Stack overflows are silent killers — they corrupt adjacent memory, crash the system days after the bug was introduced, and are nearly impossible to reproduce without tooling. FreeRTOS provides three detection methods. This part shows how to use all of them and how to measure actual stack usage with high-water marks.

✓Enable stack overflow detection methods 1 and 2
✓Implement vApplicationStackOverflowHook() to catch overflows at runtime
✓Measure real stack usage with uxTaskGetStackHighWaterMark() and right-size stacks
✓Build a monitoring task that reports memory health over UART

Detection Methods

🔍

Method 1 — SP Check

After each context switch, checks if the stack pointer passed the boundary. Fast but can miss transient overflows.

configCHECK_FOR_STACK_OVERFLOW = 1

🎨

Method 2 — Paint Pattern

Fills the last 20 stack bytes with a known pattern at creation. Checks the pattern on every context switch — catches far more overflows.

configCHECK_FOR_STACK_OVERFLOW = 2

📊

High Water Mark

Returns the minimum remaining stack in words. Zero means the stack is full. Call after full load testing to right-size every task.

uxTaskGetStackHighWaterMark()

Code

Overflow hook + watermark monitor

stack_monitor.c

#define configCHECK_FOR_STACK_OVERFLOW 2

void vApplicationStackOverflowHook(
    TaskHandle_t xTask, char *pcName)
{
    uart_puts("[FATAL] Stack overflow: ");
    uart_puts(pcName); uart_puts("
");
    taskDISABLE_INTERRUPTS();
    for(;;);
}

TaskHandle_t xSensorH, xProcessH, xStorageH;

void vMonitorTask(void *pv) {
    while(1) {
        uart_printf("Sensor  watermark: %u words
",
            uxTaskGetStackHighWaterMark(xSensorH));
        uart_printf("Process watermark: %u words
",
            uxTaskGetStackHighWaterMark(xProcessH));
        uart_printf("Free heap: %u bytes
",
            xPortGetFreeHeapSize());
        vTaskDelay(pdMS_TO_TICKS(10000));
    }
}

💡

Right-sizing stacks

Run under worst-case load, read each watermark, then keep at least 20% headroom above it. Stack is measured in words (4 bytes each on ARM). A watermark of 50 = 200 bytes remaining.

Continue the Series

Work through all 8 parts of the FreeRTOS Deep Dive to master real-time embedded systems programming.

Part 7: Low-Power RTOS & Tickless Idle → Related Project →