wasm32-wasi target is a new and still (as of April 2019) an
experimental target. The definition in this file is likely to be tweaked
over time and shouldn’t be relied on too much.
wasi target is a proposal to define a standardized set of syscalls
that WebAssembly files can interoperate with. This set of syscalls is
intended to empower WebAssembly binaries with native capabilities such as
filesystem access, network access, etc.
You can see more about the proposal at https://wasi.dev.
The Rust target definition here is interesting in a few ways. We want to serve two use cases here with this target:
First, we want Rust usage of the target to be as hassle-free as possible, ideally avoiding the need to configure and install a local wasm32-wasi toolchain.
Second, one of the primary use cases of LLVM’s new wasm backend and the wasm support in LLD is that any compiled language can interoperate with any other. To that the
wasm32-wasitarget is the first with a viable C standard library and sysroot common definition, so we want Rust and C/C++ code to interoperate when compiled to
You’ll note, however, that the two goals above are somewhat at odds with one
another. To attempt to solve both use cases in one go we define a target
that (ab)uses the
crt-static target feature to indicate which one you’re
By default the
crt-static target feature is enabled, and when enabled
this means that the bundled version of
libc.a found in
is used. This isn’t intended really for interoperation with a C because it
may be the case that Rust’s bundled C library is incompatible with a
foreign-compiled C library. In this use case, though, we use
some copied crt startup object files to ensure that you can download the
wasi target for Rust and you’re off to the races, no further configuration
All in all, by default, no external dependencies are required. You can
wasm32-wasi binaries straight out of the box. You can’t, however,
reliably interoperate with C code in this mode (yet).
For the second goal we repurpose the
target-feature flag, meaning that
you’ll need to do a few things to have C/Rust code interoperate.
All Rust code needs to be compiled with
-C target-feature=-crt-static, indicating that the bundled C standard library in the Rust sysroot will not be used.
If you’re using rustc to build a linked artifact then you’ll need to specify
-C linkerto a
clangbinary that supports
wasm32-wasiand is configured with the
wasm32-wasisysroot. This will cause Rust code to be linked against the libc.a that the specified
If you’re building a staticlib and integrating Rust code elsewhere, then compiling with
-C target-feature=-crt-staticis all you need to do.
You can configure the linker via Cargo using the
CARGO_TARGET_WASM32_WASI_LINKER env var. Be sure to also set
CC_wasm32-wasi if any crates in the dependency graph are using the
The wasi target is very new in its specification. It’s likely going to be a long effort to get it standardized and stable. We’ll be following it as best we can with this target. Don’t start relying on too much here unless you know what you’re getting in to!