lib

APEX reference implementation

A small, embeddable, portable C library implementing the APEX protocol — both Host and Device roles for Core (framing, discovery, heartbeat) and for the first device class, Activation.

Goals

• Pure C99, no dynamic allocation, no stdio, no platform headers.
• Transport-agnostic: the caller pushes received bytes in and registers a TX callback. There is no built-in UART driver.
• Drop-in source: copy src/ and include/ into your project and add the .c files to your build. No submodules, no third-party deps.
• Trivial standalone build: a 20-line Makefile produces libapex.a.

Build

make            # produces build/libapex.a
make test       # builds and runs unit tests (needs GoogleTest; see test/Makefile)
make clean

The library compiles with any reasonable C99 toolchain. There is no configure, no CMake, no autotools. To cross-compile, override CC and AR:

make CC=arm-none-eabi-gcc AR=arm-none-eabi-ar CFLAGS="-mcpu=cortex-m7 -mthumb -Os"

Layout

include/apex/        public headers (this is what consumers #include)
  apex.h               umbrella header
  apex_core.h          constants, types, and outer-header layout from spec §3.1
  apex_cobs.h          COBS encode/decode
  apex_crc.h           CRC-16/CCITT-FALSE
  apex_framer.h        decoded-frame pack/unpack + byte-stream ingress
  apex_host.h          host: device table, discovery, heartbeat
  apex_device.h        device: discovery client, heartbeat
  apex_activation.h    Activation class — both Host and Device sides
src/                 one .c per public header
test/                gtest-based unit tests; mirrors the byte-level worked
                     example from APEX_Device_Class_Activation.md §9.4

Consumption patterns

As a static library

make
gcc my_app.c -Iapex/include -Lapex/build -lapex

As drop-in source (e.g. into flight-controller firmware)

Append the .c files to your source list:

APEX_SRC = \
    apex/src/apex_cobs.c \
    apex/src/apex_crc.c \
    apex/src/apex_framer.c \
    apex/src/apex_host.c \
    apex/src/apex_device.c \
    apex/src/apex_activation.c

Add apex/include to your include path. That's it.

Scope of v1

Feature	Status
COBS framing, CRC-16/CCITT-FALSE	✅
Outer header pack/unpack	✅
Host discovery + device table + lifecycle (NEW → CONNECTED → EXPENDED / FAULT)	✅
Device discovery + 1 Hz heartbeat + 5 s watchdog	✅
NAME_REQUEST / NAME_REPLY	✅
HOST_STATE broadcast	✅
Activation class — Device state machine + COMMAND/ACK/STATUS/CAPABILITY	✅
Activation class — Host driver	✅
Multi-class on a single link (host side)	✅
Passthrough / hub nodes	⛔ — spec marks the area as not well defined
Baud-rate negotiation	⛔ — spec does not define a CONFIG message for this yet

License

MIT. See LICENSE.