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#47 in WebAssembly

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Used in 534 crates (161 directly)

Apache-2.0 WITH LLVM-exception

37K SLoC


A standalone runtime for WebAssembly

A Bytecode Alliance project


This crate is the Rust embedding API for the Wasmtime project: a cross-platform engine for running WebAssembly programs. Notable features of Wasmtime are:

  • Fast. Wasmtime is built on the optimizing Cranelift code generator to quickly generate high-quality machine code either at runtime or ahead-of-time. Wasmtime's runtime is also optimized for cases such as efficient instantiation, low-overhead transitions between the embedder and wasm, and scalability of concurrent instances.

  • Secure. Wasmtime's development is strongly focused on the correctness of its implementation with 24/7 fuzzing donated by Google's OSS Fuzz, leveraging Rust's API and runtime safety guarantees, careful design of features and APIs through an RFC process, a security policy in place for when things go wrong, and a release policy for patching older versions as well. We follow best practices for defense-in-depth and known protections and mitigations for issues like Spectre. Finally, we're working to push the state-of-the-art by collaborating with academic researchers to formally verify critical parts of Wasmtime and Cranelift.

  • Configurable. Wastime supports a rich set of APIs and build time configuration to provide many options such as further means of restricting WebAssembly beyond its basic guarantees such as its CPU and Memory consumption. Wasmtime also runs in tiny environments all the way up to massive servers with many concurrent instances.

  • WASI. Wasmtime supports a rich set of APIs for interacting with the host environment through the WASI standard.

  • Standards Compliant. Wasmtime passes the official WebAssembly test suite, implements the official C API of wasm, and implements future proposals to WebAssembly as well. Wasmtime developers are intimately engaged with the WebAssembly standards process all along the way too.


An example of using the Wasmtime embedding API for running a small WebAssembly module might look 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" "host_func" (func $host_hello (param i32)))

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

    // Create a `Linker` which will be later used to instantiate this module.
    // Host functionality is defined by name within the `Linker`.
    let mut linker = Linker::new(&engine);
    linker.func_wrap("host", "host_func", |caller: Caller<'_, u32>, param: i32| {
        println!("Got {} from WebAssembly", param);
        println!("my host state is: {}", caller.data());

    // 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 instance = linker.instantiate(&mut store, &module)?;
    let hello = instance.get_typed_func::<(), ()>(&mut store, "hello")?;

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


More examples and information can be found in the wasmtime crate's online documentation as well.


📚 Read the Wasmtime guide here! 📚

The wasmtime guide is the best starting point to learn about what Wasmtime can do for you or help answer your questions about Wasmtime. If you're curious in contributing to Wasmtime, it can also help you do that!


~386K SLoC