wasm32-wasip1

Tier: 2

The wasm32-wasip1 target is a WebAssembly compilation target which assumes that the WASIp1 (aka "WASI preview1") set of "syscalls" are available for use in the standard library. This target explicitly supports interop with non-Rust code such as C and C++.

The WASIp1 set of syscalls is standard insofar as it was written down once by a set of folks and has not changed since then. Additionally the WASIp1 syscalls have been adapted and adopted into a number of runtimes and embeddings. It is not standard in the sense that there are no formal semantics for each syscall and APIs are no longer receiving any maintenance (e.g. no new APIs, no new documentation, etc). After WASIp1 was originally developed in 2019 the WASI standard effort has since been "rebased" on top of the Component Model. This was a large change to the WASI specification and was released as 0.2.0 ("WASIp2" colloquially) in January 2024. Current standardization efforts are focused on the Component Model-based definition of WASI. At this point the wasm32-wasip1 Rust target is intended for historical compatibility with WASIp1 set of syscalls.

Today the wasm32-wasip1 target will generate core WebAssembly modules which will import functions from the wasi_snapshot_preview1 module for OS-related functionality (e.g. printing).

Note: Prior to March 2024 this target was known as wasm32-wasi with some historical context found in old MCPs:

• Rename wasm32-wasi target to wasm32-wasip1
• Smooth the renaming transition of wasm32-wasi

Target maintainers

When this target was added to the compiler platform-specific documentation here was not maintained at that time. This means that the list below is not exhaustive and there are more interested parties in this target. That being said since when this document was last updated those interested in maintaining this target are:

@alexcrichton @loganek

Requirements

This target is cross-compiled. The target includes support for std itself, but not all of the standard library works. For example spawning a thread will always return an error (see the wasm32-wasip1-threads target for example). Another example is that spawning a process will always return an error. Operations such as opening a file, however, will be implemented by calling WASI-defined APIs.

The WASI targets for Rust are explicitly intended to interoperate with other languages compiled to WebAssembly, for example C/C++. Any ABI differences or mismatches are considered bugs that need to be fixed.

By default the WASI targets in Rust ship in rustup with a precompiled copy of wasi-libc meaning that a WebAssembly-targeting-Clang is not required to use the WASI targets from Rust. If there is no actual interoperation with C then rustup target add wasm32-wasip1 is all that's needed to get started with WASI.

Note that this behavior can be controlled with -Clinker and -Clink-self-contained, however. By specifying clang as a linker and disabling the link-self-contained option an external version of libc.a can be used instead.

Building the target

To build this target first acquire a copy of wasi-sdk. At this time version 22 is the minimum needed.

Next configure the WASI_SDK_PATH environment variable to point to where this is installed. For example:

export WASI_SDK_PATH=/path/to/wasi-sdk-22.0

Next be sure to enable LLD when building Rust from source as LLVM's wasm-ld driver for LLD is required when linking WebAssembly code together. Rust's build system will automatically pick up any necessary binaries and programs from WASI_SDK_PATH.

Building Rust programs

The wasm32-wasip1 target is shipped with rustup so users can install the target with:

rustup target add wasm32-wasip1

Rust programs can be built for that target:

rustc --target wasm32-wasip1 your-code.rs

Cross-compilation

This target can be cross-compiled from any hosts.

Testing

This target is tested in rust-lang/rust CI on all merges. A subset of tests are run in the test-various builder such as the UI tests and libcore tests. This can be tested locally, for example, with:

./x.py test --target wasm32-wasip1 tests/ui

Conditionally compiling code

It's recommended to conditionally compile code for this target with:

#[cfg(all(target_os = "wasi", target_env = "p1"))]

Note that the target_env = "p1" condition first appeared in Rust 1.80. Prior to Rust 1.80 the target_env condition was not set.

Enabled WebAssembly features

The default set of WebAssembly features enabled for compilation is currently the same as wasm32-unknown-unknown. See the documentation there for more information.