Capnproto on wasi/emscripten?

[pepij...@gmail.com]

Hey,

I was playing around a bit to see if I could compile capnproto with the wasi-sdk or emscripten. wasi is a "web assembly system interface" that allows running wasm code completely without a JS runtime, while emscripten requires a browser/nodejs runtime. I think both options could be interesting for distributing software in a platform independent way.

However, it seems capnproto doesn't compile out of the box with either. When targeting wasi, it is missing a bunch of things such as pipes, signals, and exceptions as far as I understand from compile errors in kj exception and unix abstraction code. When targeting emscripting it actually compiles but has some missing symbols during linking, also related to exceptions: https://github.com/emscripten-core/emscripten/issues/11985#issuecomment-1019317541

Would there be a way to compile capnproto without these problematic features, or a feasible way to provide wasi-specific stubs for them?

Cheers,

Pepijn

[Kenton Varda]

WASI is essentially a different operating system, POSIX-like but not exactly the same. We will need to add support for it alongside POSIX and Windows.

There's actually a PR open to create a build mode which mostly avoids OS dependencies altogether, which might be a good starting point:

https://github.com/capnproto/capnproto/pull/1376

I'm a little confused about exceptions, though. Exceptions are not an OS feature, they're a programming language feature... how could they not be supported? Cap'n Proto actually does support building with `-fno-exceptions`, so maybe that's a path forward, but there are a lot of caveats...

-Kenton

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