WebAssembly interpreter in C
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wac - WebAssembly in C

A Minimal WebAssembly interpreter written in C. Supports the WebAssembly MVP (minimum viable product) version of the WebAssembly specification.

There are two different builds of wac:

  • wac: (WebAssembly in C) Minimal client with an interactive REPL mode. Designed to run standalone wasm files compiled with wast2wasm or wasm-as. Passes most spec tests apart from some multi-module import/export tests.
  • wace: (WebAssembly in C with Emscripten) Client with host library/memory integration. Designed to run wasm code that has been built with Emscripten (using -s SIDE_MODULE=1 -s LEGALIZE_JS_FFI=0).


To build wac and wace you need a 32-bit version of gcc and 32-bit versions of SDL2 and libedit. On 64-bit Ubuntu/Debian these can be installed like this:

dpkg --add-architecture i386
apt-get update
apt-get install lib32gcc-4.9-dev libSDL2-dev:i386 libedit-dev:i386

To compile wast source files to binary wasm modules you will need the wasm-as tool from Binaryen. To compile C programs to wasm modules you will need a patched version of emscripten, the incoming branch of fastcomp and the master branch of binaryen.

As an alternative to downloading and building the above tools, the kanaka/emscripten docker image (1.7GB) has 32-bit gcc compiler/libraries, patched emscripten, and binaryen preinstalled. The docker image can be started with appropriate file mounts like this:

docker run -v `pwd`:/wac -w /wac -it kanaka/emscripten bash

All the build commands below can be run within the docker container.

wac usage

Build wac:

$ make wac

Use wasm-as to compile a simple wast program to a wasm:

$ make examples_wast/arith.wasm

Now load the compiled wasm file and invoke some functions:

$./wac examples_wast/arith.wasm add 2 3
$./wac examples_wast/arith.wasm mul 7 8

wac also supports a very simple REPL (read-eval-print-loop) mode that runs commands in the form of FUNC ARG...:

$ ./wac --repl examples_wast/arith.wasm
> sub 10 5
> div 13 4

wace usage

Build wace:

$ make wace

Use emscripten/binaryen to compile some simple C programs and run them using wace:

$ make examples_c/hello1.wasm
$ ./wace examples_c/hello1.wasm
hello world

$ make examples_c/hello2.wasm
$ ./wace examples_c/hello2.wasm
hello malloc people

Use emscripten/binaryen to compile some C SDL programs and run them using wace:

$ make examples_c/hello_sdl.wasm
$ ./wace examples_c/hello_sdl.wasm
INFO: OpenGL shaders: ENABLED
INFO: Created renderer: opengl
# Blue Window displayed for 2 seconds

$ make examples_c/triangle.wasm
$ ./wace examples_c/triangle.wasm
# A colorfully shaded triangle is rendered

$ make examples_c/hello_owl/hello_owl.wasm
$ ./wace examples_c/hello_owl/hello_owl.wasm
# An Owl image displayed for 2 seconds

Running WebAssembly spec tests

wac includes a runtest.py test driver which can be used for running tests from the WebAssembly specification.

Check out the spec:

git clone https://github.com/WebAssembly/spec

You will need wast2wasm to compile the spec tests. Check-out and build wabt (wabbit):

git clone --recursive https://github.com/WebAssembly/wabt
make -C wabt gcc-release

Run the func.wast test file (to test function calls) from the spec:

./runtest.py --wast2wasm ./wabt/out/gcc/Release/wast2wasm --interpreter ./wac spec/test/core/func.wast