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Wasmtime Embedding API

The wasmtime crate is an embedding API of the wasmtime WebAssembly runtime. This is intended to be used in Rust projects and provides a high-level API of working with WebAssembly modules.

If you're interested in embedding wasmtime in other languages, you may wish to take a look a the C embedding API instead!


Wasmtime's embedding API

This crate contains an API used to interact with WebAssembly modules. For example you can compile modules, instantiate them, call them, etc. As an embedder of WebAssembly you can also provide WebAssembly modules functionality from the host by creating host-defined functions, memories, globals, etc, which can do things that WebAssembly cannot (such as print to the screen).

The wasmtime crate has similar concepts to the the JS WebAssembly API as well as the proposed C API, but the Rust API is designed for efficiency, ergonomics, and expressivity in Rust. As with all other Rust code you're guaranteed that programs will be safe (not have undefined behavior or segfault) so long as you don't use unsafe in your own program. With wasmtime you can easily and conveniently embed a WebAssembly runtime with confidence that the WebAssembly is safely sandboxed.

An example of using Wasmtime looks like:

use anyhow::Result;
use wasmtime::*;

fn main() -> Result<()> {
    // Modules can be compiled through either the text or binary format
    let engine = Engine::default();
    let wat = r#"
            (import "host" "hello" (func $host_hello (param i32)))

            (func (export "hello")
                i32.const 3
                call $host_hello)
    let module = Module::new(&engine, wat)?;

    // All wasm objects operate within the context of a "store". Each
    // `Store` has a type parameter to store host-specific data, which in
    // this case we're using `4` for.
    let mut store = Store::new(&engine, 4);
    let host_hello = Func::wrap(&mut store, |caller: Caller<'_, u32>, param: i32| {
        println!("Got {} from WebAssembly", param);
        println!("my host state is: {}", caller.data());

    // Instantiation of a module requires specifying its imports and then
    // afterwards we can fetch exports by name, as well as asserting the
    // type signature of the function with `get_typed_func`.
    let instance = Instance::new(&mut store, &module, &[host_hello.into()])?;
    let hello = instance.get_typed_func::<(), (), _>(&mut store, "hello")?;

    // And finally we can call the wasm!
    hello.call(&mut store, ())?;


Core Concepts

There are a number of core types and concepts that are important to be aware of when using the wasmtime crate:

  • [Engine] - a global compilation environment for WebAssembly. An [Engine] is an object that can be shared concurrently across threads and is created with a [Config] to tweak various settings. Compilation of any WebAssembly requires first configuring and creating an [Engine].

  • [Module] - a compiled WebAssembly module. This structure represents in-memory JIT code which is ready to execute after being instantiated. It's often important to cache instances of a [Module] because creation (compilation) can be expensive. Note that [Module] is safe to share across threads, and can be created from a WebAssembly binary and an [Engine] with [Module::new]. Caching can either happen with [Engine::precompile_module] or [Module::serialize], feeding those bytes back into [Module::deserialize].

  • [Store] - container for all information related to WebAssembly objects such as functions, instances, memories, etc. A [Store<T>][Store] allows customization of the T to store arbitrary host data within a [Store]. This host data can be accessed through host functions via the [Caller] function parameter in host-defined functions. A [Store] is required for all WebAssembly operations, such as calling a wasm function. The [Store] is passed in as a "context" to methods like [Func::call]. Dropping a [Store] will deallocate all memory associated with WebAssembly objects within the [Store].

  • [Instance] - an instantiated WebAssembly module. An instance is where you can actually acquire a [Func] from, for example, to call.

  • [Func] - a WebAssembly (or host) function. This can be acquired as the export of an [Instance] to call WebAssembly functions, or it can be created via functions like [Func::wrap] to wrap host-defined functionality and give it to WebAssembly.

  • [Table], [Global], [Memory] - other WebAssembly objects which can either be defined on the host or in wasm itself (via instances). These all have various ways of being interacted with like [Func].

All "store-connected" types such as [Func], [Memory], etc, require the store to be passed in as a context to each method. Methods in wasmtime frequently have their first parameter as either [impl AsContext][AsContext] or [impl AsContextMut][AsContextMut]. These traits are implemented for a variety of types, allowing you to, for example, pass the following types into methods:

  • &Store<T>
  • &mut Store<T>
  • &Caller<'_, T>
  • &mut Caller<'_, T>
  • StoreContext<'_, T>
  • StoreContextMut<'_, T>

A [Store] is the sole owner of all WebAssembly internals. Types like [Func] point within the [Store] and require the [Store] to be provided to actually access the internals of the WebAssembly function, for instance.


WebAssembly modules almost always require functionality from the host to perform I/O-like tasks. They might refer to quite a few pieces of host functionality, WASI, or maybe even a number of other wasm modules. To assist with managing this a [Linker] type is provided to instantiate modules.

A [Linker] performs name-based resolution of the imports of a WebAssembly module so the [Linker::instantiate] method does not take an imports argument like [Instance::new] does. Methods like [Linker::define] or [Linker::func_wrap] can be used to define names within a [Linker] to later be used for instantiation.

For example we can reimplement the above example with a Linker:

use anyhow::Result;
use wasmtime::*;

fn main() -> Result<()> {
    let engine = Engine::default();
    let wat = r#"
            (import "host" "hello" (func $host_hello (param i32)))

            (func (export "hello")
                i32.const 3
                call $host_hello)
    let module = Module::new(&engine, wat)?;

    // Create a `Linker` and define our host function in it:
    let mut linker = Linker::new(&engine);
    linker.func_wrap("host", "hello", |caller: Caller<'_, u32>, param: i32| {
        println!("Got {} from WebAssembly", param);
        println!("my host state is: {}", caller.data());

    // Use the `linker` to instantiate the module, which will automatically
    // resolve the imports of the module using name-based resolution.
    let mut store = Store::new(&engine, 0);
    let instance = linker.instantiate(&mut store, &module)?;
    let hello = instance.get_typed_func::<(), (), _>(&mut store, "hello")?;
    hello.call(&mut store, ())?;


The [Linker] type also transparently handles Commands and Reactors as defined by WASI.

Example Architecture

To better understand how Wasmtime types interact with each other let's walk through, at a high-level, an example of how you might use WebAssembly. In our use case let's say we have a web server where we'd like to run some custom WebAssembly on each request. To ensure requests are entirely isolated from each other, though, we'll be creating a new [Store] for each request.

When the server starts, we'll start off by creating an [Engine] (and maybe tweaking [Config] settings if necessary). This [Engine] will be the only engine for the lifetime of the server itself. Next, we can compile our WebAssembly. You'd create a [Module] through the [Module::new] API. This will generate JIT code and perform expensive compilation tasks up-front. Finally the last step of initialization would be to create a [Linker] which will later be used to instantiate modules, adding functionality like WASI to the linker too.

After that setup, the server starts up as usual and is ready to receive requests. Upon receiving a request you'd then create a [Store] with [Store::new] referring to the original [Engine]. Using your [Module] and [Linker] from before you'd then call [Linker::instantiate] to instantiate our module for the request. Both of these operations are designed to be as cheap as possible.

With an [Instance] you can then invoke various exports and interact with the WebAssembly module. Once the request is finished the [Store], is dropped and everything will be deallocated. Note that if the same [Store] were used for every request then that would have all requests sharing resources and nothing would ever get deallocated, causing memory usage to baloon and would achive less isolation between requests.


The wasmtime crate does not natively provide support for WASI, but you can use the wasmtime-wasi crate for that purpose. With wasmtime-wasi all WASI functions can be added to a [Linker] and then used to instantiate WASI-using modules. For more information see the WASI example in the documentation.

Cross-store usage of items

In wasmtime wasm items such as [Global] and [Memory] "belong" to a [Store]. The store they belong to is the one they were created with (passed in as a parameter) or instantiated with. This store is the only store that can be used to interact with wasm items after they're created.

The wasmtime crate will panic if the [Store] argument passed in to these operations is incorrect. In other words it's considered a programmer error rather than a recoverable error for the wrong [Store] to be used when calling APIs.

Crate Features

The wasmtime crate comes with a number of compile-time features that can be used to customize what features it supports. Some of these features are just internal details, but some affect the public API of the wasmtime crate. Be sure to check the API you're using to see if any crate features are enabled.

  • cranelift - Enabled by default, this features enables using Cranelift at runtime to compile a WebAssembly module to native code. This feature is required to process and compile new WebAssembly modules. If this feature is disabled then the only way to create a [Module] is to use the [Module::deserialize] function with a precompiled artifact (typically compiled with the same version of Wasmtime, just somewhere else).

  • cache - Enabled by default, this feature adds support for wasmtime to perform internal caching of modules in a global location. This must still be enabled explicitly through [Config::cache_config_load] or [Config::cache_config_load_default].

  • wat - Enabled by default, this feature adds support for accepting the text format of WebAssembly in [Module::new]. The text format will be automatically recognized and translated to binary when compiling a module.

  • parallel-compilation - Enabled by default, this feature enables support for compiling functions of a module in parallel with rayon.

  • async - Enabled by default, this feature enables APIs and runtime support for defining asynchronous host functions and calling WebAssembly asynchronously.

  • jitdump - Enabled by default, this feature compiles in support for the jitdump runtime profilng format. The profiler can be selected with [Config::profiler].

  • vtune - Not enabled by default, this feature compiles in support for supporting VTune profiling of JIT code.

  • uffd - Not enabled by default. This feature enables userfaultfd support when using the pooling instance allocator. As handling page faults in user space comes with a performance penalty, this feature should only be enabled when kernel lock contention is hampering multithreading throughput. This feature is only supported on Linux and requires a Linux kernel version 4.11 or higher.

  • all-arch - Not enabled by default. This feature compiles in support for all architectures for both the JIT compiler and the wasmtime compile CLI command.


In addition to the examples below be sure to check out the online embedding documentation as well as the online list of examples

An example of using WASI looks like:

# use anyhow::Result;
# use wasmtime::*;
use wasmtime_wasi::sync::WasiCtxBuilder;

# fn main() -> Result<()> {
// Compile our module and create a `Linker` which has WASI functions defined
// within it.
let engine = Engine::default();
let module = Module::from_file(&engine, "foo.wasm")?;
let mut linker = Linker::new(&engine);
wasmtime_wasi::add_to_linker(&mut linker, |cx| cx)?;

// Configure and create a `WasiCtx`, which WASI functions need access to
// through the host state of the store (which in this case is the host state
// of the store)
let wasi_ctx = WasiCtxBuilder::new().inherit_stdio().build();
let mut store = Store::new(&engine, wasi_ctx);

// Instantiate our module with the imports we've created, and run it.
let instance = linker.instantiate(&mut store, &module)?;
// ...

# Ok(())
# }

An example of reading a string from a wasm module:

use std::str;

# use wasmtime::*;
# fn main() -> anyhow::Result<()> {
let mut store = Store::default();
let log_str = Func::wrap(&mut store, |mut caller: Caller<'_, ()>, ptr: i32, len: i32| {
    // Use our `caller` context to learn about the memory export of the
    // module which called this host function.
    let mem = match caller.get_export("memory") {
        Some(Extern::Memory(mem)) => mem,
        _ => return Err(Trap::new("failed to find host memory")),

    // Use the `ptr` and `len` values to get a subslice of the wasm-memory
    // which we'll attempt to interpret as utf-8.
    let data = mem.data(&caller)
        .get(ptr as u32 as usize..)
        .and_then(|arr| arr.get(..len as u32 as usize));
    let string = match data {
        Some(data) => match str::from_utf8(data) {
            Ok(s) => s,
            Err(_) => return Err(Trap::new("invalid utf-8")),
        None => return Err(Trap::new("pointer/length out of bounds")),
    assert_eq!(string, "Hello, world!");
    println!("{}", string);
let module = Module::new(
            (import "" "" (func $log_str (param i32 i32)))
            (func (export "foo")
                i32.const 4   ;; ptr
                i32.const 13  ;; len
                call $log_str)
            (memory (export "memory") 1)
            (data (i32.const 4) "Hello, world!"))
let instance = Instance::new(&mut store, &module, &[log_str.into()])?;
let foo = instance.get_typed_func::<(), (), _>(&mut store, "foo")?;
foo.call(&mut store, ())?;
# Ok(())
# }


~227K SLoC