System Hooks

Overview

FreeRTOS can call user functions at specific kernel events:

• vApplicationIdleHook() — CPU has no runnable tasks
• vApplicationTickHook() — every system tick interrupt
• vApplicationMallocFailedHook() — heap allocation failed
• vApplicationStackOverflowHook() — a task overflowed its stack

Enable hooks in FreeRTOSConfig.h:

#define configUSE_IDLE_HOOK            1
#define configUSE_TICK_HOOK            1
#define configUSE_MALLOC_FAILED_HOOK   1
#define configCHECK_FOR_STACK_OVERFLOW 2
#define configUSE_TRACE_FACILITY       1  // useful for diagnostics

Idle Hook

Runs when no task is ready to run. Keep it short and non-blocking.

void vApplicationIdleHook(void){
  // Optionally enter low power, blink a LED slowly, or run background checks
  // Never block here. Do minimal work.
}

Tick Hook

Called on each tick interrupt. Keep it extremely short; use only ...FromISR APIs.

void vApplicationTickHook(void){
  // DO: increment counters; very small work
  // DO NOT: call blocking APIs or heavy code
}

Caution

Tick hook runs in interrupt context. Use FromISR variants only.

Malloc Failed Hook

Called if a pvPortMalloc() fails (e.g., not enough heap for creating tasks/queues).

void vApplicationMallocFailedHook(void){
  // Stop safely and aid debugging
  taskDISABLE_INTERRUPTS();
  for(;;){}
}

Checklist when this triggers:

• Increase configTOTAL_HEAP_SIZE or reduce allocations
• Check task stack sizes (don’t over-allocate)
• Ensure the chosen heap implementation (e.g., heap_4.c) is linked

Stack Overflow Hook

Detects task stack overflow (method depends on configCHECK_FOR_STACK_OVERFLOW).

void vApplicationStackOverflowHook(TaskHandle_t xTask, char *pcTaskName){
  (void)xTask; (void)pcTaskName;
  taskDISABLE_INTERRUPTS();
  for(;;){}
}

How to avoid overflows:

• Use uxTaskGetStackHighWaterMark() during development
• Avoid large stack arrays; use static/global or heap when appropriate
• Keep ISRs short; do heavy work in tasks