Clarification on FreeRTOS Integration with openbsw

[saanjh-sengupta]

I’m new to the openbsw repository and would like some clarification:

1. How is FreeRTOS integrated with the openbsw stack?
2. In the POSIX build, does the application rely on FreeRTOS timers, or does it use native POSIX timing instead?

[nhuvaoanh123]

Hi @saanjh-sengupta — I was digging into exactly this recently. Sharing what I found in case it's useful while maintainers weigh in.

1) FreeRTOS integration architecture

openbsw wraps FreeRTOS behind a four-layer async abstraction, so application code never calls FreeRTOS APIs directly:

• Abstract API — libs/bsw/async/include/async/Async.h exposes RTOS-agnostic execute(), schedule(), scheduleAtFixedRate().
• FreeRTOS adapter — libs/bsw/asyncFreeRtos/include/async/FreeRtosAdapter.h bridges the abstract API to FreeRTOS primitives (xTaskCreateStatic, xTaskNotify, idle/timer task lifecycle, ISR safety via enterIsr/leaveIsr).
• Configuration — libs/bsw/asyncFreeRtos/freeRtosConfiguration/FreeRTOSConfig.h sets up static allocation, idle hook, tick rate, and pulls in the platform-specific os/FreeRtosPlatformConfig.h.
• Application binding — executables/referenceApp/asyncBinding/include/async/AsyncBinding.h picks the adapter at compile time via SUPPORT_FREERTOS and wraps it with runtime monitoring.

One detail worth noting: per-task scheduling (schedule() / scheduleAtFixedRate()) is handled by a software TimerType inside TaskContext.h, driven by getSystemTimeUs32Bit(). It doesn't go through FreeRTOS's global xTimerCreate API even though configUSE_TIMERS=1 in the config.

2) POSIX build timing

Both, layered. FreeRTOS actually runs on the POSIX build — it's just emulated on top of pthreads — but the wall clock is native POSIX.

• RTOS selection: executables/referenceApp/platforms/posix/CMakeLists.txt honors BUILD_TARGET_RTOS. With FREERTOS it pulls in the Cambridge Consultants pthread-backed port at platforms/posix/3rdparty/freeRtosPosix/.
• FreeRTOS APIs are real: xTaskCreateStatic, xTaskNotify, etc. all work, but underneath they're pthread_create, pthread_cond_signal, and signals (SIGUSR1 for task switches, SIGALRM for the tick).
• Time source is POSIX: getSystemTimeUs32Bit() in platforms/posix/bsp/bspSystemTime/src/bsp/time/systemTimer.cpp uses std::chrono::steady_clock, not FreeRTOS ticks. Condition-variable waits inside the port use clock_gettime(CLOCK_REALTIME) + pthread_cond_timedwait (see freeRtosPosix/src/utils/wait_for_event.c).

So your schedule() calls flow: application → TaskContext software timer → std::chrono::steady_clock. FreeRTOS kernel is genuinely there, but on POSIX its time source is your host clock, not an emulated tick counter.