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What's this

A simple wasi vfs implementation backed by littlefs.

Prerequisites

Checkout littlefs source code as "littlefs" in this directory.

git clone https://github.com/littlefs-project/littlefs

Or, you can override the location of littlefs source code with the TOYWASM_LITTLEFS_SOURCE_DIR cmake variable.

Build

Build toywasm with the following options.

TOYWASM_ENABLE_WASI=ON
TOYWASM_ENABLE_WASI_LITTLEFS=ON

Usage

You can use the --wasi-littlefs-dir cli option to add preopens backed by a littlefs file image.

Or, make your embedder use the wasi_instance_prestat_add_littlefs api.

Example

Copy a file from a littlefs image to host /tmp.

toywasm --wasi --wasi-dir /tmp::host --wasi-littlefs-dir littlefs.bin::/::lfs cp.wasm lfs/srcfile host/dstfile

WASI incompatibility notes

Littlefs have some incompatibilities with POSIX/WASI semantics.

While some of them can be emulated with user-attributes, we don't bother to do so.

  • littlefs doesn't have unique file-id suitable for WASI file "inode number".
  • littlefs doesn't have file timestamps.
  • littlefs doesn't have pread/pwrite.
  • littlefs doesn't have fallocate.
  • littlefs doesn't have symlinks.
  • littlefs doesn't have hardlinks.
  • In littlefs, opened files are snapshot of the files at the time of open.

Caveats

Curretly, block size (4096) and other filesystem parameters are hardcoded.