args[1].kind = WASMTIME_I32;
args[1].of.i32 = arg2;
check_trap(store, func, args, 2, 0);
}
int main(int argc, const char* argv[]) {
// Initialize.
printf("Initializing...\n");
wasm_engine_t* engine = wasm_engine_new();
wasmtime_store_t* store = wasmtime_store_new(engine, NULL, NULL);
wasmtime_context_t *context = wasmtime_store_context(store);
// Load our input file to parse it next
FILE* file = fopen("examples/memory.wat", "r");
if (!file) {
printf("> Error loading file!\n");
return 1;
}
fseek(file, 0L, SEEK_END);
size_t file_size = ftell(file);
fseek(file, 0L, SEEK_SET);
wasm_byte_vec_t wat;
wasm_byte_vec_new_uninitialized(&wat, file_size);
if (fread(wat.data, file_size, 1, file) != 1) {
printf("> Error loading module!\n");
return 1;
}
fclose(file);
// Parse the wat into the binary wasm format
wasm_byte_vec_t binary;
wasmtime_error_t *error = wasmtime_wat2wasm(wat.data, wat.size, &binary);
if (error != NULL)
exit_with_error("failed to parse wat", error, NULL);
wasm_byte_vec_delete(&wat);
// Compile.
printf("Compiling module...\n");
wasmtime_module_t* module = NULL;
error = wasmtime_module_new(engine, (uint8_t*) binary.data, binary.size, &module);
if (error)
exit_with_error("failed to compile module", error, NULL);
wasm_byte_vec_delete(&binary);
// Instantiate.
printf("Instantiating module...\n");
wasmtime_instance_t instance;
wasm_trap_t *trap = NULL;
error = wasmtime_instance_new(context, module, NULL, 0, &instance, &trap);
if (error != NULL || trap != NULL)
exit_with_error("failed to instantiate", error, trap);
wasmtime_module_delete(module);
// Extract export.
printf("Extracting exports...\n");
wasmtime_memory_t memory;
wasmtime_func_t size_func, load_func, store_func;
wasmtime_extern_t item;
bool ok;
ok = wasmtime_instance_export_get(context, &instance, "memory", strlen("memory"), &item);
assert(ok && item.kind == WASMTIME_EXTERN_MEMORY);
memory = item.of.memory;
ok = wasmtime_instance_export_get(context, &instance, "size", strlen("size"), &item);
assert(ok && item.kind == WASMTIME_EXTERN_FUNC);
size_func = item.of.func;
ok = wasmtime_instance_export_get(context, &instance, "load", strlen("load"), &item);
assert(ok && item.kind == WASMTIME_EXTERN_FUNC);
load_func = item.of.func;
ok = wasmtime_instance_export_get(context, &instance, "store", strlen("store"), &item);
assert(ok && item.kind == WASMTIME_EXTERN_FUNC);
store_func = item.of.func;
// Check initial memory.
printf("Checking memory...\n");
check(wasmtime_memory_size(context, &memory) == 2);
check(wasmtime_memory_data_size(context, &memory) == 0x20000);
check(wasmtime_memory_data(context, &memory)[0] == 0);
check(wasmtime_memory_data(context, &memory)[0x1000] == 1);
check(wasmtime_memory_data(context, &memory)[0x1003] == 4);
check_call0(context, &size_func, 2);
check_call1(context, &load_func, 0, 0);
check_call1(context, &load_func, 0x1000, 1);
check_call1(context, &load_func, 0x1003, 4);
check_call1(context, &load_func, 0x1ffff, 0);
check_trap1(context, &load_func, 0x20000);
// Mutate memory.
printf("Mutating memory...\n");
wasmtime_memory_data(context, &memory)[0x1003] = 5;
check_ok2(context, &store_func, 0x1002, 6);
check_trap2(context, &store_func, 0x20000, 0);
check(wasmtime_memory_data(context, &memory)[0x1002] == 6);
check(wasmtime_memory_data(context, &memory)[0x1003] == 5);
check_call1(context, &load_func, 0x1002, 6);
check_call1(context, &load_func, 0x1003, 5);
// Grow memory.
printf("Growing memory...\n");
uint32_t old_size;
error = wasmtime_memory_grow(context, &memory, 1, &old_size);
if (error != NULL)
exit_with_error("failed to grow memory", error, trap);
check(wasmtime_memory_size(context, &memory) == 3);
check(wasmtime_memory_data_size(context, &memory) == 0x30000);
check_call1(context, &load_func, 0x20000, 0);
check_ok2(context, &store_func, 0x20000, 0);
check_trap1(context, &load_func, 0x30000);
check_trap2(context, &store_func, 0x30000, 0);
error = wasmtime_memory_grow(context, &memory, 1, &old_size);
assert(error != NULL);
wasmtime_error_delete(error);
error = wasmtime_memory_grow(context, &memory, 0, &old_size);
if (error != NULL)
exit_with_error("failed to grow memory", error, trap);
// Create stand-alone memory.
printf("Creating stand-alone memory...\n");
wasm_limits_t limits = {5, 5};
wasm_memorytype_t* memorytype = wasm_memorytype_new(&limits);
wasmtime_memory_t memory2;
error = wasmtime_memory_new(context, memorytype, &memory2);
if (error != NULL)
exit_with_error("failed to create memory", error, trap);
wasm_memorytype_delete(memorytype);
check(wasmtime_memory_size(context, &memory2) == 5);
// Shut down.
printf("Shutting down...\n");
wasmtime_store_delete(store);
wasm_engine_delete(engine);
// All done.
printf("Done.\n");
return 0;
}
static void exit_with_error(const char *message, wasmtime_error_t *error, wasm_trap_t *trap) {
fprintf(stderr, "error: %s\n", message);
wasm_byte_vec_t error_message;
if (error != NULL) {
wasmtime_error_message(error, &error_message);
wasmtime_error_delete(error);
} else {
wasm_trap_message(trap, &error_message);
wasm_trap_delete(trap);
}
fprintf(stderr, "%.*s\n", (int) error_message.size, error_message.data);
wasm_byte_vec_delete(&error_message);
exit(1);
}
WASI
You can also browse this source code online and clone the wasmtime repository to run the example locally.
This example shows off how to instantiate a wasm module using WASI imports.
Wasm Source code
fn main() {
println!("Hello, world!");
}
wasi.c
/*
Example of instantiating a WebAssembly which uses WASI imports.
You can compile and run this example on Linux with:
cargo build --release -p wasmtime-c-api
cc examples/wasi/main.c \
-I crates/c-api/include \
-I crates/c-api/wasm-c-api/include \
target/release/libwasmtime.a \
-lpthread -ldl -lm \
-o wasi
./wasi
Note that on Windows and macOS the command will be similar, but you'll need
to tweak the `-lpthread` and such annotations.
*/
#include <assert.h>
#include <stdio.h>
#include <stdlib.h>
#include <wasm.h>
#include <wasi.h>
#include <wasmtime.h>
#define MIN(a, b) ((a) < (b) ? (a) : (b))
static void exit_with_error(const char *message, wasmtime_error_t *error, wasm_trap_t *trap);
int main() {
// Set up our context
wasm_engine_t *engine = wasm_engine_new();
assert(engine != NULL);
wasmtime_store_t *store = wasmtime_store_new(engine, NULL, NULL);
assert(store != NULL);
wasmtime_context_t *context = wasmtime_store_context(store);
// Create a linker with WASI functions defined
wasmtime_linker_t *linker = wasmtime_linker_new(engine);
wasmtime_error_t *error = wasmtime_linker_define_wasi(linker);
if (error != NULL)
exit_with_error("failed to link wasi", error, NULL);
wasm_byte_vec_t wasm;
// Load our input file to parse it next
FILE* file = fopen("target/wasm32-wasi/debug/wasi.wasm", "rb");
if (!file) {
printf("> Error loading file!\n");
exit(1);
}
fseek(file, 0L, SEEK_END);
size_t file_size = ftell(file);
wasm_byte_vec_new_uninitialized(&wasm, file_size);
fseek(file, 0L, SEEK_SET);
if (fread(wasm.data, file_size, 1, file) != 1) {
printf("> Error loading module!\n");
exit(1);
}
fclose(file);
// Compile our modules
wasmtime_module_t *module = NULL;
error = wasmtime_module_new(engine, (uint8_t*)wasm.data, wasm.size, &module);
if (!module)
exit_with_error("failed to compile module", error, NULL);
wasm_byte_vec_delete(&wasm);
// Instantiate wasi
wasi_config_t *wasi_config = wasi_config_new();
assert(wasi_config);
wasi_config_inherit_argv(wasi_config);
wasi_config_inherit_env(wasi_config);
wasi_config_inherit_stdin(wasi_config);
wasi_config_inherit_stdout(wasi_config);
wasi_config_inherit_stderr(wasi_config);
wasm_trap_t *trap = NULL;
error = wasmtime_context_set_wasi(context, wasi_config);
if (error != NULL)
exit_with_error("failed to instantiate WASI", error, NULL);
// Instantiate the module
error = wasmtime_linker_module(linker, context, "", 0, module);
if (error != NULL)
exit_with_error("failed to instantiate module", error, NULL);
// Run it.
wasmtime_func_t func;
error = wasmtime_linker_get_default(linker, context, "", 0, &func);
if (error != NULL)
exit_with_error("failed to locate default export for module", error, NULL);
error = wasmtime_func_call(context, &func, NULL, 0, NULL, 0, &trap);
if (error != NULL || trap != NULL)
exit_with_error("error calling default export", error, trap);
// Clean up after ourselves at this point
wasmtime_module_delete(module);
wasmtime_store_delete(store);
wasm_engine_delete(engine);
return 0;
}
static void exit_with_error(const char *message, wasmtime_error_t *error, wasm_trap_t *trap) {
fprintf(stderr, "error: %s\n", message);
wasm_byte_vec_t error_message;
if (error != NULL) {
wasmtime_error_message(error, &error_message);
wasmtime_error_delete(error);
} else {
wasm_trap_message(trap, &error_message);
wasm_trap_delete(trap);
}
fprintf(stderr, "%.*s\n", (int) error_message.size, error_message.data);
wasm_byte_vec_delete(&error_message);
exit(1);
}
Linking modules
You can also browse this source code online and clone the wasmtime repository to run the example locally.
This example shows off how to compile and instantiate modules which link together.
linking1.wat
(module
(import "linking2" "double" (func $double (param i32) (result i32)))
(import "linking2" "log" (func $log (param i32 i32)))
(import "linking2" "memory" (memory 1))
(import "linking2" "memory_offset" (global $offset i32))
(func (export "run")
;; Call into the other module to double our number, and we could print it
;; here but for now we just drop it
i32.const 2
call $double
drop
;; Our `data` segment initialized our imported memory, so let's print the
;; string there now.
global.get $offset
i32.const 14
call $log
)
(data (global.get $offset) "Hello, world!\n")
)
linking2.wat
(module
(type $fd_write_ty (func (param i32 i32 i32 i32) (result i32)))
(import "wasi_snapshot_preview1" "fd_write" (func $fd_write (type $fd_write_ty)))
(func (export "double") (param i32) (result i32)
local.get 0
i32.const 2
i32.mul
)
(func (export "log") (param i32 i32)
;; store the pointer in the first iovec field
i32.const 4
local.get 0
i32.store
;; store the length in the first iovec field
i32.const 4
local.get 1
i32.store offset=4
;; call the `fd_write` import
i32.const 1 ;; stdout fd
i32.const 4 ;; iovs start
i32.const 1 ;; number of iovs
i32.const 0 ;; where to write nwritten bytes
call $fd_write
drop
)
(memory (export "memory") 2)
(global (export "memory_offset") i32 (i32.const 65536))
)
linking.c
/*
Example of compiling, instantiating, and linking two WebAssembly modules
together.
You can compile and run this example on Linux with:
cargo build --release -p wasmtime-c-api
cc examples/linking.c \
-I crates/c-api/include \
-I crates/c-api/wasm-c-api/include \
target/release/libwasmtime.a \
-lpthread -ldl -lm \
-o linking
./linking
Note that on Windows and macOS the command will be similar, but you'll need
to tweak the `-lpthread` and such annotations.
*/
#include <assert.h>
#include <stdio.h>
#include <stdlib.h>
#include <wasm.h>
#include <wasi.h>
#include <wasmtime.h>
#define MIN(a, b) ((a) < (b) ? (a) : (b))
static void exit_with_error(const char *message, wasmtime_error_t *error, wasm_trap_t *trap);
static void read_wat_file(wasm_engine_t *engine, wasm_byte_vec_t *bytes, const char *file);
int main() {
// Set up our context
wasm_engine_t *engine = wasm_engine_new();
assert(engine != NULL);
wasmtime_store_t *store = wasmtime_store_new(engine, NULL, NULL);
assert(store != NULL);
wasmtime_context_t *context = wasmtime_store_context(store);
wasm_byte_vec_t linking1_wasm, linking2_wasm;
read_wat_file(engine, &linking1_wasm, "examples/linking1.wat");
read_wat_file(engine, &linking2_wasm, "examples/linking2.wat");
// Compile our two modules
wasmtime_error_t *error;
wasmtime_module_t *linking1_module = NULL;
wasmtime_module_t *linking2_module = NULL;
error = wasmtime_module_new(engine, (uint8_t*) linking1_wasm.data, linking1_wasm.size, &linking1_module);
if (error != NULL)
exit_with_error("failed to compile linking1", error, NULL);
error = wasmtime_module_new(engine, (uint8_t*) linking2_wasm.data, linking2_wasm.size, &linking2_module);
if (error != NULL)
exit_with_error("failed to compile linking2", error, NULL);
wasm_byte_vec_delete(&linking1_wasm);
wasm_byte_vec_delete(&linking2_wasm);
// Configure WASI and store it within our `wasmtime_store_t`
wasi_config_t *wasi_config = wasi_config_new();
assert(wasi_config);
wasi_config_inherit_argv(wasi_config);
wasi_config_inherit_env(wasi_config);
wasi_config_inherit_stdin(wasi_config);
wasi_config_inherit_stdout(wasi_config);
wasi_config_inherit_stderr(wasi_config);
wasm_trap_t *trap = NULL;
error = wasmtime_context_set_wasi(context, wasi_config);
if (error != NULL)
exit_with_error("failed to instantiate wasi", NULL, trap);
// Create our linker which will be linking our modules together, and then add
// our WASI instance to it.
wasmtime_linker_t *linker = wasmtime_linker_new(engine);
error = wasmtime_linker_define_wasi(linker);
if (error != NULL)
exit_with_error("failed to link wasi", error, NULL);
// Instantiate `linking2` with our linker.
wasmtime_instance_t linking2;
error = wasmtime_linker_instantiate(linker, context, linking2_module, &linking2, &trap);
if (error != NULL || trap != NULL)
exit_with_error("failed to instantiate linking2", error, trap);
// Register our new `linking2` instance with the linker
error = wasmtime_linker_define_instance(linker, context, "linking2", strlen("linking2"), &linking2);
if (error != NULL)
exit_with_error("failed to link linking2", error, NULL);
// Instantiate `linking1` with the linker now that `linking2` is defined
wasmtime_instance_t linking1;
error = wasmtime_linker_instantiate(linker, context, linking1_module, &linking1, &trap);
if (error != NULL || trap != NULL)
exit_with_error("failed to instantiate linking1", error, trap);
// Lookup our `run` export function
wasmtime_extern_t run;
bool ok = wasmtime_instance_export_get(context, &linking1, "run", 3, &run);
assert(ok);
assert(run.kind == WASMTIME_EXTERN_FUNC);
error = wasmtime_func_call(context, &run.of.func, NULL, 0, NULL, 0, &trap);
if (error != NULL || trap != NULL)
exit_with_error("failed to call run", error, trap);
// Clean up after ourselves at this point
wasmtime_linker_delete(linker);
wasmtime_module_delete(linking1_module);
wasmtime_module_delete(linking2_module);
wasmtime_store_delete(store);
wasm_engine_delete(engine);
return 0;
}
static void read_wat_file(
wasm_engine_t *engine,
wasm_byte_vec_t *bytes,
const char *filename
) {
wasm_byte_vec_t wat;
// Load our input file to parse it next
FILE* file = fopen(filename, "r");
if (!file) {
printf("> Error loading file!\n");
exit(1);
}
fseek(file, 0L, SEEK_END);
size_t file_size = ftell(file);
wasm_byte_vec_new_uninitialized(&wat, file_size);
fseek(file, 0L, SEEK_SET);
if (fread(wat.data, file_size, 1, file) != 1) {
printf("> Error loading module!\n");
exit(1);
}
fclose(file);
// Parse the wat into the binary wasm format
wasmtime_error_t *error = wasmtime_wat2wasm(wat.data, wat.size, bytes);
if (error != NULL)
exit_with_error("failed to parse wat", error, NULL);
wasm_byte_vec_delete(&wat);
}
static void exit_with_error(const char *message, wasmtime_error_t *error, wasm_trap_t *trap) {
fprintf(stderr, "error: %s\n", message);
wasm_byte_vec_t error_message;
if (error != NULL) {
wasmtime_error_message(error, &error_message);
wasmtime_error_delete(error);
} else {
wasm_trap_message(trap, &error_message);
wasm_trap_delete(trap);
}
fprintf(stderr, "%.*s\n", (int) error_message.size, error_message.data);
wasm_byte_vec_delete(&error_message);
exit(1);
}
Debugging
You can also browse this source code online and clone the wasmtime repository to run the example locally.
This example shows off how to set up a module for dynamic runtime debugging via a native debugger like GDB or LLDB.
main.c
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <inttypes.h>
#include <wasm.h>
#include <wasmtime.h>
#define own
static void exit_with_error(const char *message, wasmtime_error_t *error, wasm_trap_t *trap);
int main(int argc, const char* argv[]) {
// Configuring engine to support generating of DWARF info.
// lldb can be used to attach to the program and observe
// original fib-wasm.c source code and variables.
wasm_config_t* config = wasm_config_new();
wasmtime_config_debug_info_set(config, true);
// Initialize.
printf("Initializing...\n");
wasm_engine_t* engine = wasm_engine_new_with_config(config);
wasmtime_store_t* store = wasmtime_store_new(engine, NULL, NULL);
wasmtime_context_t* context = wasmtime_store_context(store);
// Load binary.
printf("Loading binary...\n");
FILE* file = fopen("target/wasm32-unknown-unknown/debug/fib.wasm", "rb");
if (!file) {
printf("> Error opening module!\n");
return 1;
}
fseek(file, 0L, SEEK_END);
size_t file_size = ftell(file);
fseek(file, 0L, SEEK_SET);
wasm_byte_vec_t binary;
wasm_byte_vec_new_uninitialized(&binary, file_size);
if (fread(binary.data, file_size, 1, file) != 1) {
printf("> Error reading module!\n");
return 1;
}
fclose(file);
// Compile.
printf("Compiling module...\n");
wasmtime_module_t *module = NULL;
wasmtime_error_t* error = wasmtime_module_new(engine, (uint8_t*) binary.data, binary.size, &module);
if (!module)
exit_with_error("failed to compile module", error, NULL);
wasm_byte_vec_delete(&binary);
// Instantiate.
printf("Instantiating module...\n");
wasmtime_instance_t instance;
wasm_trap_t *trap = NULL;
error = wasmtime_instance_new(context, module, NULL, 0, &instance, &trap);
if (error != NULL || trap != NULL)
exit_with_error("failed to instantiate", error, trap);
wasmtime_module_delete(module);
// Extract export.
wasmtime_extern_t fib;
bool ok = wasmtime_instance_export_get(context, &instance, "fib", 3, &fib);
assert(ok);
// Call.
printf("Calling fib...\n");
wasmtime_val_t params[1];
params[0].kind = WASMTIME_I32;
params[0].of.i32 = 6;
wasmtime_val_t results[1];
error = wasmtime_func_call(context, &fib.of.func, params, 1, results, 1, &trap);
if (error != NULL || trap != NULL)
exit_with_error("failed to call function", error, trap);
assert(results[0].kind == WASMTIME_I32);
printf("> fib(6) = %d\n", results[0].of.i32);
// Shut down.
printf("Shutting down...\n");
wasmtime_store_delete(store);
wasm_engine_delete(engine);
// All done.
printf("Done.\n");
return 0;
}
static void exit_with_error(const char *message, wasmtime_error_t *error, wasm_trap_t *trap) {
fprintf(stderr, "error: %s\n", message);
wasm_byte_vec_t error_message;
if (error != NULL) {
wasmtime_error_message(error, &error_message);
} else {
wasm_trap_message(trap, &error_message);
}
fprintf(stderr, "%.*s\n", (int) error_message.size, error_message.data);
wasm_byte_vec_delete(&error_message);
exit(1);
}
Using multi-value
You can also browse this source code online and clone the wasmtime repository to run the example locally.
This example shows off how to interact with a wasm module that uses multi-value exports and imports.
multi.wat
(module
(func $f (import "" "f") (param i32 i64) (result i64 i32))
(func $g (export "g") (param i32 i64) (result i64 i32)
(call $f (local.get 0) (local.get 1))
)
(func $round_trip_many
(export "round_trip_many")
(param i64 i64 i64 i64 i64 i64 i64 i64 i64 i64)
(result i64 i64 i64 i64 i64 i64 i64 i64 i64 i64)
local.get 0
local.get 1
local.get 2
local.get 3
local.get 4
local.get 5
local.get 6
local.get 7
local.get 8
local.get 9)
)
multi.c
/*
Example of instantiating of the WebAssembly module and invoking its exported
function.
You can compile and run this example on Linux with:
cargo build --release -p wasmtime-c-api
cc examples/multi.c \
-I crates/c-api/include \
-I crates/c-api/wasm-c-api/include \
target/release/libwasmtime.a \
-lpthread -ldl -lm \
-o multi
./multi
Note that on Windows and macOS the command will be similar, but you'll need
to tweak the `-lpthread` and such annotations.
Also note that this example was taken from
https://github.com/WebAssembly/wasm-c-api/blob/master/example/multi.c
originally
*/
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <inttypes.h>
#include <wasm.h>
#include <wasmtime.h>
static void exit_with_error(const char *message, wasmtime_error_t *error, wasm_trap_t *trap);
// A function to be called from Wasm code.
wasm_trap_t* callback(
void *env,
wasmtime_caller_t *caller,
const wasmtime_val_t* args,
size_t nargs,
wasmtime_val_t* results,
size_t nresults
) {
printf("Calling back...\n");
printf("> %"PRIu32" %"PRIu64"\n", args[0].of.i32, args[1].of.i64);
printf("\n");
results[0] = args[1];
results[1] = args[0];
return NULL;
}
// A function closure.
wasm_trap_t* closure_callback(
void* env,
wasmtime_caller_t *caller,
const wasmtime_val_t* args,
size_t nargs,
wasmtime_val_t* results,
size_t nresults
) {
int i = *(int*)env;
printf("Calling back closure...\n");
printf("> %d\n", i);
results[0].kind = WASMTIME_I32;
results[0].of.i32 = (int32_t)i;
return NULL;
}
int main(int argc, const char* argv[]) {
// Initialize.
printf("Initializing...\n");
wasm_engine_t* engine = wasm_engine_new();
wasmtime_store_t* store = wasmtime_store_new(engine, NULL, NULL);
wasmtime_context_t *context = wasmtime_store_context(store);
// Load our input file to parse it next
FILE* file = fopen("examples/multi.wat", "r");
if (!file) {
printf("> Error loading file!\n");
return 1;
}
fseek(file, 0L, SEEK_END);
size_t file_size = ftell(file);
fseek(file, 0L, SEEK_SET);
wasm_byte_vec_t wat;
wasm_byte_vec_new_uninitialized(&wat, file_size);
if (fread(wat.data, file_size, 1, file) != 1) {
printf("> Error loading module!\n");
return 1;
}
fclose(file);
// Parse the wat into the binary wasm format
wasm_byte_vec_t binary;
wasmtime_error_t *error = wasmtime_wat2wasm(wat.data, wat.size, &binary);
if (error != NULL)
exit_with_error("failed to parse wat", error, NULL);
wasm_byte_vec_delete(&wat);
// Compile.
printf("Compiling module...\n");
wasmtime_module_t* module = NULL;
error = wasmtime_module_new(engine, (uint8_t*) binary.data, binary.size, &module);
if (error)
exit_with_error("failed to compile module", error, NULL);
wasm_byte_vec_delete(&binary);
// Create external print functions.
printf("Creating callback...\n");
wasm_functype_t* callback_type = wasm_functype_new_2_2(
wasm_valtype_new_i32(),
wasm_valtype_new_i64(),
wasm_valtype_new_i64(),
wasm_valtype_new_i32()
);
wasmtime_func_t callback_func;
wasmtime_func_new(context, callback_type, callback, NULL, NULL, &callback_func);
wasm_functype_delete(callback_type);
// Instantiate.
printf("Instantiating module...\n");
wasmtime_extern_t imports[1];
imports[0].kind = WASMTIME_EXTERN_FUNC;
imports[0].of.func = callback_func;
wasmtime_instance_t instance;
wasm_trap_t* trap = NULL;
error = wasmtime_instance_new(context, module, imports, 1, &instance, &trap);
if (error != NULL || trap != NULL)
exit_with_error("failed to instantiate", error, trap);
wasmtime_module_delete(module);
// Extract export.
printf("Extracting export...\n");
wasmtime_extern_t run;
bool ok = wasmtime_instance_export_get(context, &instance, "g", 1, &run);
assert(ok);
assert(run.kind == WASMTIME_EXTERN_FUNC);
// Call.
printf("Calling export...\n");
wasmtime_val_t args[2];
args[0].kind = WASMTIME_I32;
args[0].of.i32 = 1;
args[1].kind = WASMTIME_I64;
args[1].of.i64 = 2;
wasmtime_val_t results[2];
error = wasmtime_func_call(context, &run.of.func, args, 2, results, 2, &trap);
if (error != NULL || trap != NULL)
exit_with_error("failed to call run", error, trap);
// Print result.
printf("Printing result...\n");
printf("> %"PRIu64" %"PRIu32"\n",
results[0].of.i64, results[1].of.i32);
assert(results[0].kind == WASMTIME_I64);
assert(results[0].of.i64 == 2);
assert(results[1].kind == WASMTIME_I32);
assert(results[1].of.i32 == 1);
// Shut down.
printf("Shutting down...\n");
wasmtime_store_delete(store);
wasm_engine_delete(engine);
// All done.
printf("Done.\n");
return 0;
}
static void exit_with_error(const char *message, wasmtime_error_t *error, wasm_trap_t *trap) {
fprintf(stderr, "error: %s\n", message);
wasm_byte_vec_t error_message;
if (error != NULL) {
wasmtime_error_message(error, &error_message);
wasmtime_error_delete(error);
} else {
wasm_trap_message(trap, &error_message);
wasm_trap_delete(trap);
}
fprintf(stderr, "%.*s\n", (int) error_message.size, error_message.data);
wasm_byte_vec_delete(&error_message);
exit(1);
}
Using WebAssembly from your Language
Wasmtime can be used as a library to embed WebAssembly execution support within applications. Wasmtime is written in Rust, but bindings are available through a C API for a number of other languages too:
Using WebAssembly from Rust
This document shows an example of how to embed Wasmtime using the Rust
API to execute a simple wasm program. Be sure to also check out the
full API documentation for a full listing of what the wasmtime
crate has to offer and the book examples for
Rust for more information.
Creating the WebAssembly to execute
Creation of a WebAssembly file is generally covered by the Writing
WebAssembly chapter, so we'll just assume that you've already got a
wasm file on hand for the rest of this tutorial. To make things simple we'll
also just assume you've got a hello.wat file which looks like this:
(module
(func (export "answer") (result i32)
i32.const 42
)
)
Here we're just exporting one function which returns an integer that we'll read from Rust.
Hello, World!
First up let's create a rust project
$ cargo new --bin wasmtime_hello
$ cd wasmtime_hello
Next you'll want to add hello.wat to the root of your project.
We will be using the wasmtime crate to run the wasm file, so next up we need a
dependency in Cargo.toml:
[dependencies]
wasmtime = "0.18.0"
Next up let's write the code that we need to execute this wasm file. The simplest version of this looks like so:
extern crate wasmtime; use std::error::Error; use wasmtime::*; fn main() -> Result<(), Box<dyn Error>> { // An engine stores and configures global compilation settings like // optimization level, enabled wasm features, etc. let engine = Engine::default(); if false { // We start off by creating a `Module` which represents a compiled form // of our input wasm module. In this case it'll be JIT-compiled after // we parse the text format. let module = Module::from_file(&engine, "hello.wat")?; } let module = Module::new(&engine, r#"(module (func (export "answer") (result i32) i32.const 42))"#)?; // A `Store` is what will own instances, functions, globals, etc. All wasm // items are stored within a `Store`, and it's what we'll always be using to // interact with the wasm world. Custom data can be stored in stores but for // now we just use `()`. let mut store = Store::new(&engine, ()); // With a compiled `Module` we can then instantiate it, creating // an `Instance` which we can actually poke at functions on. let instance = Instance::new(&mut store, &module, &[])?; // The `Instance` gives us access to various exported functions and items, // which we access here to pull out our `answer` exported function and // run it. let answer = instance.get_func(&mut store, "answer") .expect("`answer` was not an exported function"); // There's a few ways we can call the `answer` `Func` value. The easiest // is to statically assert its signature with `typed` (in this case // asserting it takes no arguments and returns one i32) and then call it. let answer = answer.typed::<(), i32, _>(&store)?; // And finally we can call our function! Note that the error propagation // with `?` is done to handle the case where the wasm function traps. let result = answer.call(&mut store, ())?; println!("Answer: {:?}", result); Ok(()) }
We can build and execute our example with cargo run. Note that by depending on
wasmtime you're depending on a JIT compiler, so it may take a moment to build
all of its dependencies:
$ cargo run
Compiling ...
...
Finished dev [unoptimized + debuginfo] target(s) in 42.32s
Running `wasmtime_hello/target/debug/wasmtime_hello`
Answer: 42
and there we go! We've now executed our first WebAssembly in wasmtime and
gotten the result back.
Importing Host Functionality
What we've just seen is a pretty small example of how to call a wasm function and take a look at the result. Most interesting wasm modules, however, are going to import some functions to do something a bit more interesting. For that you'll need to provide imported functions from Rust for wasm to call!
Let's take a look at a wasm module which imports a logging function as well as some simple arithmetic from the environment.
(module
(import "" "log" (func $log (param i32)))
(import "" "double" (func $double (param i32) (result i32)))
(func (export "run")
i32.const 0
call $log
i32.const 1
call $log
i32.const 2
call $double
call $log
)
)
This wasm module will call our "log" import a few times and then also call the
"double" import. We can compile and instantiate this module with code that
looks like this:
extern crate wasmtime; use std::error::Error; use wasmtime::*; struct Log { integers_logged: Vec<u32>, } fn main() -> Result<(), Box<dyn Error>> { let engine = Engine::default(); if false { let module = Module::from_file(&engine, "hello.wat")?; } let module = Module::new(&engine, r#"(module (import "" "log" (func $log (param i32))) (import "" "double" (func $double (param i32) (result i32))) (func (export "run") i32.const 0 call $log i32.const 1 call $log i32.const 2 call $double call $log))"#)?; // For host-provided functions it's recommended to use a `Linker` which does // name-based resolution of functions. let mut linker = Linker::new(&engine); // First we create our simple "double" function which will only multiply its // input by two and return it. linker.func_wrap("", "double", |param: i32| param * 2); // Next we define a `log` function. Note that we're using a // Wasmtime-provided `Caller` argument to access the state on the `Store`, // which allows us to record the logged information. linker.func_wrap("", "log", |mut caller: Caller<'_, Log>, param: u32| { println!("log: {}", param); caller.data_mut().integers_logged.push(param); }); // As above, instantiation always happens within a `Store`. This means to // actually instantiate with our `Linker` we'll need to create a store. Note // that we're also initializing the store with our custom data here too. // // Afterwards we use the `linker` to create the instance. let data = Log { integers_logged: Vec::new() }; let mut store = Store::new(&engine, data); let instance = linker.instantiate(&mut store, &module)?; // Like before, we can get the run function and execute it. let run = instance.get_typed_func::<(), (), _>(&mut store, "run")?; run.call(&mut store, ())?; // We can also inspect what integers were logged: println!("logged integers: {:?}", store.data().integers_logged); Ok(()) }
Note that there's a number of ways to define a Func, be sure to consult its
documentation for other ways to create a host-defined function.
C
... more coming soon
In the meantime, have a look at the C API docs.
Using WebAssembly from Python
Wasmtime is available on PyPI and can be used programmatically or as a python module loader, which allows almost any WebAssembly module to be used as a python module. This guide will go over adding Wasmtime to your project, and some provided examples of what can be done with WebAssembly modules.
Make sure you've got Python 3.5 or newer installed locally, and we can get started!
Getting started and simple example
First, copy this example WebAssembly text module into your project. It exports a function for calculating the greatest common denominator of two numbers.
(module
(func $gcd (param i32 i32) (result i32)
(local i32)
block ;; label = @1
block ;; label = @2
local.get 0
br_if 0 (;@2;)
local.get 1
local.set 2
br 1 (;@1;)
end
loop ;; label = @2
local.get 1
local.get 0
local.tee 2
i32.rem_u
local.set 0
local.get 2
local.set 1
local.get 0
br_if 0 (;@2;)
end
end
local.get 2
)
(export "gcd" (func $gcd))
)
Next, install the Wasmtime package from PyPi. It can be installed as a dependency through Pip or related tools such as Pipenv.
pip install wasmtime
Or
pipenv install wasmtime
After you have Wasmtime installed and you've imported wasmtime, you can import
WebAssembly modules in your project like any other python module.
import wasmtime.loader
import gcd
print("gcd(27, 6) =", gcd.gcd(27, 6))
This script should output
gcd(27, 6) = 3
If this is the output you see, congrats! You've successfully ran your first WebAssembly code in python!
You can also alternatively use the wasmtime package's
API:
from wasmtime import Store, Module, Instance
store = Store()
module = Module.from_file(store, 'gcd.wat')
instance = Instance(module, [])
gcd = instance.get_export('gcd')
print("gcd(27, 6) =", gcd(27, 6))
More examples and contributing
The wasmtime Python package currently lives in its own repository outside of
wasmtime and has a number
of other more advanced
examples
as well. Feel free to browse those, but if you find anything missing don't
hesitate to open an
issue and let us
know if you have any questions!
Using WebAssembly from .NET
The Wasmtime NuGet package can be used to programmatically interact with WebAssembly modules.
This guide will go over adding Wasmtime to your project and demonstrate a simple example of using a WebAssembly module from C#.
Make sure you have a .NET Core SDK 3.0 SDK or later installed before we get started!
Getting started and simple example
Start by creating a new .NET Core console project:
$ mkdir gcd
$ cd gcd
$ dotnet new console
Next, add a reference to the Wasmtime NuGet package to your project:
$ dotnet add package --version 0.19.0-preview1 wasmtime
Copy this example WebAssembly text module into your project directory as gcd.wat.
(module
(func $gcd (param i32 i32) (result i32)
(local i32)
block ;; label = @1
block ;; label = @2
local.get 0
br_if 0 (;@2;)
local.get 1
local.set 2
br 1 (;@1;)
end
loop ;; label = @2
local.get 1
local.get 0
local.tee 2
i32.rem_u
local.set 0
local.get 2
local.set 1
local.get 0
br_if 0 (;@2;)
end
end
local.get 2
)
(export "gcd" (func $gcd))
)
This module exports a function for calculating the greatest common denominator of two numbers.
Replace the code in Program.cs with the following:
using System;
using Wasmtime;
namespace Tutorial
{
class Program
{
static void Main(string[] args)
{
using var engine = new Engine();
using var module = Module.FromTextFile(engine, "gcd.wat");
using var host = new Host(engine);
using dynamic instance = host.Instantiate(module);
Console.WriteLine($"gcd(27, 6) = {instance.gcd(27, 6)}");
}
}
}
Run the .NET core program:
$ dotnet run
The program should output:
gcd(27, 6) = 3
If this is the output you see, congrats! You've successfully ran your first WebAssembly code in .NET!
More examples and contributing
The .NET embedding of Wasmtime repository contains the source code for the Wasmtime NuGet package.
The repository also has more examples as well.
Feel free to browse those, but if you find anything missing don't hesitate to open an issue and let us know if you have any questions!
Using WebAssembly from Go
Wasmtime is available as a Go Module. This guide will go over adding Wasmtime to your project, and some provided examples of what can be done with WebAssembly modules.
Make sure you're using Go 1.12 or later with modules support.
Getting started and simple example
First up you'll want to start a new module:
$ mkdir hello-wasm
$ cd hello-wasm
$ go mod init hello-wasm
$ go get github.com/bytecodealliance/wasmtime-go
Next, copy this example WebAssembly text module into your project. It exports a function for calculating the greatest common denominator of two numbers.
(module
(func $gcd (param i32 i32) (result i32)
(local i32)
block ;; label = @1
block ;; label = @2
local.get 0
br_if 0 (;@2;)
local.get 1
local.set 2
br 1 (;@1;)
end
loop ;; label = @2
local.get 1
local.get 0
local.tee 2
i32.rem_u
local.set 0
local.get 2
local.set 1
local.get 0
br_if 0 (;@2;)
end
end
local.get 2
)
(export "gcd" (func $gcd))
)
Next, we can write our code in main.go which reads this file and runs it:
package main
import (
"fmt"
"github.com/bytecodealliance/wasmtime-go"
)
func main() {
engine := wasmtime.NewEngine()
store := wasmtime.NewStore(engine)
module, err := wasmtime.NewModuleFromFile(engine, "gcd.wat")
check(err)
instance, err := wasmtime.NewInstance(store, module, []wasmtime.AsExtern{})
check(err)
gcd := instance.GetExport(store, "gcd").Func()
val, err := gcd.Call(store, 6, 27)
check(err)
fmt.Printf("gcd(6, 27) = %d\n", val.(int32))
}
func check(err error) {
if err != nil {
panic(err)
}
}
And finally we can build and run it:
$ go run main.go
gcd(6, 27) = 3
If this is the output you see, congrats! You've successfully ran your first WebAssembly code in Go!
More examples and contributing
The wasmtime Go package lives in its own
repository and has a number
of other more advanced
examples
as well. Feel free to browse those, but if you find anything missing don't
hesitate to open an
issue and let us
know if you have any questions!
Using WebAssembly from Bash
Getting started and simple example
First up you'll want to start a new module:
$ mkdir -p gcd-bash
$ cd gcd-bash
$ touch gcd.wat gcd.sh
Next, copy this example WebAssembly text module into your project. It exports a function for calculating the greatest common denominator of two numbers.
gcd.wat
(module
(func $gcd (param i32 i32) (result i32)
(local i32)
block ;; label = @1
block ;; label = @2
local.get 0
br_if 0 (;@2;)
local.get 1
local.set 2
br 1 (;@1;)
end
loop ;; label = @2
local.get 1
local.get 0
local.tee 2
i32.rem_u
local.set 0
local.get 2
local.set 1
local.get 0
br_if 0 (;@2;)
end
end
local.get 2
)
(export "gcd" (func $gcd))
)
Create a bash script that will invoke GCD three times.
gcd.sh
#!/bin/bash
function gcd() {
# Cast to number; default = 0
local x=$(($1))
local y=$(($2))
# Invoke GCD from module; suppress stderr
local result=$(wasmtime examples/gcd.wat --invoke gcd $x $y 2>/dev/null)
echo "$result"
}
# main
for num in "27 6" "6 27" "42 12"; do
set -- $num
echo "gcd($1, $2) = $(gcd "$1" "$2")"
done
Using the wasmtime CLI
In addition to the embedding API which allows you to use Wasmtime as a
library, the Wasmtime project also provides a wasmtime CLI tool to conveniently
execute WebAssembly modules from the command line.
This section will provide a guide to the wasmtime CLI and major functionality
that it contains. In short, however, you can execute a WebAssembly file
(actually doing work as part of the start function) like so:
$ wasmtime foo.wasm
Or similarly if you want to invoke a "start" function, such as with WASI modules, you can execute
$ wasmtime --invoke _start foo.wasm
For more information be sure to check out how to install the CLI, the list of options you can pass, and how to enable logging.
Installing wasmtime
Here we'll show you how to install the wasmtime command line tool. Note that
this is distinct from embedding the Wasmtime project into another, for that
you'll want to consult the embedding documentation.
The easiest way to install the wasmtime CLI tool is through our installation
script. Linux and macOS users can execute the following:
$ curl https://wasmtime.dev/install.sh -sSf | bash
This will download a precompiled version of wasmtime and place it in
$HOME/.wasmtime, and update your shell configuration to place the right
directory in PATH.
Windows users will want to visit our releases page and can download
the MSI installer (wasmtime-dev-x86_64-windows.msi for example) and use that
to install.
You can confirm your installation works by executing:
$ wasmtime -V
wasmtime 0.12.0
And now you're off to the races! Be sure to check out the various CLI options as well.
Download Precompiled Binaries
If you'd prefer to not use an installation script, or you're perhaps
orchestrating something in CI, you can also download one of our precompiled
binaries of wasmtime. We have two channels of releases right now for
precompiled binaries:
- Each tagged release will have a full set of release artifacts on the GitHub releases page.
- The
devrelease is also continuously updated with the latest build of themainbranch. If you want the latest-and-greatest and don't mind a bit of instability, this is the release for you.
When downloading binaries you'll likely want one of the following archives (for
the dev release)
- Linux users - [
wasmtime-dev-x86_64-linux.tar.xz] - macOS users - [
wasmtime-dev-x86_64-macos.tar.xz] - Windows users - [
wasmtime-dev-x86_64-windows.zip]
Each of these archives has a wasmtime binary placed inside which can be
executed normally as the CLI would.
Compiling from Source
If you'd prefer to compile the wasmtime CLI from source, you'll want to
consult the contributing documentation for building.
Be sure to use a --release build if you're curious to do benchmarking!
CLI Options for wasmtime
The wasmtime CLI is organized into a few subcommands. If no subcommand is
provided it'll assume run, which is to execute a wasm file. The subcommands
supported by wasmtime are:
help
This is a general subcommand used to print help information to the terminal. You can execute any number of the following:
$ wasmtime help
$ wasmtime --help
$ wasmtime -h
$ wasmtime help run
$ wasmtime run -h
When in doubt, try running the help command to learn more about functionality!
run
This is the wasmtime CLI's main subcommand, and it's also the default if no
other subcommand is provided. The run command will execute a WebAssembly
module. This means that the module will be compiled to native code,
instantiated, and then optionally have an export executed.
The wasmtime CLI will automatically hook up any WASI-related imported
functionality, but at this time if your module imports anything else it will
fail instantiation.
The run command takes one positional argument which is the name of the module
to run:
$ wasmtime run foo.wasm
$ wasmtime foo.wasm
Note that the wasmtime CLI can take both a binary WebAssembly file (*.wasm)
as well as the text format for WebAssembly (*.wat):
$ wasmtime foo.wat
wast
The wast command executes a *.wast file which is the test format for the
official WebAssembly spec test suite. This subcommand will execute the script
file which has a number of directives supported to instantiate modules, link
tests, etc.
Executing this looks like:
$ wasmtime wast foo.wast
config
This subcommand is used to control and edit local Wasmtime configuration settings. The primary purpose of this currently is to configure how Wasmtime's code caching works. You can create a new configuration file for you to edit with:
$ wasmtime config new
And that'll print out the path to the file you can edit.
wasm2obj
This is an experimental subcommand to compile a WebAssembly module to native code. Work for this is still heavily under development, but you can execute this with:
$ wasmtime wasm2obj foo.wasm foo.o
compile
This subcommand is used to Ahead-Of-Time (AOT) compile a WebAssembly module to produce a "compiled wasm" (.cwasm) file.
The wasmtime run subcommand can then be used to run a AOT-compiled WebAssembly module:
$ wasmtime compile foo.wasm
$ wasmtime foo.cwasm
AOT-compiled modules can be run from hosts that are compatible with the target environment of the AOT-completed module.
settings
This subcommand is used to print the available Cranelift settings for a given target.
When run without options, it will print the settings for the host target and also display what Cranelift settings are inferred for the host:
$ wasmtime settings
Cache Configuration of wasmtime
The configuration file uses the toml format. You can create a configuration file at the default location with:
$ wasmtime config new
It will print the location regardless of the success.
Please refer to the --help message for using a custom location.
All settings, except enabled, are optional.
If the setting is not specified, the default value is used.
Thus, if you don't know what values to use, don't specify them.
The default values might be tuned in the future.
Wasmtime assumes all the options are in the cache section.
Example config:
[cache]
enabled = true
directory = "/nfs-share/wasmtime-cache/"
cleanup-interval = "30m"
files-total-size-soft-limit = "1Gi"
Please refer to the cache system section to learn how it works.
If you think some default value should be tuned, some new settings should be introduced or some behavior should be changed, you are welcome to discuss it and contribute to the Wasmtime repository.
Setting enabled
- type: boolean
- format:
true | false - default:
true
Specifies whether the cache system is used or not.
This field is mandatory. The default value is used when configuration file is not specified and none exists at the default location.
Setting directory
- type: string (path)
- default: look up
cache_dirin directories crate
Specifies where the cache directory is. Must be an absolute path.
Setting worker-event-queue-size
- type: string (SI prefix)
- format:
"{integer}(K | M | G | T | P)?" - default:
"16"
Size of cache worker event queue. If the queue is full, incoming cache usage events will be dropped.
Setting baseline-compression-level
- type: integer
- default:
3, the default zstd compression level
Compression level used when a new cache file is being written by the cache system. Wasmtime uses zstd compression.
Setting optimized-compression-level
- type: integer
- default:
20
Compression level used when the cache worker decides to recompress a cache file. Wasmtime uses zstd compression.
Setting optimized-compression-usage-counter-threshold
- type: string (SI prefix)
- format:
"{integer}(K | M | G | T | P)?" - default:
"256"
One of the conditions for the cache worker to recompress a cache file is to have usage count of the file exceeding this threshold.
Setting cleanup-interval
- type: string (duration)
- format:
"{integer}(s | m | h | d)" - default:
"1h"
When the cache worker is notified about a cache file being updated by the cache system and this interval has already passed since last cleaning up, the worker will attempt a new cleanup.
Please also refer to allowed-clock-drift-for-files-from-future.
Setting optimizing-compression-task-timeout
- type: string (duration)
- format:
"{integer}(s | m | h | d)" - default:
"30m"
When the cache worker decides to recompress a cache file, it makes sure that
no other worker has started the task for this file within the last
optimizing-compression-task-timeout interval.
If some worker has started working on it, other workers are skipping this task.
Please also refer to the allowed-clock-drift-for-files-from-future section.
Setting allowed-clock-drift-for-files-from-future
- type: string (duration)
- format:
"{integer}(s | m | h | d)" - default:
"1d"
Locks
When the cache worker attempts acquiring a lock for some task, it checks if some other worker has already acquired such a lock. To be fault tolerant and eventually execute every task, the locks expire after some interval. However, because of clock drifts and different timezones, it would happen that some lock was created in the future. This setting defines a tolerance limit for these locks. If the time has been changed in the system (i.e. two years backwards), the cache system should still work properly. Thus, these locks will be treated as expired (assuming the tolerance is not too big).
Cache files
Similarly to the locks, the cache files or their metadata might have modification time in distant future. The cache system tries to keep these files as long as possible. If the limits are not reached, the cache files will not be deleted. Otherwise, they will be treated as the oldest files, so they might survive. If the user actually uses the cache file, the modification time will be updated.
Setting file-count-soft-limit
- type: string (SI prefix)
- format:
"{integer}(K | M | G | T | P)?" - default:
"65536"
Soft limit for the file count in the cache directory.
This doesn't include files with metadata. To learn more, please refer to the cache system section.
Setting files-total-size-soft-limit
- type: string (disk space)
- format:
"{integer}(K | Ki | M | Mi | G | Gi | T | Ti | P | Pi)?" - default:
"512Mi"
Soft limit for the total size* of files in the cache directory.
This doesn't include files with metadata. To learn more, please refer to the cache system section.
*this is the file size, not the space physically occupied on the disk.
Setting file-count-limit-percent-if-deleting
- type: string (percent)
- format:
"{integer}%" - default:
"70%"
If file-count-soft-limit is exceeded and the cache worker performs the cleanup task,
then the worker will delete some cache files, so after the task,
the file count should not exceed
file-count-soft-limit * file-count-limit-percent-if-deleting.
This doesn't include files with metadata. To learn more, please refer to the cache system section.
Setting files-total-size-limit-percent-if-deleting
- type: string (percent)
- format:
"{integer}%" - default:
"70%"
If files-total-size-soft-limit is exceeded and cache worker performs the cleanup task,
then the worker will delete some cache files, so after the task,
the files total size should not exceed
files-total-size-soft-limit * files-total-size-limit-percent-if-deleting.
This doesn't include files with metadata. To learn more, please refer to the cache system section.
How does the cache work?
This is an implementation detail and might change in the future. Information provided here is meant to help understanding the big picture and configuring the cache.
There are two main components - the cache system and the cache worker.
Cache system
Handles GET and UPDATE cache requests.
- GET request - simply loads the cache from disk if it is there.
- UPDATE request - compresses received data with zstd and
baseline-compression-level, then writes the data to the disk.
In case of successful handling of a request, it notifies the cache worker about this
event using the queue.
The queue has a limited size of worker-event-queue-size. If it is full, it will drop
new events until the cache worker pops some event from the queue.
Cache worker
The cache worker runs in a single thread with lower priority and pops events from the queue in a loop handling them one by one.
On GET request
-
Read the statistics file for the cache file, increase the usage counter and write it back to the disk.
-
Attempt recompressing the cache file if all of the following conditions are met:
- usage counter exceeds
optimized-compression-usage-counter-threshold, - the file is compressed with compression level lower than
optimized-compression-level, - no other worker has started working on this particular task within the last
optimizing-compression-task-timeoutinterval.
When recompressing,
optimized-compression-levelis used as a compression level. - usage counter exceeds
On UPDATE request
- Write a fresh statistics file for the cache file.
- Clean up the cache if no worker has attempted to do this within the last
cleanup-interval. During this task:- all unrecognized files and expired task locks in cache directory will be deleted
- if
file-count-soft-limitorfiles-total-size-soft-limitis exceeded, then recognized files will be deleted according tofile-count-limit-percent-if-deletingandfiles-total-size-limit-percent-if-deleting. Wasmtime uses Least Recently Used (LRU) cache replacement policy and requires that the filesystem maintains proper mtime (modification time) of the files. Files with future mtimes are treated specially - more details inallowed-clock-drift-for-files-from-future.
Metadata files
- every cached WebAssembly module has its own statistics file
- every lock is a file
Writing WebAssembly
Wasmtime is a runtime for executing WebAssembly but you also at some point need to actually produce the WebAssembly module to feed into Wasmtime! This section of the guide is intended to provide some introductory documentation for compiling source code to WebAssembly to later run in Wasmtime. There's plenty of other documentation on the web for doing this, so you'll want to be sure to check out your language's documentation for WebAssembly as well.
Rust
The Rust Programming Language supports WebAssembly
as a compilation target. If you're not familiar with Rust it's recommended to
start with its introductory documentation.
Compiling to WebAssembly will involve specifying the desired target via the
--target flag, and to do this there are a number of "target triples" for
WebAssembly compilation in Rust:
wasm32-wasi- when usingwasmtimethis is likely what you'll be using. The WASI target is integrated into the standard library and is intended on producing standalone binaries.wasm32-unknown-unknown- this target, like the WASI one, is focused on producing single*.wasmbinaries. The standard library, however, is largely stubbed out since the "unknown" part of the target means libstd can't assume anything. This means that while binaries will likely work inwasmtime, common conveniences likeprintln!orpanic!won't work.wasm32-unknown-emscripten- this target is intended to work in a web browser and produces a*.wasmfile coupled with a*.jsfile, and it is not compatible withwasmtime.
For the rest of this documentation we'll assume that you're using the
wasm32-wasi target for compiling Rust code and executing inside of wasmtime.
Hello, World!
Cross-compiling to WebAssembly involves a number of knobs that need
configuration, but you can often gloss over these internal details by using
build tooling intended for the WASI target. For example we can start out writing
a WebAssembly binary with cargo wasi.
First up we'll install cargo wasi:
$ cargo install cargo-wasi
Next we'll make a new Cargo project:
$ cargo new hello-world
$ cd hello-world
Inside of src/main.rs you'll see the canonical Rust "Hello, World!" using
println!. We'll be executing this for the wasm32-wasi target, so you'll want
to make sure you're previously built wasmtime and inserted it into
PATH;
$ cargo wasi run
info: downloading component 'rust-std' for 'wasm32-wasi'
info: installing component 'rust-std' for 'wasm32-wasi'
Compiling hello-world v0.1.0 (/hello-world)
Finished dev [unoptimized + debuginfo] target(s) in 0.16s
Running `/.cargo/bin/cargo-wasi target/wasm32-wasi/debug/hello-world.wasm`
Running `target/wasm32-wasi/debug/hello-world.wasm`
Hello, world!
And we're already running our first WebAssembly code inside of wasmtime!
While it's automatically happening for you as part of cargo wasi, you can also
run wasmtime yourself:
$ wasmtime target/wasm32-wasi/debug/hello-world.wasm
Hello, world!
You can check out the introductory documentation of
cargo-wasi as
well for some more information.
Writing Libraries
Previously for "Hello, World!" we created a binary project which used
src/main.rs. Not all *.wasm binaries are intended to be executed like
commands, though. Some are intended to be loaded into applications and called
through various APIs, acting more like libraries. For this use case you'll want
to add this to Cargo.toml:
# in Cargo.toml ...
[lib]
crate-type = ['cdylib']
and afterwards you'll want to write your code in src/lib.rs like so:
#![allow(unused)] fn main() { #[no_mangle] pub extern "C" fn print_hello() { println!("Hello, world!"); } }
When you execute cargo wasi build that'll generate a *.wasm file which has
one exported function, print_hello. We can then run it via the CLI like so:
$ cargo wasi build
Compiling hello-world v0.1.0 (/home/alex/code/hello-world)
Finished dev [unoptimized + debuginfo] target(s) in 0.08s
$ wasmtime --invoke print_hello target/wasm32-wasi/debug/hello_world.wasm
Hello, world!
As a library crate one of your primary consumers may be other languages as well.
You'll want to consult the section of this book for using wasmtime from
Python` and after running through the basics there you can
execute our file in Python:
$ cp target/wasm32-wasi/debug/hello_world.wasm .
$ python3
>>> import wasmtime
>>> import hello_world
>>> hello_world.print_hello()
Hello, world!
()
>>>
Note that this form of using #[no_mangle] Rust functions is pretty primitive.
You're only able to work with primitive datatypes like integers and floats.
While this works for some applications if you need to work with richer types
like strings or structs, then you'll want to use the support in wasmtime for
interface types.
WebAssembly Interface Types
Note: support for interface types has temporarily removed from Wasmtime. This documentation is somewhat up to date but will no longer work with recent versions of Wasmtime. For more information see https://github.com/bytecodealliance/wasmtime/issues/677
Working with WebAssembly modules at the bare-bones level means that you're only
dealing with integers and floats. Many APIs, however, want to work with things
like byte arrays, strings, structures, etc. To facilitate these interactions the
WebAssembly Interface Types
Proposal comes into play. The
wasmtime runtime has support for interface types, and the Rust toolchain has
library support in a crate called
wasm-bindgen.
Note: WebAssembly Interface Types is still a WebAssembly proposal and is under active development. The toolchain may not match the exact specification, and during development you'll generally need to make sure tool versions are all kept up to date to ensure everything aligns right. This'll all smooth over as the proposal stabilizes!
To get started with WebAssembly interface types let's write a library module which will generate a greeting for us. The module itself won't do any printing, we'll simply be working with some strings.
To get starts let's add this to our Cargo.toml:
[lib]
crate-type = ['cdylib']
[dependencies]
wasm-bindgen = "0.2.54"
Using this crate, we can then update our src/lib.rs with the following:
use wasm_bindgen::prelude::*;
#[wasm_bindgen]
pub fn greet(name: &str) -> String {
format!("Hello, {}!", name)
}
Then we can build this with:
$ cargo wasi build --release
Updating crates.io index
...
Finished dev [unoptimized + debuginfo] target(s) in 9.57s
Downloading precompiled wasm-bindgen v0.2.54
and we have our new wasm binary!
Note: for now when using
wasm-bindgenyou must use--releasemode to build wasi binaries with interface types.
We can then test out support for this with the CLI:
$ wasmtime --invoke greet ./target/wasm32-wasi/release/hello_world.wasm "Wasmtime CLI"
warning: using `--invoke` with a function that takes arguments is experimental and may break in the future
warning: using `--invoke` with a function that returns values is experimental and may break in the future
Hello, Wasmtime CLI!
Here we can see some experimental warnings, but we got our error message printed
out! The first CLI parameter, "Wasmtime CLI", was passed as the first argument
of the greet function. The resulting string was then printed out to the
console.
Like before, we can also execute this with Python:
$ cp target/wasm32-wasi/release/hello_world.wasm .
$ python3
>>> import wasmtime
>>> import hello_world
>>> hello_world.greet('python interpreter')
'Hello, python interpreter!'
>>>
Note that wasm-bindgen was originally developed for JS and usage in a browser,
but a subset of its implementation (such as arguments which are strings) are
supported for WebAssembly interface types. You can also check out the reference
documentation for wasm-bindgen for
more information about how it works. Note that the wasm-bindgen support for
wasm interface type is still in its nascent phase and is likely to be greatly
improved in the future.
Exporting Rust functionality
Currently only Rust functions can be exported from a wasm module. Rust functions
must be #[no_mangle] to show up in the final binary, but if you're using
#[wasm_bindgen] that will happen automatically for you.
Memory is by default exported from Rust modules under the name memory. This
can be tweaked with the -Clink-arg flag to rustc to pass flags to LLD, the
WebAssembly code linker.
Tables cannot be imported at this time. When using rustc directly there is no
support for anyref and only one function table is supported. When using
wasm-bindgen it may inject an anyref table if necessary, but this table is
an internal detail and is not exported. The function table can be exported by
passing the --export-table argument to LLD (via -C link-arg) or can be
imported with the --import-table.
Rust currently does not have support for exporting or importing custom global
values.
Importing host functionality
Only functions can be imported in Rust at this time, and they can be imported via raw interfaces like:
#![allow(unused)] fn main() { struct MyStruct; #[link(wasm_import_module = "the-wasm-import-module")] extern "C" { // imports the name `foo` from `the-wasm-import-module` fn foo(); // functions can have integer/float arguments/return values fn translate(a: i32) -> f32; // Note that the ABI of Rust and wasm is somewhat in flux, so while this // works, it's recommended to rely on raw integer/float values where // possible. fn translate_fancy(my_struct: MyStruct) -> u32; // you can also explicitly specify the name to import, this imports `bar` // instead of `baz` from `the-wasm-import-module`. #[link_name = "bar"] fn baz(); } }
When you're using wasm-bindgen you would instead use:
use wasm_bindgen::prelude::*;
#[wasm_bindgen(module = "the-wasm-import-module")]
extern "C" {
fn foo();
fn baz();
// ...
}
Note that unless you're using interface types you likely don't need
wasm-bindgen.
C/C++
All the parts needed to support wasm are included in upstream clang, lld, and compiler-rt, as of the LLVM 8.0 release. However, to use it, you'll need to build WebAssembly-targeted versions of the library parts, and it can be tricky to get all the CMake invocations lined up properly.
To make things easier, we provide prebuilt packages that provide builds of Clang and sysroot libraries.
WASI doesn't yet support setjmp/longjmp or C++ exceptions, as it is
waiting for unwinding support in WebAssembly.
By default, the C/C++ toolchain orders linear memory to put the globals first,
the stack second, and start the heap after that. This reduces code size,
because references to globals can use small offsets. However, it also means
that stack overflow will often lead to corrupted globals. The
-Wl,--stack-first flag to clang instructs it to put the stack first, followed
by the globals and the heap, which may produce slightly larger code, but will
more reliably trap on stack overflow.
See the wasm-ld documentation for more information and additional flags. Note
flags related to dynamic linking, such -shared and --export-dynamic are
not yet stable and are expected to change behavior in the future.
AssemblyScript
AssemblyScript 0.10.0 includes support for targeting WASI. To use it, add
import "wasi" at the top of your entrypoint file.
To create a program which can be run directly as a command, pass --runtime half
to the AssemblyScript linker. This selects the half runtime, which ensures that
the generated wasm module doesn't contain any extraneous exports. (This isn't
strictly required today, but the handling of extraneous exports may change in
the future, so it's encouraged. As a bonus, it also reduces code size.)
To create a program which can be loaded as a library and used from other modules, no special options are needed.
Let's walk through a simple hello world example.
wasi-hello-world.ts
import "wasi"
import {Console} from "as-wasi"
Console.log('Hello World!\n');
This uses as-wasi as a dependency to make working with the AssemblyScript WASI bindings easier. Then, you can run:
asc wasi-hello-world.ts -b wasi-hello-world.wasm
to compile it to wasm, and
wasmtime wasi-hello-world.wasm
to run it from the command-line. Or you can instantiate it using the Wasmtime API.
package.json
It can also be packaged using a package.json file:
{
"name": "wasi-hello-world",
"version": "1.0.0",
"description": "Hello world in Wasi with AS and as-wasi",
"main": "index.js",
"scripts": {
"build": "asc wasi-hello-world.ts -b wasi-hello-world.wasm -t wasi-hello-world.wat --runtime half",
"wasmtime": "wasmtime wasi-hello-world.wasm"
},
"author": "Aaron Turner",
"license": "MIT",
"devDependencies": {
"assemblyscript": "^0.10.0"
},
"dependencies": {
"as-wasi": "^0.1.1"
}
}
You can also browse this source code online and clone the wasmtime repository to run the example locally.
WebAssembly Text Format (*.wat)
While not necessarily a full-blown language you might be curious how Wasmtime
interacts with the *.wat text format! The wasmtime CLI and Rust
embedding API both support the *.wat text format by default.
"Hello, World!" is pretty nontrivial in the *.wat format since it's
assembly-like and not really intended to be a primary programming language. That
being said we can create a simple add function to call it!
For example if you have a file add.wat like so:
(module
(func (export "add") (param i32 i32) (result i32)
local.get 0
local.get 1
i32.add))
Then you can execute this on the CLI with:
$ wasmtime add.wat --invoke add 1 2
warning: ...
warning: ...
3
And we can see that we're already adding numbers!
You can also see how this works in the Rust API like so:
extern crate wasmtime; extern crate anyhow; use wasmtime::*; fn main() -> anyhow::Result<()> { let mut store = Store::<()>::default(); let wat = r#" (module (func (export "add") (param i32 i32) (result i32) local.get 0 local.get 1 i32.add)) "#; let module = Module::new(store.engine(), wat)?; let instance = Instance::new(&mut store, &module, &[])?; let add = instance.get_typed_func::<(i32, i32), i32, _>(&mut store, "add")?; println!("1 + 2 = {}", add.call(&mut store, (1, 2))?); Ok(()) }
Example: Markdown Parser
Stability
... more coming soon
Release Process
... more coming soon
Platform Support
The wasmtime project is a configurable and lightweight runtime for WebAssembly
which has a number of ways it can be configured. Not all features are supported
on all platforms, but it is intended that wasmtime can run in some capacity on
almost all platforms! The matrix of what's being tested, what works, and what's
supported where is evolving over time, and this document hopes to capture a
snapshot of what the current state of the world looks like.
All features of wasmtime should work on the following platforms:
- Linux x86_64
- Linux aarch64
- macOS x86_64
- Windows x86_64
For more detailed information about supported platforms, please check out the sections below!
JIT compiler support
The JIT compiler, backed by Cranelift, supports the x86_64 and aarch64 architectures at this time. Support for at least ARM and x86 is planned as well.
Usage of the JIT compiler will require a host operating system which supports
creating executable memory pages on-the-fly. In Rust terms this generally means
that std needs to be supported on this platform.
Interpreter support
At this time wasmtime does not have a mode in which it simply interprets
WebAssembly code. It is planned to add support for an interpreter, however, and
this will have minimal system dependencies. It is planned that the system will
need to support some form of dynamic memory allocation, but other than that not
much else will be needed.
What about #[no_std]?
The wasmtime project does not currently use #[no_std] for its crates, but
this is not because it won't support it! At this time we're still gathering use
cases for for what #[no_std] might entail, so if you're interested in this
we'd love to hear about your use case! Feel free to open an
issue on the
wasmtime repository to discuss this.
This is a common question we are asked, however, so to provide some more context
on why Wasmtime is the way it is, here's some responses to frequent points
raised about #![no_std]:
-
What if my platform doesn't have
std? - For platforms without support for the Rust standard library the JIT compiler of Wasmtime often won't run on the platform as well. The JIT compiler requiresmmap(or an equivalent), and presence ofmmapoften implies presence of a libc which means Rust'sstdlibrary works.Cargo's
-Z build-stdfeature feature is also intended to help easily build the standard library for all platforms. With this feature you can recompile the standard library (using Nightly Rust for now) with a custom target specification if necessary. Additionally the intention at this time is to getstdbuilding for all platforms, regardless of what the platform actually supports. This change is taking time to implement, but rust-lang/rust#74033 is an example of this support growing over time.We're also interested in running Wasmtime without a JIT compiler in the future, but that is not implemented at this time. Implementing this will require a lot more work than tagging crates
#![no_std]. The Wasmtime developers are also very interested in supporting as many targets as possible, so if Wasmtime doesn't work on your platform yet we'd love to learn why and what we can do to support that platform, but the conversation here is typically more nuanced than simply makingwasmtimecompile withoutstd. -
Doesn't
#![no_std]have smaller binary sizes? - There's a lot of factors that affect binary size in Rust. Compilation options are a huge one but beyond that idioms and libraries linked matter quite a lot as well. Code is not inherently large when usingstdinstead ofcore, it's just that often code usingstdhas more dependencies (likestd::thread) which requires code to bind. Code size improvements can be made to code usingstdandcoreequally, and switching to#![no_std]is not a silver bullet for compile sizes. -
The patch to switch to
#![no_std]is small, why not accept it? - PRs to switch to#![no_std]are often relatively small or don't impact too many parts of the system. There's a lot more to developing a#![no_std]WebAssembly runtime than switching a few crates, however. Maintaining a#![no_std]library over time has a number of costs associated with it:-
Rust has no stable way to diagnose
no_stderrors in an otherwisestdbuild, which means that to supoprt this feature it must be tested on CI with ano_stdtarget. This is costly in terms of CI time, CI maintenance, and developers having to do extra builds to avoid CI errors. Note that this isn't more costly than any other platform supported by Wasmtime, but it's a cost nonetheless. -
Idioms in
#![no_std]are quite different than normal Rust code. You'll import from different crates (coreinstead ofstd) and data structures have to all be manually imported fromalloc. These idioms are difficult to learn for newcomers to the project and are not well documented in the ecosystem. This cost of development and maintenance is not unique to Wasmtime but in general affects the#![no_std]ecosystem at large, unfortunately. -
Currently Wasmtime does not have a target use case which requires
#![no_std]support, so it's hard to justify these costs of development. We're very interested in supporting as many use cases and targets as possible, but the decision to support a target needs to take into account the costs associated so we can plan accordingly. Effectively we need to have a goal in mind instead of taking on the costs of#![no_std]blindly. -
At this time it's not clear whether
#![no_std]will be needed long-term, so eating short-term costs may not pay off in the long run. Features like Cargo's-Z build-stdmay mean that#![no_std]is less and less necessary over time.
-
-
How can Wasmtime support
#![no_std]if it uses X? - Wasmtime as-is today is not suitable for many#![no_std]contexts. For example it might usemmapfor allocating JIT code memory, leverage threads for caching, or use thread locals when calling into JIT code. These features are difficult to support in their full fidelity on all platforms, but the Wasmtime developers are very much aware of this! Wasmtime is intended to be configurable where many of these features are compile-time or runtime options. For example caches can be disabled, JITs can be removed and replaced with interpreters, or users could provide a callback to allocate memory instead of using the OS. This is sort of a long-winded way of saying that Wasmtime on the surface may today look like it won't support#![no_std], but this is almost always simply a matter of time and development priorities rather than a fundamental reason why Wasmtime couldn't support#![no_std].
Note that at this time these guidelines apply not only to Wasmtime but also to
some of its dependencies developed by the Bytecode Alliance such as the
wasm-tools repository. These
projects don't have the same runtime requirements as Wasmtime (e.g. wasmparser
doesn't need mmap), but we're following the same guidelines above at this
time. Patches to add #![no_std], while possibly small, incur many of the same
costs and also have an unclear longevity as features like -Z build-std evolve.
WebAssembly Proposals Support
The following table summarizes Wasmtime's support for WebAssembly proposals as
well as the command line flag and wasmtime::Config method you can
use to enable or disable support for a proposal.
If a proposal is not listed, then it is not supported by Wasmtime.
Wasmtime will never enable a proposal by default unless it has reached phase 4 of the WebAssembly standardizations process and its implementation in Wasmtime has been thoroughly vetted.
| WebAssembly Proposal | Supported in Wasmtime? | Command Line Flag | Config Method |
|---|---|---|---|
| Import and Export Mutable Globals | Yes. Always enabled. | (none) | (none) |
| Sign-Extension Operations | Yes. Always enabled. | (none) | (none) |
| Non-Trapping Float-to-Int Conversions | Yes. Always enabled. | (none) | (none) |
| Multi-Value | Yes. Enabled by default. | --enable-multi-value | wasm_multi_value |
| Bulk Memory Operations | Yes. Enabled by default. | --enable-bulk-memory | wasm_bulk_memory |
| Reference Types | Yes. Enabled by default. | --enable-reference-types | wasm_reference_types |
| Fixed-Width SIMD | In progress. | --enable-simd | wasm_simd |
| Threads and Atomics | In progress. | --enable-threads | wasm_threads |
| Multi-Memory | Yes. | --enable-multi-memory | wasm_multi_memory |
| Module Linking | Yes. | --enable-module-linking | wasm_module_linking |
Security
Please refer to the Bytecode Alliance security policy for details on how to report security issues in Wasmtime, our disclosure policy, and how to receive notifications about security issues.
Disclosure Policy
The disclosure policy for security issues in Wasmtime is documented on the Bytecode Alliance website.
Sandboxing
One of WebAssembly (and Wasmtime's) main goals is to execute untrusted code in a safe manner inside of a sandbox. WebAssembly is inherently sandboxed by design (must import all functionality, etc). This document is intended to cover the various sandboxing implementation strategies that Wasmtime has as they are developed.
At this time Wasmtime implements what's necessary for the WebAssembly specification, for example memory isolation between instances. Additionally the safe Rust API is intended to mitigate accidental bugs in hosts.
Different sandboxing implementation techniques will also come with different tradeoffs in terms of performance and feature limitations, and Wasmtime plans to offer users choices of which tradeoffs they want to make.
More will be added here over time!
WebAssembly Core
The core WebAssembly spec has several features which create a unique sandboxed environment:
-
The callstack is inaccessible. Unlike most native execution environments, return addresses from calls and spilled registers are not stored in memory accessible to applications. They are stored in memory that only the implementation has access to, which makes traditional stack-smashing attacks targeting return addresses impossible.
-
Pointers, in source languages which have them, are compiled to offsets into linear memory, so implementations details such as virtual addresses are hidden from applications. And all accesses within linear memory are checked to ensure they stay in bounds.
-
All control transfers—direct and indirect branches, as well as direct and indirect calls—are to known and type-checked destinations, so it's not possible to accidentally call into the middle of a function or branch outside of a function.
-
All interaction with the outside world is done through imports and exports. There is no raw access to system calls or other forms of I/O; the only thing a WebAssembly instance can do is what is available through interfaces it has been explicitly linked with.
-
There is no undefined behavior. Even where the WebAssembly spec permits multiple possible behaviors, it doesn't permit arbitrary behavior.
Filesystem Access
Wasmtime implements the WASI APIs for filesystem access, which follow a capability-based security model, which ensures that applications can only access files and directories they've been given access to. WASI's security model keeps users safe today, and also helps us prepare for shared-nothing linking and nanoprocesses in the future.
Wasmtime developers are intimately engaged with the WASI standards process, libraries, and tooling development, all along the way too.
Terminal Output
If untrusted code is allowed to print text which is displayed to a terminal, it may emit ANSI-style escape sequences and other control sequences which, depending on the terminal the user is using and how it is configured, can have side effects including writing to files, executing commands, injecting text into the stream as if the user had typed it, or reading the output of previous commands. ANSI-style escape sequences can also confuse or mislead users, making other vulnerabilities easier to exploit.
Our first priority is to protect users, so Wasmtime now filters writes to output streams when they are connected to a terminal to translate escape sequences into inert replacement sequences.
Some applications need ANSI-style escape sequences, such as terminal-based editors and programs that use colors, so we are also developing a proposal for the WASI Subgroup for safe and portable ANSI-style escape sequence support, which we hope to post more about soon.
Spectre
Wasmtime does not yet implement Spectre mitigations, however this is a subject of ongoing research.
Contributing
We're excited to work on Wasmtime and/or Cranelift together with you! This guide should help you get up and running with Wasmtime and Cranelift development. But first, make sure you've read the Code of Conduct!
Wasmtime and Cranelift are very ambitious projects with many goals, and while we're confident we can achieve some of them, we see many opportunities for people to get involved and help us achieve even more.
Join Our Chat
We chat about Wasmtime and Cranelift development on Zulip — join us!. You can also join specific streams:
If you're having trouble building Wasmtime or Cranelift, aren't sure why a test is failing, or have any other questions, feel free to ask on Zulip. Not everything we hope to do with these projects is reflected in the code or documentation yet, so if you see things that seem missing or that don't make sense, or even that just don't work the way you expect them to, we're also interested to hear about that!
As always, you're more than welcome to open an issue too!
Finding Something to Hack On
If you're looking for something to do, these are great places to start:
-
Issues labeled "good first issue" — these issues tend to be simple, what needs to be done is well known, and are good for new contributors to tackle. The goal is to learn Wasmtime's development workflow and make sure that you can build and test Wasmtime.
-
Issues labeled "help wanted" — these are issues that we need a little help with!
If you're unsure if an issue is a good fit for you or not, feel free to ask in a comment on the issue, or in chat.
Mentoring
We're happy to mentor people, whether you're learning Rust, learning about compiler backends, learning about machine code, learning about wasm, learning about how Cranelift does things, or all together at once.
We categorize issues in the issue tracker using a tag scheme inspired by Rust's issue tags. For example, the E-easy marks good beginner issues, and E-rust marks issues which likely require some familiarity with Rust, though not necessarily Cranelift-specific or even compiler-specific experience. E-compiler-easy marks issues good for beginners who have some familiarity with compilers, or are interested in gaining some :-).
See also the full list of Cranelift labels.
Also, we encourage people to just look around and find things they're interested in. This a good time to get involved, as there aren't a lot of things set in stone yet.
Architecture of Wasmtime
This document is intended to give an overview of the implementation of Wasmtime.
This will explain the purposes of the various wasmtime-* crates that the main
wasmtime crate depends on. For even more detailed information it's recommended
to review the code itself and find the comments contained within.
The wasmtime crate
The main entry point for Wasmtime is the wasmtime crate itself. Wasmtime is
designed such that the wasmtime crate is nearly a 100% safe API (safe in the
Rust sense) modulo some small and well-documented functions as to why they're
unsafe. The wasmtime crate provides features and access to WebAssembly
primitives and functionality, such as compiling modules, instantiating them,
calling functions, etc.
At this time the wasmtime crate is the first crate that is intended to be
consumed by users. First in this sense means that everything wasmtime depends
on is thought of as an internal dependency. We publish crates to crates.io but
put very little effort into having a "nice" API for internal crates or worrying
about breakage between versions of internal crates. This primarily means that
all the other crates discussed here are considered internal dependencies of
Wasmtime and don't show up in the public API of Wasmtime at all. To use some
Cargo terminology, all the wasmtime-* crates that wasmtime depends on are
"private" dependencies.
Additionally at this time the safe/unsafe boundary between Wasmtime's internal
crates is not the most well-defined. There are methods that should be marked
unsafe which aren't, and unsafe methods do not have exhaustive documentation
as to why they are unsafe. This is an ongoing matter of improvement, however,
where the goal is to have safe methods be actually safe in the Rust sense,
as well as having documentation for unsafe methods which clearly lists why
they are unsafe.
Important concepts
To preface discussion of more nitty-gritty internals, it's important to have a few concepts in the back of your head. These are some of the important types and their implications in Wasmtime:
-
wasmtime::Engine- this is a global compilation context which is sort of the "root context". AnEngineis typically created once per program and is expected to be shared across many threads (internally it's atomically reference counted). EachEnginestores configuration values and other cross-thread data such as type interning forModuleinstances. The main thing to remember forEngineis that any mutation of its internals typically involves acquiring a lock, whereas forStorebelow no locks are necessary. -
wasmtime::Store- this is the concept of a "store" in WebAssembly. While there's also a formal definition to go of this it can be thought of as a bag of related WebAssembly objects. This includes instances, globals, memories, tables, etc. AStoredoes not implement any form of garbage collection of the internal items (there is agcfunction but that's just forexternrefvalues). This means that once you create anInstanceor aTablethe memory is not actually released until theStoreitself is deallocated. AStoreis sort of a "context" used for almost all wasm operations.Storealso contains instance handles which recursively refer back to theStore, leading to a good bit of aliasing of pointers within theStore. The important thing for now, though, is to know thatStoreis a unit of isolation. WebAssembly objects are always entirely contained within aStore, and at this time nothing can cross between stores (except scalars if you manually hook it up). In other words, wasm objects from different stores cannot interact with each other. AStorecannot be used simultaneously from mulitple threads (almost all operations require&mut self). -
wasmtime_runtime::InstanceHandle- this is the low-level representation of a WebAssembly instance. At the same time this is also used as the representation for all host-defined object. For example if you callwasmtime::Memory::newit'll create anInstanceHandleunder the hood. This is a veryunsafetype that should probably have all of its functions markedunsafeor otherwise have more strict guarantees documented about it, but it's an internal type that we don't put much thought into for public consumption at this time. AnInstanceHandledoesn't know how to deallocate itself and relies on the caller to manage its memory. Currently this is either allocated on-demand (withmalloc) or in a pooling fashion (using the pooling allocator). Thedeallocatemethod is different in these two paths (as well as theallocatemethod).An
InstanceHandleis laid out in memory with some Rust-owned values first capturing the dynamic state of memories/tables/etc. Most of these fields are unused for host-defined objects that serve one purpose (e.g. awasmtime::Table::new), but for an instantiated WebAssembly module these fields will have more information. After anInstanceHandlein memory is aVMContext, which will be discussed next.InstanceHandlevalues are the main internal runtime representation and what thewasmtime_runtimecrate works with. Thewasmtime::Storeholds onto all theseInstanceHandlevalues and deallocates them at the appropriate time. From the runtime perspective it simplifies things so the graph of wasm modules communicating to each other is reduced to simplyInstanceHandlevalues all talking to themselves. -
wasmtime_runtime::VMContext- this is a raw pointer, within an allocation of anInstanceHandle, that is passed around in JIT code. AVMContextdoes not have a structure defined in Rust (it's a 0-sized structure) because its contents are dynamically determined based on theVMOffsets, or the source wasm module it came from. EachInstanceHandlehas a "shape" of aVMContextcorresponding with it. For example aVMContextstores all values of WebAssembly globals, but if a wasm module has no globals then the size of this array will be 0 and it won't be allocated. The intention of aVMContextis to be an efficient in-memory representation of all wasm module state that JIT code may access. The layout ofVMContextis dynamically determined by a module and JIT code is specialized for this one structure. This means that the structure is efficiently accessed by JIT code, but less efficiently accessed by native host code. A non-exhaustive list of purposes of theVMContextis to:- Store WebAssembly instance state such as global values, pointers to tables, pointers to memory, and pointers to other JIT functions.
- Separate wasm imports and local state. Imported values have pointers stored to their actual values, and local state has the state defined inline.
- Hold a pointer to the stack limit at which point JIT code will trigger a stack overflow.
- Hold a pointer to a
VMExternRefActivationsTablefor fast-path insertion ofexternrefvalues into the table. - Hold a pointer to a
*mut dyn wasmtime_runtime::Storeso store-level operations can be performed in libcalls.
A comment about the layout of a
VMContextcan be found in thevmoffsets.rsfile. -
wasmtime::Module- this is the representation of a compiled WebAssembly module. At this time Wasmtime always assumes that a wasm module is always compiled to native JIT code.Moduleholds the results of said compilation, and currently the Cranelift and Lightbeam modes can be used for compiling. It is a goal of Wasmtime to support other modes of representing modules but those are not implemented today just yet. Additionally the Lightbeam compiler has not received maintenance in a long time, so effectively everyone uses Cranelift. -
wasmtime_environ::Module- this is a descriptor of a wasm module's type and structure without holding any actual JIT code. An instance of this type is created very early on in the compilation process, and it is not modified when functions themselves are actually compiled. This holds the internal type representation and state about functions, globals, etc. In a sense this can be thought of as the result of validation or typechecking a wasm module, although it doesn't have information such as the types of each opcode or minute function-level details like that.
Compiling a module
With a high-level overview and some background information of types, this will
next walk through the steps taken to compile a WebAssembly module. The main
entry point for this is the wasmtime::Module::from_binary API. There are a
number of other entry points that deal with surface-level details like
translation from text-to-binary, loading from the filesystem, etc.
Compilation is roughly broken down into a few phases:
-
First compilation walks over the WebAssembly module validating everything except function bodies. This synchronous pass over a wasm module creates a
wasmtime_environ::Moduleinstance and additionally prepares for function compilation. Note that with the module linking proposal one input module may end up creating a number of output modules to process. Each module is processed independently and all further steps are parallelized on a per-module basis. Note that parsing and validation of the WebAssembly module happens with thewasmparsercrate. Validation is interleaved with parsing, validating parsed values before using them. -
Next all functions within a module are validated and compiled in parallel. No inter-procedural analysis is done and each function is compiled as its own little island of code at this time. This is the point where the meat of Cranelift is invoked on a per-function basis.
-
The compilation results at this point are all woven into a
wasmtime_jit::CompilationArtifactsstructure. This holds module information (wasmtime_environ::Module), compiled JIT code (stored as an ELF image), and miscellaneous other information about functions such as platform-agnostic unwinding information, per-function trap tables (indicating which JIT instructions can trap and what the trap means), per-function address maps (mapping from JIT addresses back to wasm offsets), and debug information (parsed from DWARF information in the wasm module). These results are inert and can't actually be executed, but they're appropriate at this point to serialize to disk or begin the next phase... -
The final step is to actually place all code into a form that's ready to get executed. This starts from the
CompilationArtifactsof the previous step. Here a new memory mapping is allocated and the JIT code is copied into this memory mapping. This memory mapping is then switched from read/write to read/execute so it's actually executable JIT code at this point. This is where various hooks like loading debuginfo, informing JIT profilers of new code, etc, all happens. At this point awasmtime_jit::CompiledModuleis produced and this is itself wrapped up in awasmtime::Module. At this point the module is ready to be instantiated.
A wasmtime::Module is an atomically-reference-counted object where upon
instantiation into a Store the Store will hold a strong reference to the
internals of the module. This means that all instances of a wasmtime::Module
share the same compiled code. Additionally a wasmtime::Module is one of the
few objects that lives outside of a wasmtime::Store. This means that
wasmtime::Module's reference counting is its own form of memory management.
Note that the property of sharing a module's compiled code across all
instantiations has interesting implications on what the compiled code can
assume. For example Wasmtime implements a form of type interning, but the
interned types happen at a few different levels. Within a module we deduplicate
function types, but across modules in a Store types need to be represented
with the same value. This means that if the same module is instantiated into
many stores its same function type may take on many values, so the compiled
code can't assume a particular value for a function type. (more on type
information later). The general gist though is that compiled code leans
relatively heavily on the VMContext for contextual input because the JIT code
is intended to be so widely reusable.
Trampolines
An important aspect to also cover for compilation is the creation of trampolines. Trampolines in this case refer to code executed by Wasmtime to enter WebAssembly code. The host may not always have prior knowledge about the signature of the WebAssembly function that it wants to call. Wasmtime JIT code is compiled with native ABIs (e.g. params/results in registers according to System V on Unix), which means that a Wasmtime embedding doesn't have an easy way to enter JIT code.
This problem is what the trampolines compiled into a module solve, which is to
provide a function with a known ABI that will call into a function with a
specific other type signature/ABI. Wasmtime collects all the exported functions
of a module and creates a set of their type signatures. Note that exported in
this context actually means "possibly exported" which includes things like
insertion into a global/function table, conversion to a funcref, etc. A
trampoline is generated for each of these type signatures and stored along with
the JIT code for the rest of the module.
These trampolines are then used with the wasmtime::Func::call API where in
that specific case because we don't know the ABI of the target function the
trampoline (with a known ABI) is used and has all the parameters/results passed
through the stack.
Another point of note is that trampolines are not deduplicated at this time. Each compiled module contains its own set of trampolines, and if two compiled modules have the same types then they'll have different copies of the same trampoline.
Type Interning and VMSharedSignatureIndex
One important point to talk about with compilation is the
VMSharedSignatureIndex type and how it's used. The call_indirect opcode in
wasm compares an actual function's signature against the function signature of
the instruction, trapping if the signatures mismatch. This is implemented in
Wasmtime as an integer comparison, and the comparison happens on a
VMSharedSignatureIndex value. This index is an intern'd representation of a
function type.
The scope of interning for VMSharedSignatureIndex happens at the
wasmtime::Engine level. Modules are compiled into an Engine. Insertion of a
Module into an Engine will assign a VMSharedSignatureIndex to all of the
types found within the module.
The VMSharedSignatureIndex values for a module are local to that one
instantiation of a Module (and they may change on each insertion of a
Module into a different Engine). These are used during the instantiation
process by the runtime to assign a type ID effectively to all functions for
imports and such.
Instantiating a module
Once a module has been compiled it's typically then instantiated to actually
get access to the exports and call wasm code. Instantiation always happens
within a wasmtime::Store and the created instance (plus all exports) are tied
to the Store.
Instantiation itself (crates/wasmtime/src/instance.rs) may look complicated,
but this is primarily due to the implementation of the Module Linking proposal.
The rough flow of instantiation looks like:
-
First all imports are type-checked. The provided list of imports is cross-referenced with the list of imports recorded in the
wasmtime_environ::Moduleand all types are verified to line up and match (according to the core wasm specification's definition of type matching). -
Each
wasmtime_environ::Modulehas a list of initializers that need to be completed before instantiation is finished. For MVP wasm this only involves loading the import into the correct index array, but for module linking this could involve instantiating other modules, handlingaliasfields, etc. In any case the result of this step is awasmtime_runtime::Importsarray which has the values for all imported items into the wasm module. Note that in this case an import is typically some sort of raw pointer to the actual state plus theVMContextof the instance that was imported from. The final result of this step is anInstanceAllocationRequest, which is then submitted to the configured instance allocator, either on-demand or pooling. -
The
InstanceHandlecorresponding to this instance is allocated. How this is allocated depends on the strategy (malloc for on-demand, slab allocation for pooling). In addition to initialization of the fields ofInstanceHandlethis also initializes all the fields of theVMContextfor this handle (which as mentioned above is adjacent to theInstanceHandleallocation after it in memory). This does not process any data segments, element segments, or thestartfunction at this time. -
At this point the
InstanceHandleis stored within theStore. This is the "point of no return" where the handle must be kept alive for the same lifetime as theStoreitself. If an initialization step fails then the instance may still have had its functions, for example, inserted into an imported table via an element segment. This means that even if we fail to initialize this instance its state could still be visible to other instances/objects so we need to keep it alive regardless. -
The final step is performing wasm-defined instantiation. This involves processing element segments, data segments, the
startfunction, etc. Most of this is just translating from Wasmtime's internal representation to the specification's required behavior.
Another part worth pointing out for instantiating a module is that a
ModuleRegistry is maintained within a Store of all instantiated modules
into the store. The purpose of this registry is to retain a strong reference to
items in the module needed to run instances. This includes the JIT code
primarily but also has information such as the VMSharedSignatureIndex
registration, metadata about function addresses and such, etc. Much of this
data is stored into a GLOBAL_MODULES map for later access during traps.
Traps
Once instances have been created and wasm starts running most things are fairly
standard. Trampolines are used to enter wasm (or we can enter with a known ABI
if using wasmtime::TypedFunc) and JIT code generally does what it does to
execute wasm. An important aspect of the implementation to cover, however, is
traps.
Wasmtime today implements traps with longjmp and setjmp. The setjmp
function cannot be defined in Rust (even unsafely --
(https://github.com/rust-lang/rfcs/issues/2625) so the
crates/runtime/src/helpers.c file actually calls setjmp/longjmp. Note that in
general the operation of longjmp is not safe to execute in Rust because it
skips stack-based destructors, so after setjmp when we call back into Rust to
execute wasm we need to be careful in Wasmtime to not have any significant
destructors on the stack once wasm is called.
Traps can happen from a few different sources:
-
Explicit traps - these can happen when a host call returns a trap, for example. These bottom out in
raise_user_traporraise_lib_trap, both of which immediately calllongjmpto go back to the wasm starting point. Note that these, like when calling wasm, have to have callers be very careful to not have any destructors on the stack. -
Signals - this is the main vector for trap. Basically we use segfault and illegal instructions to implement traps in wasm code itself. Segfaults arise when linear memory accesses go out of bounds and illegal instructions are how the wasm
unreachableinstruction is implemented. In both of these cases Wasmtime installs a platform-specific signal handler to catch the signal, inspect the state of the signal, and then handle it. Note that Wasmtime tries to only catch signals that happen from JIT code itself as to not accidentally cover up other bugs. Exiting a signal handler happens vialongjmpto get back to the original wasm call-site.
The general idea is that Wasmtime has very tight control over the stack frames
of wasm (naturally via Cranelift) and also very tight control over the code that
executes just before we enter wasm (aka before the setjmp) and just after we
reenter back into wasm (aka frames before a possible longjmp).
The signal handler for Wasmtime uses the GLOBAL_MODULES map populated during
instantiation to determine whether a program counter that triggered a signal is
indeed a valid wasm trap. This should be true except for cases where the host
program has another bug that triggered the signal.
A final note worth mentioning is that Wasmtime uses the Rust backtrace crate
to capture a stack trace when a wasm exception occurs. This forces Wasmtime to
generate native platform-specific unwinding information to correctly unwind the
stack and generate a stack trace for wasm code. This does have other benefits as
well such as improving generic sampling profilers when used with Wasmtime.
Linear Memory
Linear memory in Wasmtime is implemented effectively with mmap (or the
platform equivalent thereof), but there are some subtle nuances that are worth
pointing out here too. The implementation of linear memory is relatively
configurable which gives rise to a number of situations that both the runtime
and generated code need to handle.
First there are a number of properties about linear memory which can be configured:
wasmtime::Config::static_memory_maximum_sizewasmtime::Config::static_memory_guard_sizewasmtime::Config::dynamic_memory_guard_sizewasmtime::Config::guard_before_linear_memory
The methods on Config have a good bit of documentation to go over some
nitty-gritty, but the general gist is that Wasmtime has two modes of memory:
static and dynamic. Static memories represent an address space reservation that
never moves and pages are committed to represent memory growth. Dynamic
memories represent allocations where the committed portion exactly matches the
wasm memory's size and growth happens by allocating a bigger chunk of memory.
The guard size configuration indicates the size of the guard region that happens after linear memory. This guard size affects whether generated JIT code emits bounds checks or not. Bounds checks are elided if out-of-bounds addresses provably encounter the guard pages.
The guard_before_linear_memory configuration additionally places guard pages
in front of linear memory as well as after linear memory (the same size on both
ends). This is only used to protect against possible Cranelift bugs and
otherwise serves no purpose.
At the time of this writing Wasmtime only supports WebAssembly with 32-bit memories, so a maximum of 4GB in size. Wasmtime has not implemented the memory64 proposal from upstream WebAssembly yet.
The defaults for Wasmtime on 64-bit platforms are:
- 4GB static maximum size (meaning all memories are static)
- 2GB static guard size (meaning all loads/stores with less than 2GB offset don't need bounds checks)
- Guard pages before linear memory are enabled.
Altogether this means that linear memories result in an 8GB virtual address space reservation by default in Wasmtime. With the pooling allocator where we know that linear memories are contiguous this results in a 6GB reservation per memory because the guard region after one memory is the guard region before the next.
Tables and externref
WebAssembly tables contain reference types, currently either funcref or
externref. A funcref in Wasmtime is represented as *mut VMCallerCheckedAnyfunc and an externref is represented as VMExternRef
(which is internally *mut VMExternData). Tables are consequently represented
as vectors of pointers. Table storage memory management by default goes through
Rust's Vec which uses malloc and friends for memory. With the pooling
allocator this uses preallocated memory for storage.
As mentioned previously Store has no form of internal garbage
collection for wasm objects themselves so a funcref table in wasm is pretty
simple in that there's no lifetime management of any of the pointers stored
within, they're simply assumed to be valid for as long as the table is in use.
For tables of externref the story is more complicated. The VMExternRef is a
version of Arc<dyn Any> but specialized in Wasmtime so JIT code knows where
the offset of the reference count field to directly manipulate it is.
Furthermore tables of externref values need to manage the reference count
field themselves, since the pointer stored in the table is required to have a
strong reference count allocated to it.
GC and externref
Wasmtime implements the externref type of WebAssembly with an
atomically-reference-counted pointer. Note that the atomic part is not needed
by wasm itself but rather from the Rust embedding environment where it must be
safe to send ExternRef values to other threads. Wasmtime also does not
come with a cycle collector so cycles of host-allocated ExternRef objects
will leak.
Despite reference counting, though, a Store::gc method exists. This is an
implementation detail of how reference counts are managed while wasm code is
executing. Instead of managing the reference count of an externref value
individually as it moves around on the stack Wasmtime implements "deferred
reference counting" where there's an overly conservative list of ExternRef
values that may be in use, and periodically a GC is performed to make this
overly conservative list a precise one. This leverages the stack map support
of Cranelift plus the backtracing support of backtrace to determine live
roots on the stack. The Store::gc method forces the
possibly-overly-conservative list to become a precise list of externref
values that are actively in use on the stack.
Index of crates
The main Wasmtime internal crates are:
wasmtime- the safe public API ofwasmtime.wasmtime-jit- JIT-specific support for Wasmtime. This is the concrete implementation that manages executable memory generated at Runtime. Currently all modules are compiled this way, but one day Wasmtime may be able to be compiled without JIT support where only precompiled modules can be loaded.wasmtime-runtime- low-level runtime implementation of Wasmtime. This is whereVMContextandInstanceHandlelive. This crate is theoretically agnostic to how JIT code was compiled and the runtime that it's running within.wasmtime-environ- low-level compilation support. This is where translation of theModuleand its environment happens, although no compilation actually happens in this crate (although it defines an interface for compilers). The results of this crate are handed off to other crates for actual compilation.wasmtime-cranelift- implementation of function-level compilation using Cranelift.
Note that at this time Cranelift is a required dependency of wasmtime. Most of
the types exported from wasmtime-environ use cranelift types in their API. One
day it's a goal, though, to remove the required cranelift dependency and have
wasmtime-environ be a relatively standalone crate.
In addition to the above crates there are some other miscellaneous crates that
wasmtime depends on:
wasmtime-cache- optional dependency to manage default caches on the filesystem. This is enabled in the CLI by default but not enabled in thewasmtimecrate by default.wasmtime-fiber- implementation of stack-switching used byasyncsupport in Wasmtimewasmtime-debug- implementation of mapping wasm dwarf debug information to native dwarf debug information.wasmtime-profiling- implementation of hooking up generated JIT code to standard profiling runtimes.wasmtime-obj- implementation of creating an ELF image from compiled functions.
Building
This section describes everything required to build and run Wasmtime.
Prerequisites
Before we can actually build Wasmtime, we'll need to make sure these things are installed first.
Git Submodules
The Wasmtime repository contains a number of git submodules. To build Wasmtime and most other crates in the repository, you have to ensure that those are initialized with this command:
git submodule update --init
The Rust Toolchain
Install the Rust toolchain here. This
includes rustup, cargo, rustc, etc...
libclang (optional)
The wasmtime-fuzzing crate transitively depends on bindgen, which requires
that your system has a libclang installed. Therefore, if you want to hack on
Wasmtime's fuzzing infrastructure, you'll need libclang. Details on how to
get libclang and make it available for bindgen are
here.
Building the wasmtime CLI
To make an unoptimized, debug build of the wasmtime CLI tool, go to the root
of the repository and run this command:
cargo build
The built executable will be located at target/debug/wasmtime.
To make an optimized build, run this command in the root of the repository:
cargo build --release
The built executable will be located at target/release/wasmtime.
You can also build and run a local wasmtime CLI by replacing cargo build
with cargo run.
Building the Wasmtime C API
To build the C API of Wasmtime you can run:
cargo build --release --manifest-path crates/c-api/Cargo.toml
This will place the shared library inside of target/release. On Linux it will
be called libwasmtime.{a,so}, on macOS it will be called
libwasmtime.{a,dylib}, and on Windows it will be called
wasmtime.{lib,dll,dll.lib}.
Building Other Wasmtime Crates
You can build any of the Wasmtime crates by appending -p wasmtime-whatever to
the cargo build invocation. For example, to build the wasmtime-jit crate,
execute this command:
cargo build -p wasmtime-jit
Alternatively, you can cd into the crate's directory, and run cargo build
there, without needing to supply the -p flag:
cd crates/jit/
cargo build
Cross Compiling Wasmtime
By default cargo build will build Wasmtime for the platform you're running the
build on. You might, however, want to build Wasmtime for a different platform!
Let's say for example that you want to build Wasmtime for
aarch64-unknown-linux-gnu. First you'll want to acquire the Rust standard
library for this target:
rustup target add aarch64-unknown-linux-gnu
Next you need to install a native C toolchain which has a C compiler, runtime libraries, and linker for the desired target. This is unfortunately not very easy to acquire on most platforms:
-
On Windows you can install build tools for AArch64 Windows, but targeting platforms like Linux or macOS is not easy. While toolchains exist for targeting non-Windows platforms you'll have to hunt yourself to find the right one.
-
On macOS you can install, through Xcode, toolchains for iOS but the main
x86_64-apple-darwinis really the only easy target to install. You'll need to hunt for toolchains if you want to compile for Linux or Windows. -
On Linux you can relatively easily compile for other Linux architectures most of the time. For example on Debian-based distributions you can install the
gcc-aarch64-linux-gnupackage which should come with the C compiler, runtime libraries, and linker all in one (assuming you don't explicitly request disabling recommended packages). Other Linux distributions may have differently named toolchains. Compiling for macOS from Linux will require finding your own toolchain. Compiling for Windows MSVC will require finding your own toolchain, but compiling for MinGW can work easily enough if you install the MinGW toolchain via your package manager.
For now we'll assume you're on Linux compiling for a different Linux
architecture. Once you've got the native toolchain, you'll want to find the C
compiler that came with it. On Debian, for example, this is called
aarch64-linux-gnu-gcc. Next up you'll need to configure two environment
variables to configure the Rust build:
export CARGO_TARGET_AARCH64_UNKNOWN_LINUX_GNU_LINKER=aarch64-linux-gnu-gcc
export CC_aarch64_unknown_linux_gnu=aarch64-linux-gnu-gcc
The first environment variable tells Cargo to tell rustc what the correct linker
for your target is. The second configures the cc Rust
crate for C code compiled as part of the build.
Finally you can execute.
cargo build --target aarch64-unknown-linux-gnu --release
The built executable will be located at
target/aarch64-unknown-linux-gnu/release/wasmtime. Note that you can
cross-compile the C API in the same manner as the CLI too.
Note that if you are using these invocations regularly, you can avoid the need
to set environment variables by adding some configuration to your persistent
Cargo configuration. In the file ~/.cargo/config.toml (in your home
directory), add the section:
[target.aarch64-unknown-linux-gnu]
linker = 'aarch64-linux-gnu-gcc'
Then the above cargo build --target aarch64-unknown-linux-gnu command should
work without setting any extra environment variables beforehand.
Running a Cross-Compiled Wasmtime in qemu (emulation)
Once you have cross-compiled a binary, it is possible to run it on an emulator
if you do not have access to (or do not wish to use) hardware with the given
architecture. This can be done using an emulator such as qemu. The qemu
user-space emulation support allows running, for example, a Linux/aarch64
binary on a Linux/x86-64 host, as long as you have the system libraries for
aarch64 as well.
To try this out, first install qemu, making sure that the user-space emulator
option for your target architecture is enabled. On Debian-based Linux
distributions (including Ubuntu), this is in the qemu-user package, for
example.
Next, make sure that you have system libraries for the target. You will already have these present if you cross-compiled as described above.
Finally, you can run the wasmtime binary under qemu; the following example
is for an aarch64 target. Adjust the library paths as appropriate; these are
correct for Ubuntu/Debian's cross-compilation packages.
qemu-aarch64 \
-L /usr/aarch64-linux-gnu \
-E LD_LIBRARY_PATH=/usr/aarch64-linux-gnu/lib \
target/aarch64-unknown-linux-gnu/release/wasmtime [ARGS]
You can add this to your persistent Cargo configuration as well. Extending the
above example in ~/.cargo/config.toml, you can add:
[target.aarch64-unknown-linux-gnu]
linker = 'aarch64-linux-gnu-gcc'
runner = "qemu-aarch64 -L /usr/aarch64-linux-gnu -E LD_LIBRARY_PATH=/usr/aarch64-linux-gnu/lib"
Then a simple cargo test --target aarch64-unknown-linux-gnu should work.
Testing
This section describes how to run Wasmtime's tests and add new tests.
Before continuing, make sure you can build Wasmtime successfully. Can't run the tests if you can't build it!
Running All Tests
To run all of Wasmtime's tests (excluding WASI integration tests), execute this command:
cargo test --all
To include WASI integration tests, you'll need wasm32-wasi target installed, which,
assuming you're using rustup.rs to manage your Rust versions,
can be done as follows:
rustup target add wasm32-wasi
Next, to run all tests including the WASI integration tests, execute this command:
cargo test --features test-programs/test_programs --all
You can also exclude a particular crate from testing with --exclude. For
example, if you want to avoid testing the wastime-fuzzing crate — which
requires that libclang is installed on your system, and for some reason maybe
you don't have it — you can run:
cargo test --all --exclude wasmtime-fuzzing
Testing a Specific Crate
You can test a particular Wasmtime crate with cargo test -p wasmtime-whatever. For example, to test the wasmtime-environ crate, execute
this command:
cargo test -p wasmtime-environ
Alternatively, you can cd into the crate's directory, and run cargo test
there, without needing to supply the -p flag:
cd crates/environ/
cargo test
Running the Wasm Spec Tests
The spec testsuite itself is in a git submodule, so make sure you've checked it out and initialized its submodule:
git submodule update --init
When the submodule is checked out, Wasmtime runs the Wasm spec testsuite as part
of testing the wasmtime-cli crate:
cargo test -p wasmtime-cli
Running WASI Integration Tests Only
WASI integration tests can be run separately from all other tests which
can be useful when working on the wasi-common crate. This can be done by
executing this command:
cargo test --features test-programs/test_programs -p test-programs
Adding New Tests
Adding Rust's #[test]-Style Tests
For very "unit-y" tests, we add test modules in the same .rs file as the
code that is being tested. These test modules are configured to only get
compiled during testing with #[cfg(test)].
#![allow(unused)] fn main() { // some code... #[cfg(test)] mod tests { use super::*; #[test] fn some_test_for_that_code() { // ... } } }
If you're writing a unit test and a test module doesn't already exist, you can
create one.
For more "integration-y" tests, we create a tests directory within the crate,
and put the tests inside there. For example, there are various code
cache-related tests at crates/environ/tests/cache_*.rs. Always feel free to
add a tests directory to a crate, if you want to add a new test and there
aren't any existing tests.
Adding Specification-Style Wast Tests
We use the spec testsuite as-is and without custom patches or a forked version. This probably isn't what you want to modify when adding a new Wasmtime test!
When you have a Wasmtime-specific test that you'd like to write in Wast and use
the Wast-style assertions, you can add it to our "misc testsuite". The misc
testsuite uses the same syntax and assertions as the spec testsuite, but lives
in tests/misc_testsuite. Feel free to add new tests to existing
tests/misc_testsuite/*.wast files or create new ones as needed. These tests
are run as part of the wasmtime-cli crate's tests.
If you have a new test that you think really belongs in the spec testsuite, make sure it makes sense for every Wasm implementation to run your test (i.e. it isn't Wasmtime-specific) and send a pull request upstream. Once it is accepted in the upstream repo, we can update our git submodule and we'll start running the new tests.
Adding WASI Integration Tests
When you have a WASI-specific test program that you'd like to include as a
test case to run against our WASI implementation, you can add it to our
test-programs crate. In particular, you should drop a main-style Rust source
file into crates/test-programs/wasi-tests/src/bin/some_new_test.rs with a
name of your choice. And that's it! The build script included in the
test-programs crate will automatically generate the necessary boilerplate
code for your test program so that it's run on all supported hosts.
If you would like to tweak which host to run the test program against however
(for instance, only on Unix but on Windows), you can tweak that in the build
script located under crates/test-programs/build.rs.
Fuzzing
Test Case Generators and Oracles
Test case generators and oracles live in the wasmtime-fuzzing crate, located
in the crates/fuzzing directory.
A test case generator takes raw, unstructured input from a fuzzer and translates that into a test case. This might involve interpreting the raw input as "DNA" or pre-determined choices through a decision tree and using it to generate an in-memory data structure, or it might be a no-op where we interpret the raw bytes as if they were Wasm.
An oracle takes a test case and determines whether we have a bug. For example, one of the simplest oracles is to take a Wasm binary as an input test case, validate and instantiate it, and (implicitly) check that no assertions failed or segfaults happened. A more complicated oracle might compare the result of executing a Wasm file with and without optimizations enabled, and make sure that the two executions are observably identical.
Our test case generators and oracles strive to be fuzzer-agnostic: they can be reused with libFuzzer or AFL or any other fuzzing engine or driver.
libFuzzer and cargo fuzz Fuzz Targets
We combine a test case generator and one more more oracles into a fuzz target. Because the target needs to pipe the raw input from a fuzzer into the test case generator, it is specific to a particular fuzzer. This is generally fine, since they're only a couple of lines of glue code.
Currently, all of our fuzz targets are written for
libFuzzer and cargo fuzz. They are defined in
the fuzz subdirectory.
See
fuzz/README.md
for details on how to run these fuzz targets and set up a corpus of seed inputs.
Continuous Integration (CI)
The Wasmtime and Cranelift projects heavily rely on Continuous Integration (CI) to ensure everything keeps working and keep the final end state of the code at consistently high quality. The CI setup for this repository is relatively involved and extensive, and so it's worth covering here how it's organized and what's expected of contributors.
All CI currently happens on GitHub Actions and is configured in the .github
directory of the repository.
PRs and CI
Currently the full CI test suite runs on every Pull Request. All PRs need to have that lovely green checkmark before being candidates for being merged. If a test is failing you'll want to check out the logs on CI and fix it before the PR can be merged.
PR authors are expected to fix CI failures in their PR, unless the CI failure is systemic and unrelated to the PR. In that case other maintainers should be alerted to ensure that the problem can be addressed.
Tests run on CI
While this may not be fully exhaustive, the general idea of all the checks we run on CI looks like this:
-
Code formatting - we run
cargo fmt -- --checkon CI to ensure that all code in the repository is formatted with rustfmt. All PRs are expected to be formatted with the latest stable version of rustfmt. -
Book documentation tests - code snippets (Rust ones at least) in the book documentation (the
docsfolder) are tested on CI to ensure they are working. -
Crate tests - the moral equivalent of
cargo test --allandcargo test --all --releaseis executed on CI. This means that all workspace crates have their entire test suite run, documentation tests and all, in both debug and release mode. Additionally we execute all crate tests on macOS, Windows, and Linux, to ensure that everything works on all the platforms. -
Fuzz regression tests - we take a random sampling of the fuzz corpus and run it through the fuzzers. This is mostly intended to be a pretty quick regression test and testing the fuzzers still build, most of our fuzzing happens on oss-fuzz. Found issues are recorded in the oss-fuzz bug tracker
While we do run more tests here and there, this is the general shape of what you can be expected to get tested on CI for all commits and all PRs. You can of course always feel free to expand our CI coverage by editing the CI files themselves, we always like to run more tests!
Artifacts produced on CI
Our CI system is also responsible for producing all binary releases and documentation of Wasmtime and Cranelift. Currently this consists of:
-
Tarballs of the
wasmtimeCLI - produced for macOS, Windows, and Linux we try to make these "binary compatible" wherever possible, for example producing the Linux build in a really old CentOS container to have a very low glibc requirement. -
Tarballs of the Python extension - also produced on the main three platforms these wheels are compiled on each commit.
-
Book and API documentation - the book is rendered with
mdbookand we also build all documentation withcargo doc.
Artifacts are produced for every single commit and every single PR. You should be able to find a downloadable version of all artifacts produced on the "runs" page in GitHub Actions. For example here's an example job, and if you're looking at a specific builder you can see the artifacts link in the top right. Note that artifacts don't become available until the whole run finishes.
Commits merged into the main branch will rerun CI and will also produce
artifacts as usual. On the main branch, however, documentation is pushed to
the gh-pages branch as well, and binaries are pushed to the dev release on
GitHub. Finally, tagged commits get a whole dedicated release to them too.
Coding guidelines
For the most part, Wasmtime and Cranelift follow common Rust conventions and pull request (PR) workflows, though we do have a few additional things to be aware of.
rustfmt
All PRs must be formatted according to rustfmt, and this is checked in the continuous integration tests. You can format code locally with:
$ cargo fmt
at the root of the repository. You can find more information about rustfmt online too, such as how to configure your editor.
Minimum Supported rustc Version
Wasmtime supports the current stable version of Rust.
Cranelift supports stable Rust, and follows the Rust Update Policy for Firefox.
Some of the developer scripts depend on nightly Rust, for example to run clippy and other tools, however we avoid depending on these for the main build.
Development Process
We use issues for asking questions (open one here!) and tracking bugs and unimplemented features, and pull requests (PRs) for tracking and reviewing code submissions.
Before submitting a PR
Consider opening an issue to talk about it. PRs without corresponding issues are appropriate for fairly narrow technical matters, not for fixes to user-facing bugs or for feature implementations, especially when those features might have multiple implementation strategies that usefully could be discussed.
Our issue templates might help you through the process.
When submitting PRs
-
Please fill in the pull request template as appropriate. It is usually helpful, it speeds up the review process and helps understanding the changes brought by the PR.
-
Write clear commit messages that start with a one-line summary of the change (and if it's difficult to summarize in one line, consider splitting the change into multiple PRs), optionally followed by additional context. Good things to mention include which areas of the code are affected, which features are affected, and anything that reviewers might want to pay special attention to.
-
If there is code which needs explanation, prefer to put the explanation in a comment in the code, or in documentation, rather than in the commit message.
-
For pull requests that fix existing issues, use issue keywords. Note that not all pull requests need to have accompanying issues.
-
Assign the review to somebody from the Core Team, either using suggestions in the list proposed by Github, or somebody else if you have a specific person in mind.
-
When updating your pull request, please make sure to re-request review if the request has been cancelled.
Focused commits or squashing
We generally squash sequences of incremental-development commits together into logical commits (though keeping logical commits focused). Developers may do this themselves before submitting a PR or during the PR process, or Core Team members may do it when merging a PR. Ideally, the continuous-integration tests should pass at each logical commit.
Review and merge
Anyone may submit a pull request, and anyone may comment on or review others' pull requests. However, one review from somebody in the Core Team is required before the Core Team merges it.
Even Core Team members should create PRs for every change, including minor work items (version bump, removing warnings, etc.): this is helpful to keep track of what has happened on the repository. Very minor changes may be merged without a review, although it is always preferred to have one.
Release Process
This is intended to serve as documentation for Wasmtime's release process. It's largely an internal checklist for those of us performing a Wasmtime release, but others might be curious in this as well!
To kick off the release process someone decides to do a release. Currently there's not a schedule for releases or something similar. Once the decision is made (there's also not really a body governing these decisions, it's more whimsical currently, or on request from others) then the following steps need to be executed to make the release:
git pull- make sure you've got the latest changes- Run
rustc scripts/publish.rs - Run
./publish bump
- Review and commit the changes
- Note that this bumps all cranelift/wasmtime versions as a major version bump
at this time. See the
bump_versionfunction inpublish.rsto tweak this.
- Make sure
RELEASES.mdis up-to-date, and fill it out if it doesn't have an entry yet for the current release. - Send this version update as a PR to the
wasmtimerepository, wait for a merge - After merging, tag the merge as
vA.B.C - Push the tag to the repository
- This will trigger the release CI which will create all release artifacts and publish them to GitHub releases.
- Run
./publish publish
- This will fail on some crates, but that's expected.
- Keep running this script until all crates are published. Note that crates.io won't let you publish something twice so rerunning is only for crates which need the index to be udpated and if it hasn't yet. It's recommended to wait a bit between runs of the script.
And that's it, then you've done a Wasmtime release.
Implementing WebAssembly Proposals
Adding New Support for a Wasm Proposal
The following checkboxes enumerate the steps required to add support for a new WebAssembly proposal to Wasmtime. They can be completed over the course of multiple pull requests.
-
Add support to the
wasmparsercrate. -
Add support to the
wasmprintercrate. -
Add support to the
wasm-encodercrate. -
Add support to the
wasm-smithcrate. -
Add a
wasmtime::Config::enable_foo_barmethod to thewasmtimecrate. -
Add a
--enable-foo-barcommand line flag to thewasmtimebinary. -
Enable the spec tests in
build.rsbut mark them as ignored for now. -
Stop ignoring individual spec tests and get them passing one by one.
-
Enable the proposal in the fuzz targets.
-
Add examples from the spec tests to the relevant corpora.
The
wast2jsontool from WABT is useful for this. -
Write a custom fuzz target, oracle, and/or test case generator for fuzzing this proposal in particular.
For example, we wrote a custom generator, oracle, and fuzz target for exercising
table.{get,set}instructions and their interaction with GC while implementing the reference types proposal.
-
-
Expose the proposal's new functionality in the
wasmtimecrate's API.For example, the bulk memory operations proposal introduced a
table.copyinstruction, and we exposed its functionality as thewasmtime::Table::copymethod. -
Expose the proposal's new functionality in the C API.
This may require extensions to the standard C API, and if so, should be defined in
wasmtime.hand prefixed withwasmtime_. -
Use the C API to expose the proposal's new functionality in the other language embedding APIs:
-
Document support for the proposal in
wasmtime/docs/stability-wasm-proposals-support.md.
Enabling Support for a Proposal by Default
These are the standards that must be met to enable support for a proposal by default in Wasmtime, and can be used as a review checklist.
-
The proposal must be in phase 4, or greater, of the WebAssembly standardization process.
-
All spec tests must be passing in Wasmtime.
-
No open questions, design concerns, or serious known bugs.
-
Has been fuzzed for at least a week minimum.
-
We are confident that the fuzzers are fully exercising the proposal's functionality.
For example, it would not have been enough to simply enable reference types in the
compilefuzz target to enable that proposal by default. Compiling a module that uses reference types but not instantiating it nor running any of its functions doesn't exercise any of the GC implementation and does not run the inline fast paths fortableoperations emitted by the JIT. Exercising these things was the motivation for writing the custom fuzz target fortable.{get,set}instructions. -
The proposal's functionality is exposed in the
wasmtimecrate's API. -
The proposal's functionality is exposed in the C API.
-
The proposal's functionality is exposed in at least one of the other languages' APIs.
Governance
... more coming soon
Contributor Covenant Code of Conduct
Note: this Code of Conduct pertains to individuals' behavior. Please also see the Organizational Code of Conduct.
Our Pledge
In the interest of fostering an open and welcoming environment, we as contributors and maintainers pledge to making participation in our project and our community a harassment-free experience for everyone, regardless of age, body size, disability, ethnicity, gender identity and expression, level of experience, nationality, personal appearance, race, religion, or sexual identity and orientation.
Our Standards
Examples of behavior that contributes to creating a positive environment include:
- Using welcoming and inclusive language
- Being respectful of differing viewpoints and experiences
- Gracefully accepting constructive criticism
- Focusing on what is best for the community
- Showing empathy towards other community members
Examples of unacceptable behavior by participants include:
- The use of sexualized language or imagery and unwelcome sexual attention or advances
- Trolling, insulting/derogatory comments, and personal or political attacks
- Public or private harassment
- Publishing others' private information, such as a physical or electronic address, without explicit permission
- Other conduct which could reasonably be considered inappropriate in a professional setting
Our Responsibilities
Project maintainers are responsible for clarifying the standards of acceptable behavior and are expected to take appropriate and fair corrective action in response to any instances of unacceptable behavior.
Project maintainers have the right and responsibility to remove, edit, or reject comments, commits, code, wiki edits, issues, and other contributions that are not aligned to this Code of Conduct, or to ban temporarily or permanently any contributor for other behaviors that they deem inappropriate, threatening, offensive, or harmful.
Scope
This Code of Conduct applies both within project spaces and in public spaces when an individual is representing the project or its community. Examples of representing a project or community include using an official project e-mail address, posting via an official social media account, or acting as an appointed representative at an online or offline event. Representation of a project may be further defined and clarified by project maintainers.
Enforcement
Instances of abusive, harassing, or otherwise unacceptable behavior may be reported by contacting the Bytecode Alliance CoC team at report@bytecodealliance.org. The CoC team will review and investigate all complaints, and will respond in a way that it deems appropriate to the circumstances. The CoC team is obligated to maintain confidentiality with regard to the reporter of an incident. Further details of specific enforcement policies may be posted separately.
Project maintainers who do not follow or enforce the Code of Conduct in good faith may face temporary or permanent repercussions as determined by other members of the Bytecode Alliance's leadership.
Attribution
This Code of Conduct is adapted from the Contributor Covenant, version 1.4, available at http://contributor-covenant.org/version/1/4