EventOS

A cooperative, event-driven operating system for microcontrollers. EventOS sits between a superloop and an RTOS: it gives you the structure of a real OS — actions, timers, pub/sub, a bulletin board — without the complexity and footprint of preemption, scheduling priorities, or per-task stacks. Actions run to completion in FIFO order on a single stack, and the runtime never interrupts one piece of work to start another.

A first taste

The Blinky-posix example runs on any POSIX host — no hardware required — and is the whole program:

#include "Led.h"
#include "os/os.h"
#include "posix.h"

void Blinky(os_context_t context) {
    (void)context;
    led_Toggle(LED);
    os_DoAfter(Blinky, OS_NO_CONTEXT, OS_NO_KEY, 200);
}

int main(void) {
    posix_Init();
    os_Do(Blinky, OS_NO_CONTEXT);
    for (;;) {
        posix_Tick();
        os_Exec();
    }
}

os_Do posts the first invocation; each call to Blinky toggles the LED and re-posts itself 200 ms later through the timer queue. There are no threads to create, no locks to take, no priorities to tune.

When EventOS fits

• Your firmware is already event-shaped: things happen in response to timers, interrupts, button presses, incoming messages.
• You want deterministic, in-order execution — no priority inversions, no "which task ran first" or "why is my task being starved" surprises.
• You want a small footprint (~4 KB text, ~1.2 KB RAM on STM32F103 with every feature in-use).
• You're willing to write non-blocking actions and break long-running work into steps.

When to reach for an RTOS instead

• You need preemptive scheduling — a high-priority task must be able to interrupt a lower-priority one mid-execution to meet hard real-time deadlines.
• Your application is naturally thread-shaped, with long sequential workflows and blocking APIs (recv, pthread_cond_wait, etc.).
• You need a rich IPC surface (counting semaphores, blocking mutexes, mailboxes with blocking reads).

Try it

On a laptop:

cd examples/Blinky-posix
make
./build/Blinky-posix        # Ctrl-C to stop

You'll see LED state changes printed with millisecond timestamps — the same semantics the firmware produces on real hardware, just with stdout instead of a GPIO pin.

On hardware (STM32F103 "BluePill", arm-none-eabi-gcc):

cd examples/Blinky
make
# flash build/Blinky.elf with your tool of choice

Learn more

• examples/ — runnable code for every feature. Start with Blinky-posix, then Blinky (bluepill). The rest cover contexts, pub/sub, the bulletin board, cancellable timers, ISR handoff, software debounce, chained actions, and a combined all-features demo.
• EventOS.md — the design tour: concepts, patterns, concurrency model, configuration, how to port to a new platform.
• doxygen eventos.doxy — generates the Doxygen API reference from the headers into docs/.
• ./build.sh (from the repo root) — runs the full unit test suite and regenerates the docs.

License

BSD Zero Clause License (0BSD) — use, modify, and redistribute freely; no attribution or warranty.

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