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+# WASI Libc
+
+WASI Libc is a libc for WebAssembly programs built on top of WASI system calls.
+It provides a wide array of POSIX-compatible C APIs, including support for
+standard I/O, file I/O, filesystem manipulation, memory management, time, string,
+environment variables, program startup, and many other APIs.
+
+WASI Libc is sufficiently stable and usable for many purposes, as most of the
+POSIX-compatible APIs are stable, though it is continuing to evolve to better
+align with wasm and WASI. For example, pthread support is still a work in
+progress.
+
+## Usage
+
+The easiest way to get started with this is to use [wasi-sdk], which includes a
+build of WASI Libc in its sysroot.
+
+## Building from source
+
+To build a WASI sysroot from source, obtain a WebAssembly-supporting C compiler
+(currently this is only clang 10+, though we'd like to support other compilers as well),
+and then run:
+
+```sh
+make CC=/path/to/clang/with/wasm/support \
+     AR=/path/to/llvm-ar \
+     NM=/path/to/llvm-nm
+```
+
+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:
+
+```sh
+/path/to/wasm/supporting/c/compiler --sysroot=/path/to/the/newly/built/sysroot ...
+```
+
+to run the compiler using the newly built sysroot.
+
+Note that Clang packages typically don't include cross-compiled builds of
+compiler-rt, libcxx, or libcxxabi, for `libclang_rt.builtins-wasm32.a`, libc++.a,
+or libc++abi.a, respectively, so they may not be usable without
+extra setup. This is one of the things [wasi-sdk] simplifies, as it includes
+cross-compiled builds of compiler-rt, libc++.a, and libc++abi.a.
+
+## Arch Linux AUR package
+For Arch Linux users, there's an unofficial AUR package tracking this git repo that can be installed under the name [wasi-libc-git].
+
+[wasi-sdk]: https://github.com/WebAssembly/wasi-sdk
+[wasi-libc-git]: https://aur.archlinux.org/packages/wasi-libc-git/