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 experimentally provided via the wasi-threads proposal.

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). The build is configured with:

cmake -S . -B build

You can configure the build system with the -G argument to cmake, for example -G Ninja, to build with a different build system instead.

Supported options to the configuration step are:

• -DCMAKE_C_COMPILER=clang - specify a C compiler to use.
• -DCMAKE_AR=llvm-ar - specify a specific ar program to use.
• -DCMAKE_INSTALL_PREFIX=path - where to install the sysroot.
• -DTARGET_TRIPLE=... - which target to build: for example wasm32-wasip1, wasm32-wasip2, wasm32-wasip1-threads, ...
• -DMALLOC=... - one of dlmalloc, emmalloc, or none.
• -DBUILTINS_LIB=... - optional path to compiler-rt which, if not present, will be downloaded automatically.
• -DBULK_MEMORY_THRESHOLD=... - used to tune bulk-memory-related copying functions.
• -DSETJMP=(ON|OFF) - whether or not to build libsetjmp.{a,so}
• -DBUILD_TESTS=(ON|OFF) - whether or not to build tests
• -DSIMD=(ON|OFF) - whether or not to enable simd-related string routines
• -DBUILD_SHARED=(ON|OFF) - forcibly enable or disable building shared libraries.

After the build is configured you can use the generated build system to build wasi-libc. For example.

cd build && make -j$(nproc)

This can then be installed using the install target of your build system. For example:

cd build && make install

This will install to the directory specified by -DCMAKE_INSTALL_PREFIX=.... This path can then be used as a sysroot for C:

/path/to/wasm/supporting/c/compiler --sysroot=/path/to/the/newly/built/sysroot ...

to run the compiler using the newly built sysroot.

Other default libraries

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. This means that by default just passing --sysroot may not be enough as Clang will also look for some of these libraries, notably libclang_rt.builtins-wasm32.a, by default. This is one of the things wasi-sdk simplifies, as it includes cross-compiled builds of compiler-rt, libc++.a, and libc++abi.a.

The build of wasi-libc itself depends on compiler-rt, or libclang_rt.builtins-wasm32.a. This manifests in error messages such as undefined symbol: __muloti4 for example. Specifically the PIC build of wasi-libc will require that Clang can locate libclang_rt.builtins-wasm32.a, meaning that libc.so requires this library to be present. This library is downloaded automatically as part of the build but it can also be manually specified with -DBUILTINS_LIB=....

Building in pthread support

All targets in wasi-libc have pthread_*-related symbols defined. The behavior of these symbols differ based on target, however:

• wasm32-wasip{1,2} - symbols return an error or trap as appropriate. For example spawning a thread returns an error and blocking on a condition variable traps (as it can't possibly be woken).

• wasm32-wasip1-threads - spawning a thread is done with the (now largely deprecated) wasi-threads proposal and blocking is done with WebAssembly memory.atomic.* instructions.

• wasm32-wasip3 - at this time this behaves the same as wasm32-wasip{1,2} by default. When wasi-libc is built with -DENABLE_COOP_THREADS=ON then wasi-libc will use the cooperative multithreading intrinsics of the component model (the 🧵 gate in the component model explainer). This means that in this mode all pthread_* symbols are implemented and work as expected and scheduling is implemented with cooperative switches (explicit yields or synchronization points in pthread primitives). For more information see this documentation in wasi-sdk for how to use a pre-built copy of wasi-libc and avoid needing to build your own sysroot.

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.