Freertos

FreeRTOS

Contract

Inputs

• FreeRTOS kernel version (tskKERNEL_VERSION_NUMBER in task.h; V11.3.1 is the current release on 2026-09-05).
• Port: CPU core and compiler (for example GCC on Cortex-M4, which sets configPRIO_BITS and the interrupt priority rules).
• The tasks, their periods, and which data flows between them.
• Heap budget and the RAM regions available for it.
• Debug transport, if any (OpenOCD config, see openocd-jtag).

Procedure

1. Create tasks. usStackDepth is in words, not bytes. Higher numbers are more urgent; tskIDLE_PRIORITY is 0 and the highest usable priority is configMAX_PRIORITIES - 1. A task function never returns. Done when: vTaskStartScheduler() is called after every task and queue is created, and no task function has a return path.

   #include "FreeRTOS.h"
   #include "task.h"
   
   void vSensorTask(void *pvParameters) {
       for (;;) {
           read_sensor();
           vTaskDelay(pdMS_TO_TICKS(500));
       }
   }
   
   int main(void) {
       TaskHandle_t xHandle = NULL;
       xTaskCreate(vSensorTask, "sensor",
                   configMINIMAL_STACK_SIZE + 128,   /* words */
                   NULL, tskIDLE_PRIORITY + 2, &xHandle);
       vTaskStartScheduler();                        /* returns only if the heap is too small */
       for (;;);
   }
   

   Give the task that services an interrupt the highest priority among application tasks so the ISR can stay short.

2. Pass data through queues. xQueueSend copies the item; the third argument is the block time when the queue is full. From an ISR use xQueueSendFromISR with pxHigherPriorityTaskWoken and call portYIELD_FROM_ISR on the way out. Done when: every producer and consumer pair shares one queue created before the scheduler starts.

   #include "queue.h"
   
   typedef struct { uint32_t sensor_id; float value; } SensorReading_t;
   QueueHandle_t xSensorQueue;
   
   void vProducerTask(void *p) {
       SensorReading_t r = { .sensor_id = 1 };
       for (;;) {
           r.value = read_adc();
           xQueueSend(xSensorQueue, &r, pdMS_TO_TICKS(10));
           vTaskDelay(pdMS_TO_TICKS(100));
       }
   }
   
   void vConsumerTask(void *p) {
       SensorReading_t r;
       for (;;) {
           if (xQueueReceive(xSensorQueue, &r, portMAX_DELAY) == pdTRUE) {
               process(r.value);
           }
       }
   }
   
   /* before vTaskStartScheduler() */
   xSensorQueue = xQueueCreate(10, sizeof(SensorReading_t));
   

3. Signal with a binary semaphore and guard with a mutex. A binary semaphore carries an event from an ISR to a task. A mutex protects a shared resource and provides priority inheritance, which a semaphore does not; a mutex is never taken or given from an ISR. Done when: every shared resource is behind a mutex and every ISR-to-task signal uses a semaphore, a queue, or a task notification.

   #include "semphr.h"
   
   SemaphoreHandle_t xSem = xSemaphoreCreateBinary();
   
   void UART_ISR(void) {
       BaseType_t xWoken = pdFALSE;
       xSemaphoreGiveFromISR(xSem, &xWoken);
       portYIELD_FROM_ISR(xWoken);
   }
   
   void vUartTask(void *p) {
       for (;;) {
           xSemaphoreTake(xSem, portMAX_DELAY);
           drain_uart();
       }
   }
   
   SemaphoreHandle_t xMutex = xSemaphoreCreateMutex();
   
   void update_shared(void) {
       if (xSemaphoreTake(xMutex, pdMS_TO_TICKS(100)) == pdTRUE) {
           shared_resource++;
           xSemaphoreGive(xMutex);
       }
   }
   

   xSemaphoreCreateRecursiveMutex with xSemaphoreTakeRecursive and xSemaphoreGiveRecursive lets one task take the same mutex more than once.

4. Turn on stack overflow detection and the hooks. Method 2 checks a fill pattern at the top of the stack on every context switch and catches more overflows than method 1, which checks only the stack pointer. Done when: both hooks exist and stop the core, and uxTaskGetStackHighWaterMark has been read for every task under its worst-case load.

   /* FreeRTOSConfig.h */
   #define configCHECK_FOR_STACK_OVERFLOW  2
   #define configUSE_MALLOC_FAILED_HOOK    1
   
   void vApplicationStackOverflowHook(TaskHandle_t xTask, char *pcTaskName) {
       (void)xTask; (void)pcTaskName;
       taskDISABLE_INTERRUPTS();
       for (;;);
   }
   
   void vApplicationMallocFailedHook(void) {
       taskDISABLE_INTERRUPTS();
       for (;;);
   }
   
   UBaseType_t headroom = uxTaskGetStackHighWaterMark(xHandle);  /* minimum ever free, in words */
   

   The right stack size comes from the measured high-water mark under the task's worst-case call depth plus the interrupt frame the port pushes, not from a fixed number.

5. Write FreeRTOSConfig.h. Start from the template the kernel ships in examples/template_configuration/FreeRTOSConfig.h (V11.0.0 and later) and set the values for your MCU. references/freertos-config.md explains each group. Done when: the file compiles, configASSERT stops the core, and the Cortex-M interrupt priority values match the NVIC priority bits of the part.

   #define configCPU_CLOCK_HZ              (SystemCoreClock)
   #define configTICK_RATE_HZ              1000
   #define configMAX_PRIORITIES            8
   #define configMINIMAL_STACK_SIZE        128           /* words */
   #define configTOTAL_HEAP_SIZE           (16 * 1024)   /* bytes */
   #define configMAX_TASK_NAME_LEN         16
   
   #define configUSE_TRACE_FACILITY        1
   #define configUSE_STATS_FORMATTING_FUNCTIONS 1
   #define configCHECK_FOR_STACK_OVERFLOW  2
   #define configUSE_MALLOC_FAILED_HOOK    1
   #define configASSERT(x) if ((x) == 0) { taskDISABLE_INTERRUPTS(); for (;;); }
   
   #define configUSE_MUTEXES               1
   #define configUSE_RECURSIVE_MUTEXES     1
   #define configUSE_COUNTING_SEMAPHORES   1
   #define configUSE_TIMERS                1
   #define configTIMER_TASK_STACK_DEPTH    (configMINIMAL_STACK_SIZE * 2)
   

6. Debug with thread awareness. OpenOCD's RTOS support reads the task list from kernel symbols; enable it per target and keep configUSE_TRACE_FACILITY at 1 so the symbols exist. Done when: info threads in GDB lists every task by name.

   # openocd.cfg, after the target config
   $_TARGETNAME configure -rtos auto     # or: -rtos FreeRTOS
   

   (gdb) info threads      # one row per task
   (gdb) thread 3
   (gdb) bt                # that task's stack
   (gdb) call vTaskList(buf)
   (gdb) printf "%s\n", buf
   

   The OpenOCD user guide notes that FreeRTOS may need an extra OpenOCD-specific object linked into the firmware for symbol access; follow the "RTOS Support" section for the kernel version in use.

7. Use the newer kernel APIs where they fit. Indexed task notifications (xTaskNotifyIndexed, xTaskNotifyWaitIndexed) and stream buffers (xStreamBufferSend, xStreamBufferSetTriggerLevel) are in V10.4 and later; V11.0.0 added symmetric multiprocessing to the mainline. Check tskKERNEL_VERSION_NUMBER before relying on any of them. Done when: no API call is newer than the kernel in the tree.

   xTaskNotifyIndexed(xHandle, 0, ulValue, eSetValueWithOverwrite);
   xTaskNotifyWaitIndexed(0, 0, ULONG_MAX, &ulNotified, portMAX_DELAY);
   
   xStreamBufferSend(xStream, data, len, 0);
   xStreamBufferSetTriggerLevel(xStream, 1);   /* wake the receiver on one byte */
   

8. Restrict a task with the MPU when the port supports it. xTaskCreateRestricted takes a TaskParameters_t with up to portNUM_CONFIGURABLE_REGIONS regions; a zero-length region ends the list. On ARMv8-M with TrustZone, a task that calls secure functions allocates a secure context with portALLOCATE_SECURE_CONTEXT(size) after SecureContext_Init() on the secure side, with configENABLE_TRUSTZONE set. Done when: the restricted task runs and a deliberate write outside its regions raises the MemManage fault.

   static const TaskParameters_t xRestricted = {
       .pvTaskCode = vRestrictedTask,
       .pcName = "restricted",
       .usStackDepth = configMINIMAL_STACK_SIZE,
       .pvParameters = NULL,
       .uxPriority = tskIDLE_PRIORITY + 1,
       .puxStackBuffer = stackBuffer,
       .xRegions = {
           { (void *)0x20000000, 0x10000, tskMPU_REGION_READ_ONLY | tskMPU_REGION_EXECUTE_NEVER },
           { NULL, 0, 0 },
       },
   };
   xTaskCreateRestricted(&xRestricted, &xHandle);
   

9. For TCP, use FreeRTOS+TCP after FreeRTOS_IPInit. Buffer counts and protocol switches live in FreeRTOSIPConfig.h (ipconfigUSE_TCP, ipconfigNUM_NETWORK_BUFFER_DESCRIPTORS). Done when: FreeRTOS_socket, FreeRTOS_connect, and FreeRTOS_send move bytes to a peer.

   #include "FreeRTOS_IP.h"
   #include "FreeRTOS_Sockets.h"
   
   Socket_t s = FreeRTOS_socket(FREERTOS_AF_INET, FREERTOS_SOCK_STREAM, FREERTOS_IPPROTO_TCP);
   FreeRTOS_connect(s, &xAddress, sizeof(xAddress));
   FreeRTOS_send(s, buffer, len, 0);
   

Failure and recovery

Output

A FreeRTOS application in the named directory with tasks, queues, and mutexes as designed, a FreeRTOSConfig.h with overflow detection and asserts on, and, when a probe is present, a GDB session that lists tasks with info threads.