This is a work in progress. It's usable for many purposes, though the APIs aren't stable yet.
It's several things.
First, it's a usable libc. It builds a "sysroot" which can be pointed to by compilers, such as Clang 8.0, using the wasm32-unknown-wasi target triple. It's a work in progress, but it is already sufficient to run basic programs.
Second, it's a "reference" implementation, which means the interfaces defined
here can be used by other tools and libraries, even if they don't use all the
actual implementations here. For example, we don't expect everyone will want
to use the exact malloc implementation provided here, but tools and
libraries using an ABI-compatible malloc interface will be able to
interoperate regardless of which actual implementation is used.
Third, it's an example showing the use of the WASI API. The libc functionality is implemented using calls to WASI functions.
The easiest way to get started with this is to use one of the prepackaged releases.
To build a WASI sysroot from source, obtain a WebAssembly-supporting C compiler (currently this is only clang, though we'd like to support other compilers as well), and then run:
make WASM_CC=/path/to/wasm/supporting/c/compiler
This makes a directory called "sysroot", by default. See the top of the Makefile for customization options.
To use the sysroot, use the --sysroot= option:
/path/to/wasm/supporting/c/compiler --sysroot=/path/to/the/newly/built/sysroot ...
to run the compiler using the newly built sysroot.