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.
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 \
NM=/path/to/llvm-nmThis 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. 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 may depend 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. To handle this the
default behavior of the build system is to download the latest version of
compiler-rt from wasi-sdk and use that for the build of libc.so. You can
also set BUILTINS_LIB as a path to libclang_rt.builtins-wasm32.a as well to
use that instead of downloading a version.
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.
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.
