C++ std output results in wasm runtime error: null function or function signature mismatch

[akashkgarg]

I have a simple example of a wasm module:

#define export __attribute__((visibility("default")))

#include <iostream>

extern "C" export int add(int a, int b) {
    std::cout << "adding " << a << " and " << b << std::endl;
    return a + b;
}

I'm compiling using the following (not using emscripten):

clang++ --target=wasm32-wasi -nostartfiles -ffunction-sections -fdata-sections -fno-exceptions -Wl,--no-entry -Wl,--strip-all -Wl,--export-dynamic -Wl,--import-memory -Wl,--export-memory -fvisibility=hidden -Wl,--gc-sections --sysroot wasi-sdk-22-sysroot-dir/ -o add.wasm add.cpp

This compiles fine.

I'm then incorporating this into a simple webpage using the browser wasi shim: https://github.com/bjorn3/browser_wasi_shim. Here is my index.js for reference:

  let args = [];
  let fds = [
    new OpenFile(new File([])), // stdin
    ConsoleStdout.lineBuffered(msg => console.log(`[WASI stdout] ${msg}`)),
    ConsoleStdout.lineBuffered(msg => console.warn(`[WASI stderr] ${msg}`)),
    new PreopenDirectory(".", []),
  ];
  let wasi = new WASI(args, [], fds);

  const module = await WebAssembly.compileStreaming(fetch('add.wasm'));
  const memory = new WebAssembly.Memory({ initial: 32 });
  const env = { memory };

  const instance = await WebAssembly.instantiate(module, { env, "wasi_snapshot_preview1": wasi.wasiImport });
  wasi.initialize(instance)
  const { add } = instance.exports
  let result = add(1, 2);
  console.log(`js result = ${result}`)
  result = add(2, 3);
  console.log(`js result = ${result}`)

This results in the following console output:

[WASI stdout] adding 1 and 2
js result = 3
[WASI stdout] adding 2 and 3
add.wasm:0x68963 Uncaught (in promise) RuntimeError: null function or function signature mismatch
    at add.wasm:0x68963
    at add.wasm:0x76482
    at add.wasm:0x76499
    at index.js:24:12

Interestingly, if I remove the std::cout statement from my C++ source above, this works just fine; however with the std::cout the second invocation of the add function results in this runtime error. I've also tried different WASI shims, e.g., wasmer but same result.

[sbc100]

This almost certain comes down to how/when the static constrcutors and destructors are being run.

Can you try building with -mexec-model=reactor instead of -Wl,--no-entry, and then calling _initialize before calling any other exports?

[akashkgarg]

Thank you @sbc100! I also had to remove the -nostartfiles flag, but that does seem to do the trick. Closing this ticket.

Learned something new today. If you have good references on the command/reactor model for further reading that would be appreciated.