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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 experimentally provided via the wasi-threads proposal.


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:

make CC=/path/to/clang/with/wasm/support \
     AR=/path/to/llvm-ar \

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.

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.

Building in pthread support

To enable pthreads support via the wasi-threads proposal, follow the above build directions with one addition: make ... THREAD_MODEL=posix. This creates additional artifacts in sysroot/lib/wasm32-wasi-threads to support --target wasm32-wasi-threads.

Arch Linux AUR package

For Arch Linux users, there's an official wasi-libc package tracking this Git repository. You might want to install other WASI related packages as well.