Embedding Wasmtime in C++
This is a little guide on how to execute WebAssembly inside C++, and vice versa. First, we are going to embed Wasmtime in C++. This will allow us to call WebAssembly within any C++ project. Wasmtime is a fantastic standalone runtime for WebAssembly, that comes equipped with it’s own compiler, called Cranelift. I might be a bit biased though as I have contributed to Cranelift/Wasmtime. Nevertheless, give it a try and if you have any questions, issue, etc., join us at zulip.
Anyway, let’s get back to work. Calling C++ functions from WebAssembly is a little bit more tricky. This will require us to register Host-Functions in Wasmtime. Lastly, if we are interested in passing more complex data to WebAssembly (such as a C++ pointers or classes), we can utilize WebAssembly’s externref type.
Embedding WebAssembly
Before we can use Wasmtime in our C++ project, we need to include the wasmtime.hh header file. Now, let’s suppose we have a simple WebAssembly module called wasm.wat as following:
(module
(func (export "execute") ()
)
)
This module contains one empty exported function that is correct, callable and yet does nothing. We now want to call this function from within our C++ project.
#include <wasmtime.hh>
using namespace wasmtime;
int main() {
Engine engine;
Linker linker(engine);
Store store(engine);
auto module = Module::compile(engine, readFile("wasm.wat")).unwrap();
auto instance = Instance::create(store, module, {}).unwrap();
auto execute = std::get<Func>(*instance.get(store, "execute"));
execute.call(store, {}).unwrap();
}