STK Library

A C23 library for reading EdgeTX radio joystick (HID) input on Windows. Reads raw HID input reports to access 4 analog axes and 24 buttons.

Wired for test-driven development using Unity and CMock.

All dependencies are fetched automatically via CMake FetchContent — no manual installation required beyond the tools listed below.

Requirements

Tool	Purpose
CMake >= 3.21	Build system
Ninja	Build backend
C23 compiler	GCC 14+, Clang 18+
Ruby >= 3.0	CMock mock generation
gcovr >= 6.0	Coverage reports — optional (uv tool install gcovr)

Build

Configure:

cmake -S . -B build -G Ninja

Build:

ninja -C build

Test

Full Unity output with colors:

ninja -C build check

CTest summary only:

ninja -C build test

check builds all suites and runs CTest with --output-on-failure, so assertion-level detail appears on any failure without running binaries by hand.

Coverage

Configure with coverage instrumentation:

cmake -S . -B build-cov -G Ninja -DENABLE_COVERAGE=ON

Generate the HTML report:

ninja -C build-cov coverage

Open build-cov/coverage/index.html in a browser to view results.

Only stk/ source files are measured. Unity, CMock, and generated mock files are excluded. Requires GCC or Clang with gcov support, and gcovr on PATH.

Windows note: requires GCC (e.g. scoop install gcc) or a Clang build that includes compiler-rt. A custom Clang without compiler-rt will fail at link time.

Run

Windows:

./build/main.exe

API

#include <stk/stk.h>

// Open with default VID/PID
stk_config_t config = {
    .vendor_id  = STK_DEFAULT_VID,
    .product_id = STK_DEFAULT_PID,
};

if (stk_open(&config) != 0) {
    // handle error
}

// Read state (non-blocking)
stk_state_t state;
stk_read(&state);

// Named accessors for the 4 active axes
int16_t rx = stk_right_x(&state);
int16_t ry = stk_right_y(&state);
int16_t throttle = stk_throttle(&state);
int16_t lx = stk_left_x(&state);

// Raw axes array (indices 0-7, only 1/2/3/4 active)
// state.axes[0..7] — values in range STK_AXIS_MIN..MAX (0..2047)
// AETR layout: axes 1/2 = right stick, 3 = throttle, 4 = left X (rudder)

// Button check
if (stk_button_pressed(&state, 3)) {
    // switch 4 is on
}

// Close the device
stk_close();

Radio Profile

EdgeTX radio (VID:PID 1209:4F54) HID input report:

Field	Size	Details
Axes	4 x 16-bit	Range 0-2047 (HID usage 0x30-0x37, indices 1/2/3/4)
Buttons	24 bits	Switches, rockers, paddles
Total	20 bytes	No report ID

Axis 1 = right X (aileron), 2 = right Y (elevator), 3 = throttle, 4 = left X (rudder). This matches the AETR stick layout.

Current Status

• Build system (CMake + Ninja)
• Raw HID device enumeration and opening
• Reading 4 axes and 24 buttons
• Linux support
• macOS support