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# `wasi-threads`



A proposed [WebAssembly System Interface](https://github.com/WebAssembly/WASI)

API to add native thread support.



> __NOTE__: this proposal is considered a legacy proposal, retained for engines

> that can only support WASI v0.1 (`preview1`). After much debate, future work

> on threads will happen in the [shared-everything-threads] proposal which adds

> component model [built-ins] for thread spawning, among other things. The goal

> is that WASI v0.2 and following will use [shared-everything-threads] (once

> fully implemented) and this proposal can eventually be removed. In the

> meantime, users experimenting with this proposal can continue to get help with

> questions and bugs by opening issues on this repository and tagging various

> maintainers who plan to continue supporting WASI v0.1 (e.g., @loganek, @yamt,

> @wenyongh).



[shared-everything-threads]: https://github.com/WebAssembly/shared-everything-threads

[built-ins]: https://github.com/WebAssembly/component-model/blob/main/design/mvp/Explainer.md#-threads



### Current Phase



Phase 1



### Champions



- [Alexandru Ene](https://github.com/AlexEne)



### Phase 4 Advancement Criteria



_TODO before entering Phase 2._



## Table of Contents



- [Introduction](#introduction)

- [Goals](#goals)

- [Non-goals](#non-goals)

- [API walk-through](#api-walk-through)

  - [Use case: support various languages](#use-case-support-various-languages)

  - [Use case: support thread-local storage](#use-case-support-thread-local-storage)

- [Detailed design discussion](#detailed-design-discussion)

  - [Design choice: thread IDs](#design-choice-thread-ids)

  - [Design choice: termination](#design-choice-termination)

  - [Design choice: pthreads](#design-choice-pthreads)

  - [Design choice: instance-per-thread](#design-choice-instance-per-thread)

- [Considered alternatives](#considered-alternatives)

  - [Alternative: WebAssembly threads](#alternative-webassembly-threads)

  - [Alternative: wasi-parallel](#alternative-wasi-parallel)

- [Stakeholder Interest & Feedback](#stakeholder-interest--feedback)

- [References & acknowledgements](#references--acknowledgements)



### Introduction

This proposal looks to provide a standard API for thread creation. This is a

WASI-level proposal that augments the WebAssembly-level [threads proposal]. That

WebAssembly-level proposal provides the primitives necessary for shared memory,

atomic operations, and wait/notify. This WASI-level proposal solely provides a

mechanism for spawning threads. Any other thread-like operations (thread

joining, locking, etc.) will use primitives from the WebAssembly-level proposal.



Some background: browsers already have a mechanism for spawning threads —

[Web Workers] — and the WebAssembly-level proposal avoided specifying how

thread spawning should occur. This allows other uses of WebAssembly —

i.e., outside the browser — to specify their own mechanism for spawning

threads.



[threads proposal]: https://github.com/WebAssembly/threads

[Web Workers]: https://developer.mozilla.org/en-US/docs/Web/API/Web_Workers_API/Using_web_workers



### Goals

- __`pthreads` support__: the goal of this proposal is to add the missing

  functions that are required to implement a subset of `pthreads` API. It does

  not aim to be identical to the `pthreads` API, but one must be able to create

  threads that operate on a shared Wasm memory while using the WebAssembly

  atomic instructions to synchronize on memory access.



- __library reuse__: standardizing this API would allow re-use of existing

  libraries and remove friction when porting projects from native

  execution contexts to WebAssembly and WASI environments (outside the

  browsers).



- __future-compatible__: a possible future direction for WebAssembly is towards

  supporting multiple threads per instance. We aim to expose an API that would

  be compatible with this future direction.



- __browser polyfills__: for browsers, we aim to provide a way to polyfill this

  API using Web Workers providing similar functionality to what exists in

  browsers today.



### Non-goals

- __full POSIX compatibity__: this API will not be 100% compatible with all

  functions and options described by POSIX threads standard.



- __modify core WebAssembly__: the current proposal is limited to the WASI APIs

  signatures and behavior and does not propose changes to the Wasm instruction

  set.



### API walk-through



The API consists of a single function. In pseudo-code:



```C

status wasi_thread_spawn(thread_start_arg* start_arg);

```



where the `status` is a unique non-negative integer thread ID (TID) of the new

thread (see [Design choice: thread IDs](#design-choice-thread-ids)) or a

negative number representing an error if the host failed to spawn the thread.

The host implementing `wasi_thread_spawn` will call a predetermined function

export (`wasi_thread_start`) in a new WebAssembly instance. Any necessary

locking/signaling/thread-local storage will be implemented using existing

instructions available in WebAssembly. Ideally, users will never use

`wasi_thread_spawn` directly but rather compile their threaded code from a

language that supports threads (see below).



#### Use case: support various languages



Using this API, it should be possible to implement threads in languages like:

- __C__, using the `pthreads` library (see the current work in [wasi-libc])

- __Rust__, as a part of the `std` library (in the future, e.g., [here])



The API should be able to support even more languages, but supporting these

initially is a good starting point.



[wasi-libc]: https://github.com/WebAssembly/wasi-libc

[here]: https://github.com/rust-lang/rust/blob/7308c22c6a8d77e82187e290e1f7459870e48d12/library/std/src/sys/wasm/atomics/thread.rs



#### Use case: support thread-local storage



For languages that implement thread-local storage (TLS), the start argument can

contain a language-specific structure with the address and (potentially) the

length of a TLS memory region. The host WebAssembly engine will treat this

argument as an opaque pointer — it should not introspect these

language-specific details. In C, e.g., the start function should be a static

trampoline-like wrapper (exported as `wasi_thread_start`) that reads the actual

user start function out of the start argument and calls this after doing some

TLS bookkeeping (this is not much different than how C starts threads natively).



### Detailed design discussion



Threads are tricky to implement. This proposal relies on a specific convention

in order to work correctly. When instantiating a module which is expected to run

with `wasi-threads`, the WASI host must first allocate shared memories to

satisfy the module's imports.



Upon a call to `wasi_thread_spawn`, the WASI host must:



1. instantiate the module again — this child instance will be used for the

   new thread

2. in the child instance, import all of the same WebAssembly objects,

   including the above mentioned shared memories, as the parent

3. optionally, spawn a new host-level thread (other spawning mechanisms are

   possible)

4. calculate a positive, non-duplicate thread ID, `tid`, and return it to the

   caller; any error in the previous steps is indicated by returning a negative

   error code.

5. in the new thread, call the child instance's exported entry function with the

   thread ID and the start argument: `wasi_thread_start(tid, start_arg)`



A WASI host that implements the above should be able to spawn threads for a

variety of languages.



#### Design choice: thread IDs



When `wasi_thread_spawn` successfully spawns a thread, it returns a thread ID

(TID) — 32-bit integer with several restrictions. TIDs are managed and

provided by the WASI host. To avoid leaking information, the host may choose to

return arbitrary TIDs (as opposed to leaking OS TIDs).



Valid TIDs fall in the range $[1, 2^{29})$. Some considerations apply:

- `0` is reserved for compatibility reasons with existing libraries (e.g.,

  wasi-libc) and must not be returned by `wasi_thread_spawn`

- the uppermost three bits of a valid TID must always be `0`. The most

  significant bit is the sign bit and recall that `wasi_thread_spawn` uses

  negative values to indicate errors. The remaining bits are reserved for

  compatibility with existing language implementations.



#### Design choice: termination



A `wasi-threads` module initially executes a single thread — the main

thread. As `wasi_thread_spawn` is called, more threads begin to execute. Threads

terminate in the following ways:



- __upon return__ from `wasi_thread_start`, and other threads continue to

  execute

- __upon a trap__ in any thread; all threads are immediately terminated

- __upon a `proc_exit` call__ in any thread; all threads are immediately

  terminated.



#### Design choice: pthreads



One of the goals of this API is to be able to support `pthreads` for C compiled

to WebAssembly. Given a WASI host that implements `thread_spawn` as described

above, what responsibility would the C language have (i.e., `libc`) to properly

implement `pthreads`?



`pthread_create` must not only call WASI's `wasi_thread_spawn` but is also

responsible for setting up the new thread's stack, TLS/TSD space, and updating

the `pthread_t` structure. This could be implemented by the following steps

(ignoring error conditions):

1. configure a `struct start_args` with the user's `void *(*start_func)(void *)`

   and `void *start_arg` (as done natively) but also with `pthread_t *thread`

2. call `malloc` (instead of `mmap`) to allocate TLS/TSD in the shared

   WebAssembly memory

3. define a static, exported `wasi_thread_start` function that takes as

   parameters `int tid` and `void *start_args`

4. in `pthread_create`, call `wasi_thread_spawn` with the configured

   `start_args` and use `atomic.wait` to wait for the `start_args->thread->tid`

   value to change (note that for web polyfills this may not be necessary since

   creation of web workers is not synchronous)

5. now in the child thread: once the WASI host creates the new thread instance

   and calls `wasi_thread_start`, then a) set `args->thread->tid` to the

   host-provided `tid`, b) set the `__wasilibc_pthread_self` global to point to

   `args->thread` (this is used by `pthread_self`, e.g.), c) use `atomic.notify`

   to inform the parent thread that the child now has a `tid`, d) start

   executing the user's `start_func` with the user's `start_arg` — at this

   point the new instance is executing separately in its own thread

6. back in the parent thread: once it has been notified that the child has

   recorded its TID, it can safely return with the `pthread_t` structure

   properly filled out.



`pthread_join` has a similar `wait`/`notify` implementation, but in reverse: the

parent thread can `wait` on the `thread->return` address to change and the child

thread can `notify` it of this once the user's start function finishes (i.e., at

the end of the `wasi_thread_start` wrapper).



The remainder of the `pthreads` API can be split up into what can be implemented

and what can safely be skipped until some later date.



##### What can easily be implemented



- `pthread_self` can use the `__wasilibc_pthread_self` global to return the

  address to the current thread's `pthread_t` structure; this relies on each

  thread mapping to a new instance (and thus a new set of globals) &mdash see

  discussion below on "instance per thread."

- `pthread_detach` can be implemented by using the flags already present in the

  `pthread_t` structure.

- `pthread_mutex_*`, `pthread_rwlock_*`, `pthread_cond_*`, `sem_*` can all be

  implemented using existing operations in the WebAssembly [threads proposal].

- thread-specific data (TSD), i.e., functions using `pthread_key_t`, can be

  implemented using the memory region allocated for the thread in WebAssembly

  shared memory.



##### What can be skipped

- `pthread_yield` is a [deprecated] `pthreads` function; `sched_yield` is the

  right one to use. Since it is unclear how WASI's scheduling should interact

  with the host's, this can be deferred until someone has a use case for it.

- `pthread_cancel` allows a parent thread to cancel a child thread; in

  particular, asynchronous cancellation is difficult (impossible?) to implement

  without a WebAssembly mechanism to interrupt the child thread and it

  complicates the entire implementation. It can be left for later.



[deprecated]: https://man7.org/linux/man-pages/man3/pthread_yield.3.html



##### What _has_ been implemented



`wasi-libc` contains an implementation of `pthreads` using

`wasi-threads`. Various WebAssembly engines support the proposal, including:

Wasmtime, WAMR, Wasmer, toywasm.



#### Design choice: instance-per-thread



A thread spawning mechanism for WebAssembly could be implemented in various

ways: the way chosen here, a cloned "instance-per-thread," is one option. The

other major option is to share the instance among many threads, as described in

the [Weakening WebAssembly] paper. Sharing an instance among many threads, as

described there, would require:

 - WebAssembly objects (memories, tables, globals, functions) to allow a

   `shared` attribute

 - the WebAssembly specification to grow a `fork` instruction



[Weakening WebAssembly]: https://www.researchgate.net/publication/336447205_Weakening_WebAssembly



The "instance-per-thread" approach was chosen here because a) it matches the

thread instantiation model of the browser (also "instance-per-thread") and b)

the WebAssembly specification changes required for the other approach may take

some time to materialize. In the meantime, this proposal allows threaded

WebAssembly to progress. If in the future the WebAssembly specification were to

add a "many-threads-per-instance" mechanism, the hope is that the API here

should not need to change significantly, though it is unclear how much the

changes might be.



The "instance-per-thread" approach chosen here does have its disadvantages:

* higher memory consumption (each instance is cloned)

* breaking behavior on non-standard functions such as `dlopen()` that require to

  modify the function table

* potential breaking behaviour of existing binaries once a new instruction gets

  added. This is a low risk because `shared` attributes do not yet exist on

  globals/tables/etc. having the `shared` attribute in a future WebAssembly spec

  version is not a likely approach. Most likely, no attributes would be

  interpreted as `local`/`private` as that would keep the existing behavior for

  binaries.



### Considered alternatives



#### Alternative: WebAssembly threads



Instead of exposing threads at the WASI level, thread spawning could be

specified in the WebAssembly specification. This is the approach described in

the [Weakening WebAssembly] paper. See the [Design choice:

instance-per-thread](#design-choice-instance-per-thread) discussion above for

more details.



#### Alternative: wasi-parallel



[wasi-parallel] is another WASI proposal which provides a parallel "for"

construct, similar to what, e.g., [OpenMP](https://www.openmp.org/) provides.

[wasi-parallel] spawns `N` threads at a time (though they may not all run

concurrently); this API spawns a single thread at a time.



[wasi-parallel]: https://github.com/WebAssembly/wasi-parallel/blob/main/docs/Explainer.md



### Stakeholder Interest & Feedback



TODO before entering Phase 3.



### References & acknowledgements



Many thanks for valuable feedback and advice from (alphabetical order):

* [Amanieu d'Antras](https://github.com/Amanieu)

* [Andrew Brown](https://github.com/abrown)

* [Conrad Watt](https://github.com/conrad-watt)

* [George Kulakowski](https://github.com/kulakowski-wasm)

* [Nathaniel McCallum](https://github.com/npmccallum)

* [Petr Penzin](https://github.com/penzn)

* [Sam Clegg](https://github.com/sbc100)

* [Wang Xin](https://github.com/xwang98)

* [Wenyong Huang](https://github.com/wenyongh)

* Xu Jun