Memory and stack management

Every task needs a stack size, and when one overflows it corrupts another task's memory. How to measure stack use and catch overflows.

Every task has a stack

Inside RAM each task has its own stack. If one overflows it overwrites the stack of the task next to it.

The 256 in xTaskCreate(..., 256, ...) is in words (4 bytes), so it is 1 KB. Ten tasks use 10 KB on stacks alone. Call depth, local arrays and printf (which often uses 1 KB or more) eat stack.

Measuring how much is used

UBaseType_t left = uxTaskGetStackHighWaterMark(NULL);   // smallest amount left (words)

Run every feature at its heaviest, look at what is left, and set the size at 1.5 to 2 times the usage.

RoadmapSee where this note sits in RTOS→

Overflow detection

#define configCHECK_FOR_STACK_OVERFLOW 2
void vApplicationStackOverflowHook(TaskHandle_t t, char *name) { /* log and halt */ }

Method 2 writes a pattern at the end of the stack and checks it on every context switch. Cortex-M33 and later can catch it immediately with the hardware stack-limit register (PSPLIM).

Heap

FreeRTOS objects (tasks, queues, semaphores) are created from the FreeRTOS heap by default.

Heap implementation	Characteristics
heap_1	Allocate only, no free. Safest for systems that create everything at start-up and are done
heap_4	Can free, merges adjacent blocks. General purpose

The static allocation API (xTaskCreateStatic) fixes memory at compile time with no heap at all, which is why safety-related products prefer it.