CIS241

System-Level Programming and Utilities

C Stack and Heap

Erik Fredericks, frederer@gvsu.edu
Fall 2025

Based on material provided by Erin Carrier, Austin Ferguson, and Katherine Bowers

Stack and heap

Two areas of memory

• The stack is a stack data structure

• The heap is not necessarily a heap

• Variables are in one or the other

  • Which one depends on how variable is declared!

• So far, we've only dealt with the stack

Stack

New block (scope) for each function called

• Function local variables are stored in block
• When function returns, that block is freed
  • Those variables are gone!

Heap

Dynamically allocated memory

• Not tied to a particular functions scope
• Can be allocated and freed at any time

We must free any memory we allocate

• What happens if not?
  • Memory leaks!
  • You can lose access (pointer) to allocated memory
  • Thus your program can’t free it
  • Usually cleaned up by OS when program exits

Stack vs. Heap

So, why?

Stack allocates / deallocates automatically

• Often limited in size
• All allocations on stack are static (sizes are set at compile time)

Heap allows flexibility, but require more thought

• Larger arrays
• Arrays that persist after a function returns
• Amount of memory is unknown at compile time
  • *Technically possible in stack via VLAs
  • But we’re ignoring this :)

Memory allocation!

Stack

• Static (compile time) memory allocation
• Examples: int x; char arr[20];

Heap

• Dynamic (runtime) memory allocation
• Examples: malloc(); realloc(); etc.
  • This is our next lecture :)