wasm2c-examples

Demonstrates two ways to use wasm2c — the tool that converts WebAssembly bytecode into portable C code — so the result can be compiled and run as a native executable without a WASM runtime.

Both examples implement the same two functions (add and sub) and call them from a C host program.

Example	Source language	Toolchain path
add-sub-c	C	C → WASM → C → native
add-sub-wat	WAT (WebAssembly Text)	WAT → WASM → C → native

How it works

add.c ──┐                        add.wat ──┐
sub.c ──┤→ .wasm → wasm2c → .w2c.c        ├→ .wasm → wasm2c → .w2c.c
        │                        sub.wat ──┘
        └─ main.c ──────────────────────────────┐
                                                ↓
                                    clang → add-sub (native exe)

wasm2c translates a binary WebAssembly module into C code. The generated C file, together with the small runtime in common/, links into an ordinary native binary. No WASM runtime is needed at execution time.

The common/ directory contains the wasm-rt runtime support files from wabt, which provide memory management, trap handling, and table operations for the generated code.

Prerequisites

1. LLVM with WebAssembly target support

The standard LLVM release (version 9+) includes the wasm32 target. The LLVM bundled with Visual Studio does not include the WebAssembly target and cannot be used for the C → WASM step.

On Windows, install via winget:

winget install LLVM.LLVM

On macOS:

brew install llvm

On Linux (Debian/Ubuntu):

apt install clang lld

2. wabt (WebAssembly Binary Toolkit)

Provides wasm2c and wat2wasm. Download the latest release for your platform from the wabt releases page and place it in tools/:

mkdir tools
# Download and extract to tools/wabt-<version>/

The Makefiles default to tools/wabt-1.0.41. Override with WABT_DIR=path/to/wabt.

3. GNU Make

• macOS/Linux: typically pre-installed (make --version)
• Windows: install GnuWin32 make or use WSL

Building

# add-sub-c: C source → WASM → C → native
cd add-sub-c
make WASM_CLANG=/path/to/llvm/clang CC=/path/to/clang

# add-sub-wat: WAT source → WASM → C → native
cd add-sub-wat
make WASM_CLANG=/path/to/llvm/clang CC=/path/to/clang

Makefile variables

Variable	Default	Purpose
WASM_CLANG	clang	Clang with wasm32 target (for C→WASM step)
CC	clang	Native C compiler (for final binary)
WASM2C	$(WABT_DIR)/bin/wasm2c	wasm2c tool
WAT2WASM	$(WABT_DIR)/bin/wat2wasm	wat2wasm tool (add-sub-wat only)
WABT_DIR	../tools/wabt-1.0.41	Root of wabt installation

Running

./add-sub <a> <b>

$ ./add-sub 10 3
add(10, 3) -> 13
sub(10, 3) -> 7

Project layout

wasm2c-examples/
├── common/               # wasm-rt runtime (from wabt)
│   ├── wasm-rt.h
│   ├── wasm-rt-impl.h
│   ├── wasm-rt-impl.c
│   ├── wasm-rt-impl-tableops.inc
│   ├── wasm-rt-mem-impl.c
│   └── wasm-rt-mem-impl-helper.inc
├── add-sub-c/            # C → WASM → C example
│   ├── Makefile
│   ├── add.c
│   ├── sub.c
│   └── main.c
├── add-sub-wat/          # WAT → WASM → C example
│   ├── Makefile
│   ├── add.wat
│   ├── sub.wat
│   └── main.c
└── tools/                # downloaded wabt binaries (gitignored)

Generated files (*.wasm, *.w2c.c, *.w2c.h, *.o, add-sub) are gitignored.

Generated API

wasm2c --module-name <name> generates a clean, instance-based C API:

// Declare an instance
w2c_add add_inst;

// Initialize the runtime (once per process)
wasm_rt_init();

// Instantiate the module
wasm2c_add_instantiate(&add_inst);

// Call an exported function
u32 result = w2c_add_add(&add_inst, 10, 3);

// Clean up
wasm2c_add_free(&add_inst);
wasm_rt_free();

Multiple module instances can coexist since all state is in the instance struct.