TinyOS — Ultra-Lightweight RTOS for IoT

An ultra-lightweight real-time operating system for resource-constrained IoT and embedded devices.
Kernel footprint under 10 KB, 2 KB minimum RAM, preemptive priority-based scheduling.

---

Features

Category	Details
Kernel	Preemptive priority-based scheduling (256 levels), round-robin within same priority, O(1) priority lookup via bitmap, priority inheritance
Synchronization	Mutex (with priority inheritance), semaphore, condition variable, event groups, message queues
Software Timers	One-shot and auto-reload, millisecond precision, period change at runtime
Memory	Fixed-block pool allocator, stack overflow detection, per-task high-water mark
Shell	VT100 interactive shell — 23 built-in commands, command history (↑↓), tab completion, full line editor
POSIX Compatibility	pthreads (create/join/detach/exit, mutex, cond var) · BSD socket API (socket/bind/listen/accept/connect/send/recv, inet_pton/ntop, htons/htonl)
File System	Lightweight block-device FS, POSIX-like API, wear levelling, power-fail safe
Network	Ethernet, IPv4, ICMP, UDP, TCP, HTTP client/server, DNS
TLS / DTLS	TLS 1.2/1.3 over TCP, DTLS 1.2 over UDP (mbedTLS backend)
MQTT	Full MQTT 3.1.1 — QoS 0/1/2 with in-flight retry table, offline queue, auto-reconnect with exponential back-off
CoAP	RFC 7252 compliant client/server, observe pattern
OTA	A/B partition firmware updates, CRC32 verification, rollback
Watchdog	Hardware and software watchdog, per-task timeout monitoring
Power	Idle / Sleep / Deep-sleep modes, tickless idle, CPU frequency scaling
Security	MPU-based memory protection, secure boot support

---

Supported Hardware

Architecture	Examples
ARM Cortex-M (M0/M0+/M3/M4/M7)	STM32, nRF52, Raspberry Pi Pico
RISC-V (RV32I)	ESP32-C3
AVR (experimental)	ATmega

---

Quick Start

Prerequisites

# ARM cross-compiler (required)
sudo apt-get install -y gcc-arm-none-eabi binutils-arm-none-eabi

# QEMU ARM emulator (optional — for running without hardware)
sudo apt-get install -y qemu-system

Verify installation:

arm-none-eabi-gcc --version   # 10.x or later
qemu-system-arm --version     # 6.x or later

Build

# Default example (blink_led)
make

# Specific example
make EXAMPLE=blink_led        # LED blink + task scheduler demo
make EXAMPLE=event_groups     # Event group AND/OR/NOT/SYNC demo
make EXAMPLE=iot_sensor       # Multi-sensor IoT node
make EXAMPLE=shell_demo       # Interactive UART shell
make EXAMPLE=mqtt_demo        # MQTT publish/subscribe
make EXAMPLE=condition_variable  # Producer/consumer

# Convenience aliases
make example-blink
make example-events
make example-shell
make example-mqtt
make example-iot

# Build output
make size                     # Print ROM/RAM usage

Build artifacts are placed in build/:

File	Description
build/tinyos.elf	ELF image with debug symbols
build/tinyos.bin	Raw binary for flashing
build/tinyos.map	Linker map (symbol sizes)

Run on QEMU

TinyOS runs on the QEMU mps2-an385 target (ARM Cortex-M3, 4 MB flash, 4 MB RAM):

# Run indefinitely (Ctrl-A X to quit)
qemu-system-arm \
    -machine mps2-an385 \
    -cpu cortex-m3 \
    -nographic \
    -kernel build/tinyos.elf

# Run for a fixed duration (e.g. 10 seconds)
timeout 10 qemu-system-arm \
    -machine mps2-an385 \
    -cpu cortex-m3 \
    -nographic \
    -kernel build/tinyos.elf

# Debug: trace interrupts
qemu-system-arm \
    -machine mps2-an385 \
    -cpu cortex-m3 \
    -nographic \
    -d int \
    -kernel build/tinyos.elf

Expected output from the interrupt trace: repeated successful exception return lines confirm the scheduler is running, SysTick is ticking, and PendSV context switches are completing cleanly.

Flash to Hardware

# OpenOCD (STM32 example)
openocd -f interface/stlink.cfg -f target/stm32f4x.cfg \
    -c "program build/tinyos.bin verify reset exit 0x08000000"

# pyOCD (generic ARM Cortex-M)
pyocd flash --target cortex_m build/tinyos.bin

Build with TLS (mbedTLS)

TLS support is enabled automatically when mbedTLS is present at ~/mbedtls. To use a different path:

# Clone and build mbedTLS
git clone https://github.com/Mbed-TLS/mbedtls ~/mbedtls
make -C ~/mbedtls

# Build TinyOS with TLS
make MBEDTLS_DIR=~/mbedtls

Minimal task example:

#include "tinyos.h"

void my_task(void *param) {
    while (1) {
        /* work */
        os_task_delay_ms(100);
    }
}

int main(void) {
    tcb_t task;
    os_init();
    os_task_create(&task, "my_task", my_task, NULL, PRIORITY_NORMAL);
    os_start();
}

---

API Overview

Task Management

os_task_create(tcb, name, entry, param, priority)
os_task_delete(task)
os_task_suspend(task) / os_task_resume(task)
os_task_delay(ticks) / os_task_delay_ms(ms)
os_task_set_priority(task, priority)
os_task_get_stats(task, stats)
os_task_get_stats_by_index(index, stats)   /* iterate all tasks by index */
os_task_find_by_name(name)                 /* returns tcb_t*, NULL if not found */
os_get_system_stats(stats)
os_get_memory_stats(&free, &used, &allocs, &frees)

Synchronization

os_mutex_init(mutex) / os_mutex_lock(mutex, timeout) / os_mutex_unlock(mutex)
os_semaphore_init(sem, count) / os_semaphore_wait(sem, timeout) / os_semaphore_post(sem)
os_cond_init(cond) / os_cond_wait(cond, mutex, timeout)
os_cond_signal(cond) / os_cond_broadcast(cond)
os_event_group_set_bits(eg, bits) / os_event_group_wait_bits(eg, bits, opts, out, timeout)
os_queue_init(q, buf, item_size, max) / os_queue_send(q, item, timeout)
os_queue_receive(q, item, timeout) / os_queue_peek(q, item, timeout)

Timers

os_timer_create(timer, name, type, period_ms, callback, param)
os_timer_start(timer) / os_timer_stop(timer) / os_timer_reset(timer)
os_timer_change_period(timer, ms) / os_timer_get_remaining_ms(timer)

Shell

/* Register custom commands before calling shell_start() */
shell_register_cmd(name, handler_fn, help_text)

/* Provide UART I/O callbacks and start the shell task */
shell_io_t io = { .getc = uart_getc, .puts = uart_puts };
shell_start(&io)

/* Change the prompt at any time */
shell_set_prompt("mydevice> ")

/* Execute a single line programmatically */
shell_exec(line)

Custom command example:

static int cmd_led(int argc, char *argv[]) {
    if (argc < 2) return 1;  /* non-zero → prints usage */
    bool on = (strcmp(argv[1], "on") == 0);
    gpio_write(LED_PIN, on);
    return 0;
}

/* In main(), before shell_start(): */
shell_register_cmd("led", cmd_led, "led <on|off>  Toggle LED");

Built-in Shell Commands

Command	Description
help [cmd]	List all commands, or show detailed help for cmd
clear	Clear the terminal screen (VT100)
echo <text>	Print text to the terminal
history	Show command history
ps	List all tasks (state, priority, CPU%, stack usage)
top	Task list sorted by CPU usage, descending
kill <name> [suspend|resume|delete]	Control a task by name
mem	Heap statistics (total / used / free, alloc/free counts)
ver	TinyOS version and formatted uptime
net	Network statistics (Ethernet, IP, UDP, TCP counters)
ping <ip> [count]	Send ICMP echo requests
ifconfig [ip|netmask|gw|dns <addr>]	Show or change network configuration
power [active|idle|sleep|deepsleep]	Power stats or mode change
ls [path]	List directory (default: /)
cat <file>	Display file contents
mkdir <path>	Create directory
rm <path>	Remove file or empty directory
df	Filesystem usage statistics
touch <file>	Create empty file
cp <src> <dst>	Copy file
uptime	Show system uptime (HH:MM:SS or N day(s), HH:MM:SS)
sleep <ms>	Delay the shell task for N milliseconds
reboot	Reboot the system

Line editor key bindings:

Key	Action
← / Ctrl-B	Move cursor left
→ / Ctrl-F	Move cursor right
Home / Ctrl-A	Jump to start of line
End / Ctrl-E	Jump to end of line
↑ / ↓	Navigate command history
Tab	Complete command name
Ctrl-K	Kill to end of line
Ctrl-U	Kill to start of line
Ctrl-W	Kill previous word
Ctrl-L	Clear screen and redraw
Ctrl-C	Cancel current line

Shell configuration (include/tinyos/shell.h):

#define SHELL_MAX_COMMANDS   32    /* max registered commands       */
#define SHELL_LINE_MAX       128   /* max input line length (bytes) */
#define SHELL_ARGV_MAX       16    /* max arguments per command     */
#define SHELL_HISTORY_DEPTH  8     /* command history entries       */

Network

net_init(driver, config) / net_start()
net_socket(type) / net_bind(sock, addr) / net_connect(sock, addr, timeout_ms)
net_send(sock, data, len, timeout_ms) / net_recv(sock, buf, len, timeout_ms)
net_sendto(sock, data, len, addr) / net_recvfrom(sock, buf, len, addr)
net_close(sock)
net_ping(dest_ip, timeout_ms, rtt)
net_dns_resolve(hostname, ip, timeout_ms)
net_http_get(url, response, timeout_ms)
net_http_post(url, content_type, body, len, response, timeout_ms)

POSIX Compatibility

TinyOS provides two thin compatibility layers that allow standard portable code to be compiled and run on TinyOS with minimal changes.

pthreads (include/tinyos/posix_threads.h)

Add src/posix/posix_threads.c to your build.

#include "tinyos/posix_threads.h"

/* ── Thread ── */
pthread_t tid;
pthread_attr_t attr;
pthread_attr_init(&attr);
tinyos_pthread_attr_setpriority(&attr, PRIORITY_NORMAL); /* TinyOS extension */
pthread_create(&tid, &attr, my_fn, arg);
pthread_join(tid, &retval);
pthread_detach(tid);    /* free resources automatically on exit */
pthread_exit(retval);   /* terminate calling thread */
pthread_self();         /* handle of calling thread */

/* ── Mutex ── */
pthread_mutex_t mtx = PTHREAD_MUTEX_INITIALIZER;
pthread_mutex_lock(&mtx);
pthread_mutex_trylock(&mtx);   /* returns EBUSY if already locked */
pthread_mutex_unlock(&mtx);

/* ── Condition variable ── */
pthread_cond_t cond = PTHREAD_COND_INITIALIZER;
pthread_cond_wait(&cond, &mtx);
pthread_cond_timedwait(&cond, &mtx, &abstime); /* abstime relative to boot */
pthread_cond_signal(&cond);
pthread_cond_broadcast(&cond);

Concept	Mapped to
pthread_t	Index into an internal pool of TinyOS tcb_t slots
pthread_mutex_t	Embeds mutex_t directly (zero-init / PTHREAD_MUTEX_INITIALIZER valid)
pthread_cond_t	Embeds cond_var_t directly (zero-init / PTHREAD_COND_INITIALIZER valid)
pthread_join	Waits on a per-thread semaphore_t posted by pthread_exit

Not supported: pthread_cancel, thread-local storage (pthread_key_*), recursive mutexes (returns ENOTSUP).

Configuration (include/tinyos/posix_threads.h):

#define PTHREAD_MAX_THREADS  MAX_TASKS  /* max concurrent pthreads */

BSD Sockets (include/tinyos/posix_socket.h)

Add src/posix/posix_socket.c to your build.

#include "tinyos/posix_socket.h"

/* ── TCP server ── */
int srv = socket(AF_INET, SOCK_STREAM, 0);

int reuse = 1;
setsockopt(srv, SOL_SOCKET, SO_REUSEADDR, &reuse, sizeof(reuse));

struct sockaddr_in addr = {
    .sin_family = AF_INET,
    .sin_port   = htons(8080),
    .sin_addr   = { htonl(INADDR_ANY) },
};
bind(srv, (struct sockaddr *)&addr, sizeof(addr));
listen(srv, 4);

struct sockaddr_in peer;
socklen_t plen = sizeof(peer);
int client = accept(srv, (struct sockaddr *)&peer, &plen);
recv(client, buf, sizeof(buf), 0);
send(client, response, response_len, 0);
posix_sock_close(client);   /* or define TINYOS_POSIX_WRAP_CLOSE to use close() */
posix_sock_close(srv);

/* ── TCP client ── */
int fd = socket(AF_INET, SOCK_STREAM, 0);
struct sockaddr_in dest = {
    .sin_family = AF_INET,
    .sin_port   = htons(80),
    .sin_addr   = { inet_addr("192.168.1.1") },
};
connect(fd, (struct sockaddr *)&dest, sizeof(dest));
send(fd, request, request_len, 0);
recv(fd, buf, sizeof(buf), 0);
posix_sock_close(fd);

/* ── UDP ── */
int udp = socket(AF_INET, SOCK_DGRAM, 0);
sendto(udp, data, len, 0, (struct sockaddr *)&dest, sizeof(dest));
recvfrom(udp, buf, sizeof(buf), 0, (struct sockaddr *)&src, &srclen);
posix_sock_close(udp);

/* ── Address utilities ── */
htons(port) / htonl(addr) / ntohs(n) / ntohl(n)
inet_addr("192.168.1.1")              /* → in_addr_t, network byte order */
inet_ntoa(in)                          /* → "192.168.1.1" (static buffer) */
inet_pton(AF_INET, "192.168.1.1", &in_addr)
inet_ntop(AF_INET, &in_addr, buf, sizeof(buf))

BSD call	Mapped to
socket()	net_socket()
bind()	net_bind()
listen()	net_listen()
accept()	net_accept()
connect()	net_connect()
send() / recv()	net_send() / net_recv()
sendto() / recvfrom()	net_sendto() / net_recvfrom()
posix_sock_close()	net_close()

Supported setsockopt options:

Option	Effect
SO_REUSEADDR	Accepted; no-op (always reusable in TinyOS)
SO_RCVTIMEO	Sets receive timeout per socket (struct timeval → ms)
SO_SNDTIMEO	Sets send/connect timeout per socket

close() redirection: define TINYOS_POSIX_WRAP_CLOSE before including the header to map close(fd) → posix_sock_close(fd).

Not supported: select / poll / epoll, non-blocking mode (O_NONBLOCK), IPv6 (AF_INET6 returns EAFNOSUPPORT).

---

TLS / DTLS

TLS 1.2/1.3 over TCP and DTLS 1.2 over UDP backed by mbedTLS.
Enable at build time with -DTINYOS_TLS_ENABLE and link against mbedTLS.

/* Client (TLS over TCP) */
tls_context_t tls;
tls_config_t  cfg = TLS_CONFIG_DEFAULT_CLIENT;
cfg.ca_cert     = ca_cert_pem;
cfg.ca_cert_len = sizeof(ca_cert_pem);
tls_init(&tls, &cfg);

net_socket_t sock = net_socket(SOCK_STREAM);
net_connect(sock, &broker_addr, 5000);
tls_connect(&tls, sock, "example.com", 5000);

tls_send(&tls, data, len);
tls_recv(&tls, buf, sizeof(buf), 5000);
tls_close(&tls);

/* Server (TLS accept) */
tls_config_t srv_cfg = TLS_CONFIG_DEFAULT_SERVER;
srv_cfg.cert     = server_cert_pem;
srv_cfg.cert_len = sizeof(server_cert_pem);
srv_cfg.key      = server_key_pem;
srv_cfg.key_len  = sizeof(server_key_pem);
tls_init(&tls, &srv_cfg);
tls_accept(&tls, client_sock, 5000);

/* DTLS over UDP */
tls_config_t dtls_cfg = TLS_CONFIG_DEFAULT_DTLS_CLIENT;
net_socket_t usock = net_socket(SOCK_DGRAM);
tls_connect_dtls(&tls, usock, "example.com", 5000);

MQTT

Full MQTT 3.1.1 with per-message QoS delivery guarantees.

mqtt_config_t cfg = {
    .broker_host          = "mqtt.example.com",
    .client_id            = "tinyos-01",
    .keepalive_sec        = 60,
    .clean_session        = true,
    .auto_reconnect       = true,
    .reconnect_interval_ms = 3000,   /* base; doubles each attempt (max 60 s) */
};
mqtt_client_t client;
mqtt_client_init(&client, &cfg);
mqtt_set_connection_callback(&client, on_connect, NULL);
mqtt_set_message_callback(&client, on_message, NULL);
mqtt_connect(&client);

/* Publish — QoS1/2 are buffered offline if disconnected */
mqtt_publish(&client, "sensors/temp", "23.5", 4, MQTT_QOS_1, false);

/* Inspect reliability queues */
uint8_t in_flight = mqtt_get_inflight_count(&client);  /* sent, awaiting ACK  */
uint8_t pending   = mqtt_get_pending_count(&client);   /* queued while offline */

mqtt_subscribe(&client, "cmd/#", MQTT_QOS_1);
mqtt_flush_pending(&client);   /* discard offline queue */
mqtt_disconnect(&client);

MQTT Reliability Model

QoS 0  ─── fire-and-forget; dropped if disconnected
QoS 1  ─── in-flight table tracks each PUBLISH until PUBACK
              ↳ retransmits with DUP=1 every 5 s, up to 5 times
              ↳ if offline → offline queue (up to 8 messages)
QoS 2  ─── full PUBLISH → PUBREC → PUBREL → PUBCOMP handshake
              ↳ each step is retried independently on timeout

Auto-reconnect back-off: 3 s → 6 s → 12 s → … → 60 s (cap)
On reconnect: re-subscribes all topics, flushes offline queue

MQTT reliability configuration (include/tinyos/mqtt.h):

#define MQTT_MAX_INFLIGHT          8      /* in-flight slots       */
#define MQTT_MAX_PENDING           8      /* offline queue slots   */
#define MQTT_MAX_PAYLOAD_SIZE      512    /* bytes per queued msg  */
#define MQTT_RETRY_INTERVAL_MS     5000   /* retry after (ms)      */
#define MQTT_MAX_RETRY_COUNT       5      /* retries before drop   */
#define MQTT_RECONNECT_BASE_MS     3000   /* first reconnect delay */
#define MQTT_RECONNECT_MAX_MS      60000  /* backoff ceiling       */

CoAP

coap_init(ctx, config, is_server) / coap_start(ctx) / coap_stop(ctx)
coap_get(ctx, ip, port, path, response, timeout_ms)
coap_post(ctx, ip, port, path, format, payload, len, response, timeout_ms)
coap_resource_create(ctx, path, handler, user_data)
coap_process(ctx, timeout_ms)

OTA

ota_init(config)
ota_start_update(url, callback, user_data)
ota_write_chunk(data, size, offset) / ota_finalize_update()
ota_confirm_boot() / ota_rollback()
ota_verify_partition(type)

File System

fs_format(device) / fs_mount(device) / fs_unmount()
fs_open(path, flags) / fs_close(fd)
fs_read(fd, buf, size) / fs_write(fd, buf, size)
fs_seek(fd, offset, whence) / fs_tell(fd)
fs_mkdir(path) / fs_remove(path) / fs_rmdir(path)
fs_stat(path, stat)
fs_opendir(path) / fs_readdir(dir, entry) / fs_closedir(dir)
fs_get_stats(stats) / fs_get_free_space() / fs_is_mounted()

Power Management

os_power_init()
os_power_set_mode(mode)          /* ACTIVE / IDLE / SLEEP / DEEP_SLEEP */
os_power_get_mode()
os_power_enter_sleep(duration_ms)
os_power_enter_deep_sleep(duration_ms)
os_power_enable_tickless_idle(enable)
os_power_set_cpu_frequency(freq_hz)
os_power_configure_wakeup(source, enable)
os_power_get_stats(stats)
os_power_get_consumption_mw()
os_power_estimate_battery_life_hours()

Watchdog

wdt_init(config) / wdt_start() / wdt_stop()
wdt_feed() / wdt_set_timeout(ms)
wdt_register_task(task, timeout_ms) / wdt_feed_task(task)

---

Configuration

include/tinyos.h — kernel and OS:

#define MAX_TASKS        8      /* max concurrent tasks           */
#define STACK_SIZE       256    /* stack size per task (words)    */
#define TICK_RATE_HZ     1000   /* scheduler tick frequency (Hz)  */
#define TIME_SLICE_MS    10     /* round-robin time slice (ms)    */

include/tinyos/shell.h — interactive shell:

#define SHELL_MAX_COMMANDS   32   /* max registered commands         */
#define SHELL_LINE_MAX       128  /* max input line length (bytes)   */
#define SHELL_ARGV_MAX       16   /* max arguments per command       */
#define SHELL_HISTORY_DEPTH  8    /* command history ring buffer     */

include/tinyos/mqtt.h — MQTT reliability:

#define MQTT_MAX_INFLIGHT        8      /* in-flight QoS1/2 slots    */
#define MQTT_MAX_PENDING         8      /* offline queue slots       */
#define MQTT_MAX_PAYLOAD_SIZE    512    /* max queued payload bytes  */
#define MQTT_RETRY_INTERVAL_MS   5000   /* unACKed retry interval    */
#define MQTT_MAX_RETRY_COUNT     5      /* retries before discard    */
#define MQTT_RECONNECT_BASE_MS   3000   /* initial reconnect delay   */
#define MQTT_RECONNECT_MAX_MS    60000  /* back-off ceiling          */

TLS — requires mbedTLS; enable with:

CFLAGS += -DTINYOS_TLS_ENABLE
LDFLAGS += -lmbedtls -lmbedcrypto -lmbedx509

---

Project Structure

tinyos-rtos/
├── include/
│   ├── tinyos.h              # Core API (tasks, sync, timers, memory, FS, power)
│   └── tinyos/
│       ├── shell.h           # Interactive shell API & configuration
│       ├── net.h             # Network stack
│       ├── tls.h             # TLS 1.2/1.3 + DTLS 1.2 (mbedTLS)
│       ├── mqtt.h            # MQTT 3.1.1 client
│       ├── coap.h            # CoAP RFC 7252
│       ├── ota.h             # OTA firmware updates
│       ├── watchdog.h        # Watchdog timer
│       ├── posix_threads.h   # POSIX pthreads compatibility layer
│       └── posix_socket.h    # BSD socket compatibility layer
├── src/
│   ├── startup.s             # Vector table, Reset_Handler, SysTick/SVC/PendSV stubs
│   ├── context_switch.s      # Thumb-2: PendSV_Handler, SVC_Handler, os_pend_sv
│   ├── kernel.c              # Preemptive scheduler & task management
│   ├── sync.c                # Mutex, semaphore, queue, condition var, event groups
│   ├── timer.c               # Software timers
│   ├── memory.c              # Heap allocator
│   ├── shell.c               # Interactive shell (VT100, history, tab completion)
│   ├── filesystem.c          # Block-device file system
│   ├── security.c            # MPU memory protection
│   ├── power.c               # Power management & CPU frequency scaling
│   ├── watchdog.c            # Watchdog (HW + SW, per-task monitoring)
│   ├── bootloader.c          # Secure bootloader
│   ├── ota.c                 # OTA A/B partition updates
│   ├── mqtt.c                # MQTT client (in-flight table, offline queue)
│   ├── coap.c                # CoAP client/server
│   ├── net/
│   │   ├── network.c         # Core & buffer management
│   │   ├── ethernet.c        # Ethernet / ARP
│   │   ├── ip.c              # IPv4 / ICMP
│   │   ├── socket.c          # UDP / TCP socket API
│   │   ├── http_dns.c        # HTTP client & DNS resolver
│   │   └── tls.c             # TLS/DTLS (mbedTLS wrapper, excluded when mbedTLS absent)
│   └── posix/
│       ├── posix_threads.c   # pthreads → TinyOS task/sync wrapper
│       └── posix_socket.c    # BSD socket → net_* wrapper
├── drivers/
│   ├── flash.c / flash.h     # Flash memory driver
│   ├── ramdisk.c / ramdisk.h # RAM disk (testing)
│   └── loopback_net.c        # Loopback network driver (testing)
├── linker.ld                 # Linker script (mps2-an385: Flash 0x0/4MB, RAM 0x20000000/4MB)
└── examples/
    ├── blink_led.c           # GPIO blink
    ├── iot_sensor.c          # Multi-task sensor node
    ├── shell_demo.c          # Custom shell commands over UART
    ├── network_demo.c        # TCP/UDP/HTTP/ping
    ├── tls_demo.c            # TLS client/server
    ├── mqtt_demo.c           # MQTT publish/subscribe (QoS 1/2)
    ├── coap_demo.c           # CoAP client/server
    ├── ota_demo.c            # Firmware update flow
    ├── filesystem_demo.c     # File I/O
    ├── watchdog_demo.c       # Watchdog configuration
    ├── low_power.c           # Power mode transitions
    ├── software_timers.c     # Timer creation and callbacks
    ├── event_groups.c        # Event synchronisation
    ├── event_flags_logic.c   # AND / OR / NOT(CLEAR) / SYNC(barrier) patterns
    ├── condition_variable.c  # Producer/consumer
    ├── priority_adjustment.c # Dynamic priority
    ├── task_statistics.c     # CPU and stack monitoring
    └── posix_compat_demo.c   # POSIX pthreads + socket usage examples

---

Performance

Component	ROM	RAM
Kernel	6 KB	512 B
Per task	—	~1 KB
Mutex	—	12 B
Semaphore	—	8 B
Message queue (10 items)	—	40 B + data
Shell (with 23 built-ins)	~4 KB	~2.5 KB
MQTT client (with queues)	~8 KB	~10 KB
POSIX threads layer	~2 KB	~PTHREAD_MAX_THREADS × (tcb_t + 32 B)
POSIX socket layer	~1 KB	~NET_MAX_SOCKETS × 12 B

Architecture	Context switch
Cortex-M0	~2 μs
Cortex-M4	~1 μs
RISC-V	~1.5 μs

System requirements: 2 KB RAM minimum · < 10 KB ROM for kernel alone

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Changelog

v1.1.0

New features

• EVENT_WAIT_CLEAR flag for event groups — wait until bits become clear (NOT condition).
  EVENT_WAIT_ALL | EVENT_WAIT_CLEAR wakes when all masked bits are 0; EVENT_WAIT_ANY | EVENT_WAIT_CLEAR wakes when any is 0.
• os_event_group_sync() — rendezvous / barrier primitive.
  Each participating task sets its own arrival bit and blocks until the full set is present; all unblock simultaneously.
• New example examples/event_flags_logic.c demonstrating all four modes (AND, OR, NOT, SYNC).

Build & runtime fixes

• Added src/startup.s: vector table, Reset_Handler (.data copy, .bss zero), SysTick_Handler stub, HardFault_Handler.
• Added linker.ld: memory layout for mps2-an385 (Flash 0x00000000 / 4 MB, RAM 0x20000000 / 4 MB).
• Fixed initial task stack: R4–R11 save area is now pre-allocated so PendSV's LDMIA {R4-R11} works correctly on first context switch into any newly created task.
• SVC_Handler updated to LDMIA {R4-R11} before setting PSP, keeping both SVC and PendSV paths symmetric.
• MPU_TYPE check in os_mpu_configure_default() — MPU setup is skipped silently when no MPU is present (QEMU mps2-an385).
• Renamed timer_t → os_timer_t to avoid conflict with POSIX <sys/types.h>.
• Makefile: -I. added for drivers/ includes; TLS sources excluded automatically when mbedTLS is absent; -Wno-stringop-truncation for false-positive strncpy warnings.
• Fixed sign-compare, unused-function, uninitialized-variable, and implicit-declaration warnings across coap.c, filesystem.c, mqtt.c, ota.c, security.c, net/ip.c, net/http_dns.c.

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License

MIT License — see LICENSE for details.