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113 lines
4.1 KiB
Markdown
113 lines
4.1 KiB
Markdown
# WebAssembly C and C++ API
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Work in progress! No docs yet.
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### Design Goals
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* Provide a "black box" API for embedding a Wasm engine in other C/C++ applications.
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* Be completely agnostic to VM specifics.
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* Non-goal: "white box" interoperability with embedder (such as combined GC instead of mere finalisation) -- *much* more difficult to achieve.
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* Allow creation of bindings for other languages through typical C foreign function interfaces.
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* Support a plain C API.
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* Stick to mostly manual memory management of interface objects.
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* Avoid language features that raise barrier to use.
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* E.g., no exceptions or post-C++11 features in C++ API.
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* E.g., no passing of structs by-value or post-C99 features in C API.
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* Achieve link-time compatibility between different implementations.
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* All implementation-dependent API classes are abstract and can be instantiated through factory methods only.
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### Interfaces
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* C++ API:
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* See `include/wasm.hh` for interface.
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* See `example/*.cc` for example usages.
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* C API:
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* See `include/wasm.h` for interface.
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* See `example/*.c` for example usages.
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Some random explanations:
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* The VM must be initialised by creating an instance of an *engine* (`wasm::Engine`/`wasm_engine_t`) and is shut down by deleting it. Such an instance may only be created once per process.
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* All runtime objects are tied to a specific *store* (`wasm::Store`/`wasm_store_t`). Multiple stores can be created, but their objects cannot interact. Every store and its objects must only be accessed in a single thread.
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* To exchange module objects between threads, create a *shared* module (`wasm::Shared<Module>`/`wasm_shared_module_t`). Other objects cannot be shared in current Wasm.
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* *Vector* structures (`wasm::vec<X>`/`wasm_x_vec_t`) are lightweight abstractions of a pair of a plain array and its length. The C++ API does not use `std::vector` because that does not support adopting pre-existing arrays.
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* *References* point to runtime objects, but may involve internal indirections, which may or may not be cached. Thus, pointer equality on `Ref*` or subclasses cannot be used to compare identity of the underlying objects (`Ref::eq` may be added later). However, `nullptr`/`NULL` uniquely represents null references.
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* The API already encompasses current proposals like [multiple return values](https://github.com/WebAssembly/multi-value/blob/master/proposals/multi-value/Overview.md) and [reference types](https://github.com/WebAssembly/reference-types/blob/master/proposals/reference-types/Overview.md), but not yet [threads](https://github.com/WebAssembly/threads/blob/master/proposals/threads/Overview.md).
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### Prototype Implementation
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* This repo contains a prototype implementation based on V8 is in `src`.
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* Note that this requires adding a module to V8, so it patches V8's build file.
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* The C API is implemented on top of the C++ API.
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* See `Makefile` for build recipe. Canonical steps to run examples:
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1. `make v8-checkout`
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2. `make v8`
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3. `make all`
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#### Limitations
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V8 implementation:
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* Currently requires patching V8 by adding a module.
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* Host functions (`Func::make`) create a JavaScript function internally, since V8 cannot handle raw C imports yet.
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* As a consequence, does not support multiple results in external calls or host functions.
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* Host functions and host globals are created through auxiliary modules constructed on the fly, to work around limitations in JS API.
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* `Shared<Module>` is currently implemented via serialisation, since V8 does not currently have direct support for cross-isolate sharing.
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### Other Implementations
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Currently, known implementations of this API are included in
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* V8 natively (both C and C++)
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* Wabt (only C?)
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* Wasmtime (only C?)
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* [Wasmer](https://github.com/wasmerio/wasmer/tree/master/lib/c-api) (only C, C++ coming soon)
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### TODO
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Possible API tweaks:
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* Add `Ref::eq` (or better, a subclass `EqRef::eq`) for reference equality?
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* Add a way to return error messages from `Module::make` and `Module::validate`.
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* Use `restrict` in C API?
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* Find a way to perform C callbacks through C++ without extra wrapper?
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* Add iterators to `vec` class?
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