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).

Note: C (via clang) supports the maximum WASI functions due to wasi-libc.

FunctionStatusKnown Usage
args_getTinyGo
args_sizes_getTinyGo
environ_getTinyGo
environ_sizes_getTinyGo
clock_res_get
clock_time_getTinyGo
fd_advise
fd_allocate
fd_closeTinyGo
fd_datasync
fd_fdstat_getTinyGo
fd_fdstat_set_flags
fd_fdstat_set_rights💀
fd_filestat_getZig
fd_filestat_set_size
fd_filestat_set_times
fd_preadRust,Zig
fd_prestat_getTinyGo
fd_prestat_dir_nameTinyGo
fd_pwrite
fd_readTinyGo
fd_readdirRust,Zig
fd_renumber
fd_seekTinyGo
fd_sync
fd_tell
fd_write
path_create_directory
path_filestat_get
path_filestat_set_times
path_link
path_openTinyGo
path_readlink
path_remove_directory
path_rename
path_symlink
path_unlink_file
poll_oneoffRust,TinyGo,Zig
proc_exitAssemblyScript
proc_raise💀
sched_yield
random_get
sock_accept
sock_recv
sock_send
sock_shutdown

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