Note: as of 2023, you should probably use the Zig SDK instead of this.

The zig cc command supports cross-compilation to WASI out of the box, without having to install these builtins or the WASI SDK.

zig cc --target=wasm32-wasi -rdyamic -Os -s -o module.wasm module.c

--

Starting with version 8 (and version 9 if you need WASI), LLVM can compile to WebAssembly out of the box.

If you are using macOS with Homebrew, or any operating system with up-to-date LLVM packages, you're all set.

Almost.

Unless your WebAssembly application is compute-only, you still need some kind of interface with the runtime.

In order to do so, you may want to download and extract wasi-sysroot. Note that you don't need wasi-sdk if you already have LLVM, only wasi-sysroot.

Now you should be all set. Just replace /opt/wasi-sysroot with the location you extracted wasi-sysroot to.

clang --target=wasm32-wasi --sysroot=/opt/wasi-sysroot -O2 test.c

Dang. You may see clang now complaining about a missing file:

wasm-ld: error: cannot open .../lib/wasi/libclang_rt.builtins-wasm32.a: No such file or directory

The missing libclang_rt.builtins-wasm32.a can be obtained by recompiling compiler_rt from LLVM.

Or directly download that file here: libclang_rt.builtins-wasm32.a.

It is now also distributed with the WASI SDK releases, as a separate tarball.

Copy libclang_rt.builtins-wasm32.a from the tarball into the path expected by clang, creating the lib and wasi directories if necessary, and you'll be all set!

This has been tested with:

• LLVM 10, 11, 12, 13, 14 (llvm-10-to-14 folder)
• LLVM 15, 16
• LLVM 17
• LLVM 18