Calculating the GCD

You can also browse this source code online and clone the wasmtime repository to run the example locally.

This example shows off how to run a wasm program which calculates the GCD of two numbers.

gcd.wat

(module
  (func $gcd (param i32 i32) (result i32)
    (local i32)
    block  ;; label = @1
      block  ;; label = @2
        local.get 0
        br_if 0 (;@2;)
        local.get 1
        local.set 2
        br 1 (;@1;)
      end
      loop  ;; label = @2
        local.get 1
        local.get 0
        local.tee 2
        i32.rem_u
        local.set 0
        local.get 2
        local.set 1
        local.get 0
        br_if 0 (;@2;)
      end
    end
    local.get 2
  )
  (export "gcd" (func $gcd))
)

gcd.cc

#include <fstream>
#include <iostream>
#include <sstream>
#include <wasmtime.hh>

using namespace wasmtime;

std::string readFile(const char *name) {
  std::ifstream watFile;
  watFile.open(name);
  std::stringstream strStream;
  strStream << watFile.rdbuf();
  return strStream.str();
}

int main() {
  // Load our WebAssembly (parsed WAT in our case), and then load it into a
  // `Module` which is attached to a `Store`. After we've got that we
  // can instantiate it.
  Engine engine;
  Store store(engine);
  auto module = Module::compile(engine, readFile("examples/gcd.wat")).unwrap();
  auto instance = Instance::create(store, module, {}).unwrap();

  // Invoke `gcd` export
  auto gcd = std::get<Func>(*instance.get(store, "gcd"));
  auto results = gcd.call(store, {6, 27}).unwrap();

  std::cout << "gcd(6, 27) = " << results[0].i32() << "\n";
}