#webassembly #wasm #wasi


A virtual filesystem layer for WASI

2 unstable releases

0.2.0 Aug 26, 2022
0.1.1 Jun 2, 2022

#360 in WebAssembly

MIT license

3.5K SLoC

Rust 3K SLoC // 0.0% comments C 478 SLoC // 0.0% comments Ruby 17 SLoC Shell 13 SLoC



A virtual filesystem layer for WASI.

NOTICE: This project currently supports only WASI applications on the top of wasi-libc

This project provides a language and host-agnostic virtual filesystem layer for WASI.

Supported filesystems

  • Embedded file system: a read only file system embedded in the .wasm binary.
  • to be implemented more...



A typical installation from the release binaries might look like the following:

$ export WASI_VFS_VERSION=0.2.0

# For x86_64 Linux host machine
$ curl -LO "https://github.com/kateinoigakukun/wasi-vfs/releases/download/v${WASI_VFS_VERSION}/wasi-vfs-cli-x86_64-unknown-linux-gnu.zip"
$ unzip wasi-vfs-cli-x86_64-unknown-linux-gnu.zip

# For x86_64 macOS host machine
$ curl -LO "https://github.com/kateinoigakukun/wasi-vfs/releases/download/v${WASI_VFS_VERSION}/wasi-vfs-cli-x86_64-apple-darwin.zip"
$ unzip wasi-vfs-cli-x86_64-apple-darwin.zip

# For arm64 macOS host machine
$ curl -LO "https://github.com/kateinoigakukun/wasi-vfs/releases/download/v${WASI_VFS_VERSION}/wasi-vfs-cli-aarch64-apple-darwin.zip"
$ unzip wasi-vfs-cli-aarch64-apple-darwin.zip

# See release page for more platforms: https://github.com/kateinoigakukun/wasi-vfs/releases

$ mv wasi-vfs /usr/local/bin/wasi-vfs


$ brew tap kateinoigakukun/wasi-vfs https://github.com/kateinoigakukun/wasi-vfs.git
$ brew install kateinoigakukun/wasi-vfs/wasi-vfs


To build the project, you need to install the wasi-sdk version 14.0 or later.

$ git clone https://github.com/kateinoigakukun/wasi-vfs.git --recurse-submodules
$ cd wasi-vfs
$ export WASI_VERSION=14

# For x86_64 Linux host machine
$ curl -LO https://github.com/WebAssembly/wasi-sdk/releases/download/wasi-sdk-${WASI_VERSION}/wasi-sdk-${WASI_VERSION_FULL}-linux.tar.gz
$ tar xvf wasi-sdk-${WASI_VERSION_FULL}-linux.tar.gz

# For x86_64 and arm64 macOS host machine (since wasi-sdk doesn't provide prebuilt binaries for M1, so use x64 binaries on Rosetta)
$ curl -LO https://github.com/WebAssembly/wasi-sdk/releases/download/wasi-sdk-${WASI_VERSION}/wasi-sdk-${WASI_VERSION_FULL}-macos.tar.gz
$ tar xvf wasi-sdk-${WASI_VERSION_FULL}-macos.tar.gz

$ cargo build --target wasm32-unknown-unknown



Set $WASI_SDK_PATH environment variable to the path of the wasi-sdk (version 14 or later).

Build and run WASI application

$ git clone https://github.com/kateinoigakukun/wasi-vfs.git --recurse-submodules
$ cd wasi-vfs

# Build libwasi_vfs.a
$ cargo build --target wasm32-unknown-unknown

# Build a WASI app with libwasi_vfs.a
$ $WASI_SDK_PATH/bin/clang -target wasm32-unknown-wasi -o getline.wasm examples/getline.c ./target/wasm32-unknown-unknown/debug/libwasi_vfs.a

# Run the WASI app with --mapdir
$ wasmtime run --mapdir /::./examples/mnt getline.wasm -- /hello.txt

# Pack ./examples/mnt directory into a WASM binary
$ cargo run -p wasi-vfs-cli -- pack getline.wasm --mapdir /::./examples/mnt -o getline.packed.wasm

# Run the WASM binary again without --mapdir
$ wasmtime run getline.packed.wasm -- /hello.txt


Unit tests

$ CARGO_TARGET_WASM32_WASI_RUNNER=wasmtime cargo test --target wasm32-wasi

End-to-end tests

$ cargo build --target wasm32-unknown-unknown
$ LIB_WASI_VFS_A=$PWD/target/wasm32-unknown-unknown/debug/libwasi_vfs.a ./tools/run-make-test.sh

How does it work?

wasi-vfs pack command is a wrapper of wizer, which is a pre-initializer for Wasm applications. The initialization process scans the mapped directories, then copies them into in-memory virtual filesystem.


Currently, this project only supports WASI applications on the top of wasi-libc because of the following reasons:

This project depends on wasm-ld and wasi-libc's imported symbol behavior. wasi-libc declares some external symbols to import WASI functions in C like below. When __imported_wasi_snapshot_preview1_fd_read is not defined in any input object files, wasm-ld produces a (import "wasi_snapshot_preview1" "fd_read") entry. This is how wasi-libc calls WASI functions.

int32_t __imported_wasi_snapshot_preview1_fd_read(int32_t arg0, int32_t arg1, int32_t arg2, int32_t arg3) __attribute__((

This project exploits that external symbols to hook WASI function calls by defining them in libwasi_vfs.a. If those symbols are defined, wasm-ld doesn't produce import entries, and it links symbols normally.

Therefore, this project currently doesn't support Rust application, which calls WASI functions directly without using wasi-libc.

After module-linking and interface-types will be merged, and WASI will adopt shared-nothing architecture, this project will be able to support all WASI applications.