C in 100 Seconds: Stack vs Heap — Where Variables Live

Stack vs Heap

Every variable in C lives in one of two places: the stack or the heap. Understanding the difference is essential for writing safe, efficient code.

Stack Memory

Stack variables are automatic — declare them, use them, and they vanish when the scope ends:

Stack allocation is fast (just a pointer bump) but limited in size, and the data dies with the function.

The Dangling Pointer Problem

Returning the address of a stack variable creates a dangling pointer:

The compiler warns you: . The address exists, but the data behind it is already reclaimed.

Heap Memory

allocates memory on the heap that survives the function return:

The caller is responsible for calling when done.

When to Use Which

	Stack	Heap
Speed	Fast (pointer bump)	Slower (allocator overhead)
Lifetime	Dies with scope	Lives until freed
Size	Limited (~1-8 MB)	Limited by RAM
Management	Automatic	Manual (malloc/free)

Rule of thumb: Use the stack for short-lived data. Use the heap for data that must outlive the function that created it.

Student Code

Try it yourself: episode31/stackheap.c