a653lib

Introduction

a653lib is an ARINC 653 scheduler library for Linux (POSIX-based). It supports both native partition execution (compiled C binaries) and WebAssembly partition execution via the WAMR and Wasmtime runtimes.

Features

• ARINC 653 compliant process/partition scheduling on Linux
• Inter-partition communication via ARINC 653 ports (sampling & queuing)
• Native partition execution via POSIX fork
• WebAssembly partition execution via WAMR or Wasmtime (sandboxed, portable)

WebAssembly Support

When built with -DA653LIB_BUILD_WASM=ON, the project compiles additional targets:

Target	Description
p_wamr	Partition launcher using WAMR (libiwasm)
p_wasmtime	Partition launcher using Wasmtime (C API)
*_wasm	Per-partition .wasm guest binaries (wasm32-wasip1)

The host ARINC 653 functions (sampling/queuing ports, processes, time, semaphores, etc.) are exposed to the Wasm module via the runtime's import mechanism.

Because the Wasm guest may run as a 32-bit module on a 64-bit host, and because both little-endian and big-endian architectures must be supported, struct layout and alignment cannot be assumed to match between guest and host. To handle this correctly, accessor getters/setters are generated by c-abi-lens directly from the ARINC 653 headers; the Wasm APEX host functions use these accessors internally when marshalling data across the guest/host boundary.

This code demonstrates that WAMR and Wasmtime can support ARINC 653 APEX interfaces as host functions. The proof-of-concept also targets correctness on both little-endian and big-endian architectures (such as PowerPC, still represented in avionics).

Notable: This is believed to be the first implementation of ARINC 653 APEX interfaces as WebAssembly host functions that runs on two independent runtimes (WAMR and Wasmtime). This satisfies the key advancement criterion of the WebAssembly W3C standardization process, which requires "two or more Web VMs have implemented the feature" to progress a proposal to Phase 4 (standardization). While this implementation cannot be fully WASI-compliant due to the inherent incompatibilities between the APEX interface conventions and the WebAssembly standard (see APEX / WebAssembly Compliance Caveats), it fulfils the spirit of the dual-runtime portability requirement that underpins the WASI standardization track.

Architecture

graph TD
    subgraph scheduler["a653lib Scheduler (process)"]
        subgraph partitions["Partitions"]
            PA["Partition A\n(.wasm instance / POSIX fork)"]
            PB["Partition B\n(.wasm instance / POSIX fork)"]
        end
        ports["ARINC 653 Port Layer\n(sampling & queuing ports)"]
        PA <-.->|"APEX (native mode)"| ports
        PB <-.->|"APEX (native mode)"| ports
        subgraph runtimes["Wasm Runtimes (A653LIB_BUILD_WASM=ON)"]
            WAMR["WAMR (p_wamr)"]
            WT["Wasmtime (p_wasmtime)"]
        end
        PA <-->|"APEX 'WIT' (Wasm mode)"| runtimes
        PB <-->|"APEX 'WIT' (Wasm mode)"| runtimes
        runtimes <-->|"APEX host functions"| ports
    end
    scheduler --> posix["POSIX / Linux"]

Loading

In native mode, partitions run as forked POSIX processes. In Wasm mode, each partition is instantiated as a Wasm module inside the launcher (p_wamr or p_wasmtime), with ARINC 653 host functions injected via the runtime's import API.

APEX / WebAssembly Compliance Caveats

The WebAssembly specification states that functions take a sequence of values as parameters and return a sequence of values as results, and that host functions are expressed outside WebAssembly but passed to a module as an import.

There are a few reasons why ARINC 653 APEX interfaces cannot be 100% compliant with the WebAssembly standard:

• WIT bindgen incompatibility — APEX interfaces defined in WIT will not generate the proper standardised APEX function signatures via wit-bindgen. The APEX interface is therefore exposed as a plain C header that Wasm guest sources compile against directly.
• Function pointers — CREATE_PROCESS() and CREATE_ERROR_HANDLER() pass a SYSTEM_ADDRESS_TYPE function pointer, which cannot comply with the WebAssembly standard.
• Output via pointer parameters — Wasm specifies that return values are passed through a function's return; APEX interfaces instead use plain pointers as output parameters.

As long as C, C++, and Ada remain the primary partition languages, none of the above pose a practical concern.

Operating Systems

Tested on ArchLinux, Ubuntu (see CI), and RHEL7 (native mode only). Expected to work on any Linux 64-bit host.

Mode	OS	Notes
Native	Linux 32/64	POSIX, C99
Wasm (WAMR)	Linux 64	Requires libiwasm
Wasm (Wasmtime)	Linux 64	Requires Wasmtime C API

Dependencies

All modes

• CMake ≥ 3.23, C99 compiler (GCC or Clang)
• awk, sed (for ARINC header processing)
• ARINC 653 header archive (auto-downloaded from AEEC, or supplied via -DARINC653_ZIP=<path> for offline/Nix builds)

WebAssembly mode (-DA653LIB_BUILD_WASM=ON)

• wasi-sysroot — set via -DWASI_SYSROOT= (default: /usr/share/wasi-sysroot)
• clang with wasm32-wasip1 target — set via -DWASM_CLANG=
• WAMR launcher: libiwasm — auto-detected, or set via -DIWASM_LIBRARY=
• Wasmtime launcher: libwasmtime — auto-detected, or set via -DWASMTIME_LIBRARY=
• c-abi-lens (Rust/Cargo) — auto-fetched and built, or supply a prebuilt binary via -DC_ABI_LENS_EXECUTABLE=; disable auto-fetch with -DA653LIB_FETCH_C_ABI_LENS=OFF

ArchLinux

yay -S clang lld wasi-libc wasi-compiler-rt libwasmtime iwasm

Note: The iwasm AUR PKGBUILD must be patched to not strip the static library. See the iwasm AUR package.

Ubuntu

Install base dependencies:

sudo apt-get update
sudo apt-get install -y curl cmake cargo clang llvm libclang-dev gawk

Install WAMR (libiwasm) from source (WAMR-2.4.4):

git clone https://github.com/bytecodealliance/wasm-micro-runtime.git ~/wasm-micro-runtime
cd ~/wasm-micro-runtime && git checkout WAMR-2.4.4
mkdir build && cd build
cmake .. \
  -DWAMR_BUILD_LIBC_BUILTIN=1 \
  -DWAMR_BUILD_LIBC_WASI=1 \
  -DWAMR_BUILD_SHARED_MEMORY=1 \
  -DCMAKE_BUILD_TYPE=Release
make && sudo make install

Install Wasmtime C API (v45.0.0):

curl -L https://github.com/bytecodealliance/wasmtime/releases/download/v45.0.0/wasmtime-v45.0.0-x86_64-linux-c-api.tar.xz \
  -o wasmtime-c-api.tar.xz
tar -xf wasmtime-c-api.tar.xz
cd wasmtime-v45.0.0-x86_64-linux-c-api
sudo cp -a include/. /usr/local/include/
sudo cp -a lib/. /usr/local/lib/
sudo ldconfig

Install wasi-sdk (sdk-33):

curl -L https://github.com/WebAssembly/wasi-sdk/releases/download/wasi-sdk-33/wasi-sdk-33.0-x86_64-linux.deb \
  -o wasi-sdk.deb
sudo dpkg -i wasi-sdk.deb

The sysroot will be at /opt/wasi-sdk/share/wasi-sysroot.

Compilation

Native (default)

cmake -S . -B build
cmake --build build

The following files will be present in the build/ directory:

• a653lib_main
• liba653lib.a
• liba653lib_partition_init.a
• Makefile
• partition_a
• partition_b
• ...

With WebAssembly support

cmake -S . -B build-wasm \
  -DA653LIB_BUILD_WASM=ON \
  -DWASI_SYSROOT=/opt/wasi-sdk/share/wasi-sysroot \
  -DWASM_CLANG=/opt/wasi-sdk/bin/clang
cmake --build build-wasm
cmake --build build-wasm --target wasm   # builds p_wamr, p_wasmtime + .wasm guests

Key CMake options

Option	Default	Description
A653LIB_BUILD_WASM	OFF	Enable Wasm guest and launcher targets
A653LIB_FETCH_C_ABI_LENS	ON	Auto-clone and build c-abi-lens
WASI_SYSROOT	/usr/share/wasi-sysroot	WASI sysroot for guest compilation
WASM_CLANG	clang	Clang used for wasm32-wasip1 guest compilation
IWASM_LIBRARY	(auto-detected)	Path to libiwasm.a or shared lib
WASMTIME_LIBRARY	(auto-detected)	Path to libwasmtime
C_ABI_LENS_EXECUTABLE	(built from source)	Path to a prebuilt c-abi-lens binary
ARINC653_ZIP	(downloaded)	Path to a pre-fetched arinc653.h.zip

Run Demo

Native

./build/a653lib_main

this will generate the following output:

pid: 578773 <1702486050.317812578>: Current local time and date: Wed Dec 13 17:47:30 2023

pid: 578773 <1702486050.318212993>: a653_shm id: 0x63801e ptr: 0x7f5e57802000

pid: 578773 <1702486050.318233554>: > taskset --cpu-list 0 ./partition_a & :

pid: 578773 <1702486050.349241033>: a653 start (other pid) 578775

pid: 578773 <1702486050.349254883>: > taskset --cpu-list 1 ./partition_b & :

WebAssembly (WAMR)

cd ./build-wasm
ln -sf p_wamr wasm32_rt
./a653_main_wasm

WebAssembly (Wasmtime)

cd ./build-wasm
ln -sf p_wasmtime wasm32_rt
./a653_main_wasm

The build produces a bin/ directory. Switch the wasm32_rt symlink between p_wamr and p_wasmtime to select the WebAssembly runtime.

Limitations

• Not all APEX functions are implemented; the demonstration covers partitions with processes using sampling and queuing ports. Unimplemented functions are stubbed as empty and will be added incrementally as needed.
• Linux only (no bare-metal or RTOS support)
• WebAssembly mode requires a 64-bit host (little-endian or big-endian)

Open Topics

• Ensure Wasm is also handled through NixOS CI builds
• Improve threading model — see dev/add-wasm-apex-proc-alloc

Contributing

See CONTRIBUTING.md.

License

SPDX-License-Identifier: Apache-2.0 OR MIT

See LICENSE.