Backends

oveRTOS uses a backend to map its portable API onto a specific RTOS or threading environment. Each backend lives under backends/<name>/ and provides one implementation file per subsystem (e.g. freertos_thread.c, posix_sync.c).

A backends/common/ directory contains utilities shared across all backends (ove_backend_common.h).

Available backends

Backend	Directory	RTOS	Primary use
FreeRTOS	backends/freertos/	FreeRTOS	Embedded hardware targets (Cortex-M)
POSIX	backends/posix/	pthreads	Host-PC desktop development and CI
Zephyr	backends/zephyr/	Zephyr RTOS	Zephyr-based embedded projects
NuttX	backends/nuttx/	NuttX	NuttX-based embedded projects

Feature support matrix

Each cell indicates whether the subsystem is implemented by that backend. All backends implement the full core and communication APIs.

Subsystem	FreeRTOS	POSIX	Zephyr	NuttX
Thread	Yes	Yes	Yes	Yes
Sync (mutex, semaphore, event, condvar)	Yes	Yes	Yes	Yes
Queue	Yes	Yes	Yes	Yes
Timer	Yes	Yes	Yes	Yes
EventGroup	Yes	Yes	Yes	Yes
WorkQueue	Yes	Yes	Yes	Yes
Stream	Yes	Yes	Yes	Yes
Time (monotonic clock, delays)	Yes	Yes	Yes	Yes
Console (UART)	Yes	Yes	Yes	Yes
Board init	Yes	Yes	Yes	Yes
GPIO	Yes	Yes (simulated)	Yes	Yes
LED	Yes	Yes (simulated)	Yes	Yes
Audio (I2S)	Yes	Yes (simulated)	Yes	Yes
FS (VFS)	Yes	Yes	Yes	Yes
NVS	Yes	Yes	Yes	Yes
Watchdog	Yes	Yes (simulated)	Yes	Yes
Shell	Yes	Yes	Yes	Yes
LVGL display	Yes	Yes (SDL2)	Yes	Yes

Backend file layout

Each backend follows the same naming convention: <backend>_<subsystem>.c. For example:

backends/freertos/
    freertos_thread.c
    freertos_sync.c
    freertos_queue.c
    freertos_timer.c
    freertos_eventgroup.c
    freertos_workqueue.c
    freertos_stream.c
    freertos_time.c
    freertos_console.c
    freertos_board.c
    freertos_gpio.c
    freertos_led.c        (via freertos_board.c)
    freertos_audio.c
    freertos_fs.c
    freertos_nvs.c
    freertos_watchdog.c
    freertos_shell.c
    freertos_lvgl.c
    freertos_heap_stubs.c
    include/              (backend-private headers)

The POSIX, Zephyr, and NuttX backends follow the same layout with their respective prefix.

Selecting a backend

The backend is chosen by the board's CMakeLists. It defines CONFIG_OVE_RTOS_<NAME> and adds the relevant backend source files to OVE_BACKEND_SOURCES. Application code never needs to reference a backend directly — all API calls go through ove/ove.h.

FreeRTOS backend notes

• Uses FreeRTOS tasks for threads, with the oveRTOS priority enum mapped linearly onto FreeRTOS task priorities.
• Heap mode uses pvPortMalloc / vPortFree; zero-heap mode uses static FreeRTOS object APIs.
• freertos_heap_stubs.c provides malloc/free shims for third-party libraries (e.g. LVGL) that call the C allocator directly.
• Timer callbacks run in the FreeRTOS timer service task context.

POSIX backend notes

• Maps oveRTOS threads onto pthread_t with a scheduling policy derived from the priority level.
• Binary events use a pthread_cond_t + pthread_mutex_t pair.
• Time functions use clock_gettime(CLOCK_MONOTONIC).
• GPIO and LED are simulated in memory; audio uses /dev/null or a platform audio library depending on build options.
• The scheduler "start" call on POSIX simply waits for all spawned threads to finish, which allows the application to return cleanly from main.

Zephyr backend notes

• Threads map to Zephyr kernel threads; the priority enum is translated to Zephyr cooperative/preemptive priorities.
• Synchronization primitives use k_mutex, k_sem, k_condvar, and k_poll_signal.
• Timers use k_timer.

NuttX backend notes

• Threads map to NuttX pthread or task_create depending on configuration.
• Filesystem operations delegate to the NuttX VFS layer.