Crate wasmtime

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Wasmtime’s embedding API

Wasmtime is a WebAssembly engine for JIT-complied or ahead-of-time compiled WebAssembly modules. More information about the Wasmtime project as a whole can be found in the documentation book whereas this documentation mostly focuses on the API reference of the wasmtime crate itself.

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 wasmtime::*;

fn main() -> wasmtime::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: {}",;

    // 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! 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> 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 or impl 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 wasmtime::*;

fn main() -> wasmtime::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: {}",;

    // 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")?; 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 profiling format. The profiler can be selected with Config::profiler.

  • vtune - Enabled by default, this feature compiles in support for VTune profiling of JIT code.

  • 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.

  • pooling-allocator - Enabled by default, this feature adds support for the pooling allocation strategy enabled via Config::allocation_strategy. The pooling allocator can enable more efficient reuse of resources for high-concurrency and high-instantiation-count scenarios.

  • memory-init-cow - Enabled by default, this feature builds in support for, on supported platforms, initializing wasm linear memories with copy-on-write heap mappings. This makes instantiation much faster by mmap-ing the initial memory image into place instead of copying memory into place, allowing sharing pages that end up only getting read. Note that this is simply compile-time support and this must also be enabled at run-time via Config::memory_init_cow (which is also enabled by default).


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 wasmtime_wasi::sync::WasiCtxBuilder;

// 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)?;
// ...

An example of reading a string from a wasm module:

use std::str;

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,
        _ => anyhow::bail!("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 =
        .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(_) => anyhow::bail!("invalid utf-8"),
        None => anyhow::bail!("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")?; store, ())?;


  • componentcomponent-model
    In-progress implementation of the WebAssembly component model
  • Unix-specific extension for the wasmtime crate.





  • A trait used to get shared access to a Store in Wasmtime.
  • A trait used to get exclusive mutable access to a Store in Wasmtime.
  • Implementation of an incremental compilation’s key/value cache store.
  • An object that can take callbacks when the runtime enters or exits hostcalls.
  • Internal trait implemented for all arguments that can be passed to Func::wrap and Linker::func_wrap.
  • A linear memory. This trait provides an interface for raw memory buffers which are used by wasmtime, e.g. inside [‘Memory’]. Such buffers are in principle not thread safe. By implementing this trait together with MemoryCreator, one can supply wasmtime with custom allocated host managed memory.
  • A memory creator. Can be used to provide a memory creator to wasmtime which supplies host managed memory.
  • Used by hosts to limit resource consumption of instances.
  • Used by hosts to limit resource consumption of instances, blocking asynchronously if necessary.
  • A trait used for Func::typed and with TypedFunc to represent the set of parameters for wasm functions.
  • A trait used for Func::typed and with TypedFunc to represent the set of results for wasm functions.
  • A trait implemented for types which can be returned from closures passed to Func::wrap and friends.
  • A trait implemented for types which can be arguments and results for closures passed to Func::wrap as well as parameters to Func::typed.

Type Definitions

  • A convenience wrapper for Result<T, anyhow::Error>.


  • A “raw” and unsafe representation of a WebAssembly value.