This directory documents the "preview0" and "preview1" iterations of WASI.

Preview0 corresponded to the import module name wasi_unstable. It was also called snapshot_0 in some places. It was short-lived, and the changes to preview1 were minor, so the focus here is on preview1.

Preview1 corresponds to the import module name wasi_snapshot_preview1.

There was some work under the name "ephemeral" towards an update of preview1 however it is no longer being actively developed. The name "preview2" now refers to the new wit-based iteration of WASI instead.

Preview1 was defined using the witx IDL and associated tooling. Witx was an s-expression-based IDL derived from WebAssembly's wat text format, adding several extensions. It had a low-level C-like type system that emphasized raw pointers, and callees were expected to have access to the caller's entire linear memory, exported as "memory". It also had an implied global file descriptor table.

Some features in preview1 were not widely supported by engines:

• The proc_raise function, because Wasm itself has no signal-handling facilities, and a process wishing to terminate abnormally typically uses a Wasm trap instead of calling proc_raise.

• The process_cputime_id and thread_cputime_id, because in many engines, Wasm instances are not one-to-one with host processes, so the usual host APIs aren't sufficient to implement these.

One function has been added to preview1:

• sock_accept, allowing limited socket use cases to be supported. Sockets support with listen and connect is being added in preview2.