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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 three different builds of wac:

  • wac: (WebAssembly in C) Minimal client with an interactive REPL mode. Designed to run standalone wasm files compiled with wat2wasm or wasm-as. Passes most spec tests apart from some multi-module import/export tests.
  • wax: (WebAssembly in C with WASI) Client with WebAssembly System Interface APIs (WASI).
  • 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/wax/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 wat source files to binary wasm modules you will need the wasm-as program from Binaryen. To compile C programs to wasm modules you will need Binaryen and emscripten.

As an alternative to downloading and building the above tools, the kanaka/wac docker image (1.7GB) has 32-bit gcc compiler/libraries, 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/wac 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 wat program to a wasm:

$ make examples_wat/arith.wasm

Now load the compiled wasm file and invoke some functions:

$ ./wac examples_wat/arith.wasm add 2 3
$ ./wac examples_wat/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_wat/arith.wasm
> sub 10 5
> div 13 4

wax usage

Build wax:

$ make wax

Use wasm-as to compile a wat program that uses the WASI interface:

$ make examples_wat/echo.wasm

Now run the compiled wasm file. The program reads text from stdin and echos it to stdout until an EOF (Ctrl-D) is sent.

$ ./wax examples_wat/echo.wasm
> foo
> bar
> <Ctrl-D>

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

Running WebAssembly spec tests

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

Check out the spec:

git clone

You will need wat2wasm to compile the spec tests. Check-out and build wabt (wabbit):

git clone --recursive
make -C wabt gcc-release

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

./ --wat2wasm ./wabt/out/gcc/Release/wat2wasm --interpreter ./wac spec/test/core/func.wast

Run all the spec tests apart from a few that currently fail (mostly due to missing support for some syntax used in those test files):

BROKE_TESTS="comments exports imports linking names data elem inline-module"
for t in $(ls spec/test/core/*.wast | grep -Fv "${BROKE_TESTS// /$'\n'}"); do
    echo -e "\nTESTING ${t}"
    ./ ${t} || break

Standalone Builds using Fooboot

wac and wace can be built to run as standalone bootable programs using fooboot:

cd wac
git clone
make PLATFORM=fooboot clean
make PLATFORM=fooboot wac wace examples_wat/addTwo.wasm

The fooboot/runfoo script can be used to boot wac/wace with QEMU. fooboot/runfoo also creates a connection on a serial port (COM2) that allows files to be read from the host system:

fooboot/runfoo wac --repl examples_wat/addTwo.wasm
QEMU waiting for connection on: disconnected:tcp:localhost:21118,server
webassembly> addTwo 2 3

The standalone wac/wace builds can also be built into an ISO image that can boot directly on real hardware. You will need Grub 2 and the Grub PC/BIOS binary files (grub-pc-bin) and the xorriso program to be able to do this. Also, the wasm modules that you wish to run must be built into the binary to become part of a simple in-memory file-system:

echo "examples_wat/addTwo.wasm" > mem_fs_files
make PLATFORM=fooboot \
     FOO_TARGETS="wac" \
     FOO_CMDLINE="examples_wat/addTwo.wasm addTwo 3 4" \

You can now boot the ISO with QEMU like this:

qemu-system-i386 -cdrom boot.iso

Or you can burn the ISO to a USB device and boot from it on real hardware. This will destroy any data on the USB device! Also, make completely sure that /dev/MY_USB_DEVICE is really the USB device you want to overwrite and not your hard drive. You have been warned!

sudo dd if=boot.iso of=/dev/MY_USB_DEVICE && sync
# Now boot you can boot from the USB device


MPL-2.0 (see LICENSE).

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