memory-mgmt-patterns

memory-mgmt-patterns

Knowledge Base

Embedded memory management patterns for deterministic, fragmentation-free firmware.

---

Pattern 1: Static FreeRTOS Objects

Avoid dynamic allocation for RTOS objects. All objects pre-allocated at compile time.

/* Static task */
static StackType_t  s_sensor_stack[256];
static StaticTask_t s_sensor_tcb;
static TaskHandle_t s_sensor_handle;

/* Static queue: 8 messages of 16 bytes each */
static uint8_t          s_queue_storage[8 * 16];
static StaticQueue_t    s_queue_struct;
static QueueHandle_t    s_queue;

/* Static semaphore */
static StaticSemaphore_t s_sem_struct;
static SemaphoreHandle_t s_sem;

void rtos_objects_create(void)
{
    s_sensor_handle = xTaskCreateStatic(
        sensor_task, "sensor",
        256U, NULL, TASK_PRIO_SENSOR,
        s_sensor_stack, &s_sensor_tcb);
    configASSERT(s_sensor_handle != NULL);

    s_queue = xQueueCreateStatic(8, 16, s_queue_storage, &s_queue_struct);
    configASSERT(s_queue != NULL);

    s_sem = xSemaphoreCreateBinaryStatic(&s_sem_struct);
    configASSERT(s_sem != NULL);
}

Required in FreeRTOSConfig.h: configSUPPORT_STATIC_ALLOCATION 1.

---

Pattern 2: Memory Pool for Variable-Frequency Messages

When messages arrive at variable rate, a pool prevents heap fragmentation:

/* Message pool: 32 messages, 128 bytes each */
typedef struct {
    uint8_t  data[120];
    uint8_t  len;
    uint8_t  type;
    uint16_t seq;
} msg_t;

#define MSG_POOL_SIZE 32U
static msg_t      s_msg_pool[MSG_POOL_SIZE];
static uint32_t   s_pool_used_mask = 0U;  /* Bitmask: bit N = msg N is in use */

msg_t *msg_alloc(void)
{
    taskENTER_CRITICAL();
    for (uint32_t i = 0; i < MSG_POOL_SIZE; i++) {
        if (!(s_pool_used_mask & (1U << i))) {
            s_pool_used_mask |= (1U << i);
            taskEXIT_CRITICAL();
            return &s_msg_pool[i];
        }
    }
    taskEXIT_CRITICAL();
    return NULL;  /* Pool exhausted: caller must handle */
}

void msg_free(msg_t *m)
{
    uint32_t idx = (uint32_t)(m - s_msg_pool);
    configASSERT(idx < MSG_POOL_SIZE);
    taskENTER_CRITICAL();
    s_pool_used_mask &= ~(1U << idx);
    taskEXIT_CRITICAL();
}