wazero understands that while no-one desired to create confusion, confusion exists both in what is a standard and what in practice is in fact a standard feature. To help with this, we created some guidance both on the status quo of WebAssembly portability and what we support.

The WebAssembly Core Specification is the only specification relevant to wazero, governed by a standards body. Release 1.0 is a Web Standard (REC). Release 2.0 is a Working Draft (WD), so not yet a Web Standard.

Many compilers implement system calls using the WebAssembly System Interface, WASI. WASI is a WebAssembly community subgroup, but has not published any working drafts as a result of their work. WASI’s last stable point was wasi_snapshot_preview1, tagged at the end of 2020.

While this seems scary, the confusion caused by pre-standard features is not as bad as it sounds. The WebAssembly ecosystem is generally responsive regardless of where things are written down and wazero provides tools, such as built-in support for WASI, to reduce pain.

The goal of this section isn’t to promote a W3C recommendation exclusive approach, rather to help you understand common language around portable features and which of those wazero supports at the moment. While we consider features formalized through W3C recommendation status mandatory, we actively pursue pre-standard features as well interop with commonly used infrastructure such as AssemblyScript.

In summary, we hope this section can guide you in terms of what wazero supports as well as how to classify a request for a feature we don’t yet support.

WebAssembly Core

wazero conforms with tests defined alongside WebAssembly Core Specification 1.0 and 2.0.

By default, the runtime configuration enables features in WebAssembly Core Specification, despite it not yet being a Web Standard (REC). You can select version 1.0 like so:

rConfig = wazero.NewRuntimeConfig().WithCoreFeatures(api.CoreFeaturesV1)

One current limitation of wazero is that it doesn’t implement the Text Format, e.g. compiling .wat files. Users can work around this using tools such as wat2wasm to compile the text format into the binary format. In practice, the text format is too low level for most users, so delays here have limited impact.

Post 2.0 Features

Features regardless of W3C release are inventoried in the Proposals. repository. wazero implements Finished Proposals based on user demand, using wazero.RuntimeConfig feature flags. As of late 2022, all finished proposals are included in 2.0 Working Draft.

Features not yet assigned to a W3C release are not reliable. Encourage the WebAssembly community to formalize features you rely on, so that they become assigned to a release, and reach the W3C recommendation (REC) phase.

WebAssembly System Interface (WASI)

Many compilers implement system calls using the WebAssembly System Interface, WASI. WASI is a WebAssembly community subgroup, but has not published any working drafts as a result of their work. WASI’s last stable point was wasi_snapshot_preview1, tagged at the end of 2020.

Some functions in this tag are used in practice while some others are not known to be used at all. Further confusion exists because some compilers, like GrainLang, import functions not used. Finally, some functions were added and removed after the git tag. For example, proc_raise was removed and sock_accept added.

For all of these reasons, wazero will not implement all WASI features, just to complete the below chart. If you desire something not yet implemented, please raise an issue and include your use case (ex which language you are using to compile, a.k.a. target Wasm).

Notes:

  • C (via clang) supports the maximum WASI functions due to wasi-libc
  • AssemblyScript has its own ABI which can optionally use wasi-shim
FunctionStatusKnown Usage
args_get✅TinyGo
args_sizes_get✅TinyGo
environ_get✅TinyGo
environ_sizes_get✅TinyGo
clock_res_get✅
clock_time_get✅TinyGo
fd_advise👷
fd_allocate✅Rust
fd_close✅TinyGo
fd_datasync✅Rust
fd_fdstat_get✅TinyGo
fd_fdstat_set_flags✅Rust
fd_fdstat_set_rights💀
fd_filestat_get✅Zig
fd_filestat_set_size✅Rust,Zig
fd_filestat_set_times✅Rust,Zig
fd_pread✅Zig
fd_prestat_get✅Rust,TinyGo,Zig
fd_prestat_dir_name✅Rust,TinyGo,Zig
fd_pwrite✅Rust,Zig
fd_read✅Rust,TinyGo,Zig
fd_readdir✅Rust,Zig
fd_renumber✅libc
fd_seek✅TinyGo
fd_sync✅Go
fd_tell✅Rust
fd_write✅Rust,TinyGo,Zig
path_create_directory✅Rust,TinyGo,Zig
path_filestat_get✅Rust,TinyGo,Zig
path_filestat_set_times❌
path_link✅Rust,Zig
path_open✅Rust,TinyGo,Zig
path_readlink✅Rust,Zig
path_remove_directory✅Rust,TinyGo,Zig
path_rename✅Rust,TinyGo,Zig
path_symlink✅Rust,Zig
path_unlink_file✅Rust,TinyGo,Zig
poll_oneoff✅Rust,TinyGo,Zig
proc_exit✅Rust,TinyGo,Zig
proc_raise💀
sched_yield✅Rust
random_get✅Rust,TinyGo,Zig
sock_accept❌
sock_recv❌
sock_send❌
sock_shutdown❌

Note: 💀 means the function was later removed from WASI.