Stack
- very fast access
- don't have to explicitly de-allocate variables
- space is managed efficiently by CPU, memory will not become fragmented
- local variables only
- limit on stack size (OS-dependent)
- variables cannot be resized
Heap
- variables can be accessed globally
- no limit on memory size
- (relatively) slower access
- no guaranteed efficient use of space, memory may become fragmented over time as blocks of memory are allocated, then freed
- you must manage memory (you're in charge of allocating and freeing variables)
- variables can be resized using
realloc()
When to use the Heap?
When should you use the heap, and when should you use the stack? If you need to allocate a large block of memory (e.g. a large array, or a big struct), and you need to keep that variable around a long time (like a global), then you should allocate it on the heap. If you are dealing with relatively small variables that only need to persist as long as the function using them is alive, then you should use the stack, it's easier and faster. If you need variables like arrays and structs that can change size dynamically (e.g. arrays that can grow or shrink as needed) then you will likely need to allocate them on the heap, and use dynamic memory allocation functions like malloc(), calloc(), realloc() and free() to manage that memory "by hand". We will talk about dynamically allocated data structures after we talk about pointers.
https://gribblelab.org/CBootCamp/7_Memory_Stack_vs_Heap.html