toywasm package icon


wasmer run yamt/toywasm

What's this?

A WebAssembly interpreter written in C.

On-browser demo

You can try it out on

$ curl -o cowsay.wasm
$ toywasm --wasi cowsay.wasm hello
< hello >
        \   ^__^
         \  (oo)\_______
            (__)\       )\/\
               ||----w |
                ||     ||

Note: the binary published to is built with an ancient wasi-sdk to workaround an issue.


  • Learn the spec by implementing it
  • Have a fun
  • Correctness
  • Clean code
  • Small footprint
  • Portability
    • Core wasm: C11
    • WASI: POSIX-like
    • Clang/LLVM extentions are ok where an alternative implmentation in pure C is reasonably easy.


  • Performance
  • Stable API/ABI

What are implemented?

WebAssemblyComplete 2.0 (Draft 2022-06-27)
wasi_snapshot_preview1See the top comment in wasi.c
wasi-threadsComplete (TOYWASM_ENABLE_WASI_THREADS)
dynamic-linkingComplete (TOYWASM_ENABLE_DYLD)

Note: As this runtime is relatively new, all proposals which had finished when I started this implementation are just included in the WebAssembly in the above table. It includes mutable-global, nontrapping-float-to-int-conversions, sign-extension-ops, multi-value, reference-types, bulk-memory-operations, simd.

Where can this run?

PlatformTested on CINotes
wasm32-wasiYes (on toywasm)
Ubuntu/i386Yes32-bit, smaller alignment
Ubuntu/arm64Yes (on qemu)
Ubuntu/armhfYes (on qemu)32-bit
Ubuntu/s390xYes (on qemu)Big endian
Ubuntu/riscv64Yes (on qemu)Temporarily disabled
wasm32-wasi-threadsNoOccasionally tested manually
NuttX/esp32NoOccasionally tested manually
NuttX/sim on macOS/amd64NoOccasionally tested manually
NetBSD/amd64NoOccasionally tested manually

Use as a command

See toywasm command help message.

Use as a library

See example apps:

Toywasm provides cmake config files for its libraries. If your app is using cmake, you can use find_package to find toywasm libraries as it's done in the CMakeLists.txt of the above example apps.

Release binaries

Warning Toywasm version numbers are NOT a semver. It's just an increasing number, which doesn't imply anything about compatibilities or features.

  • Release binaries are built with clang, with thin LTO where it's available.

  • For macOS, we ship a universal binary. (amd64 and arm64)

  • We ship binaries with two configurations for each platforms/architectures. The assets with full- in the filename have most features enabled. Another one is with the default configuration.

  • Each asset contains the toywasm cli command, static libraries, and cmake config files.

Build from source

  • To build toywasm, we recommend to use clang for the following features, which some parts of toywasm codebase is kind of assuming to have. They are not required for the correctness though.

  • For macOS and similar posix-like environment, you can build it with cmake in a usual way.

    % cmake -B build
    % cmake --build build

    For build-time options, see the ToywasmConfig.cmake.

    By default, it requires wabt and cmocka for tests. If you don't want to install them, you can disable tests.

    % cmake -B build -D BUILD_TESTING=OFF
    % cmake --build build

    Otherwise, after building it, you can run tests with:

    % cd build
    % ctest
  • For wasm32-wasi and wasm32-wasi-threads targets, we provide a convenient script to download wasi-sdk and build toywasm with it.

  • For NuttX, it's probably easiest to use interpreters/toywasm in NuttX apps. There are a few sample defconfig with toywasm enabled, including sim:toywasm.

How slow/fast is this?

Why is this slow?

  • Unlike many of interpreters, toywasm aims to execute wasm bytecode directly where possible. That is, it doesn't "compile" wasm bytecode into intermediate code.

    Unfortunately, wasm bytecode is simply not efficient to execute that way. It's basically designed to be somehow "compiled" at the load time.

    Many of interpreters out there translate wasm bytecode to their internal bytecode for performance reasons. Wasm3 and WAMR "fast" interpreter work exactly that way. Even WAMR "classic" interpreter replaces some critical instructions in-place.

    While toywasm maps wasm modules read-only and never modifies them in-place, it still generates a few types of offline annotations on the bytecode to avoid being too slow. While they are smaller than a full translation, you might consider them a kind of translation. See Annotations for details.

  • I don't like to use huge-switch statements or labels as values GNU C extension, which are well-known techniques to implement efficient interpreters.



A WebAssembly interpreter


Making software universally accessible