Stack and heap

Learn the difference between stack and heap memory.

Stack and heap: two memory regions

C programs split memory into two main regions. The stack holds local variables; it is cleaned automatically when a function returns. The heap is memory you allocate with malloc; you release it with free.

int a = 10;            /* stack */
int *p = malloc(4);    /* heap */

Where a variable lives depends on how you declare it. The stack is fast but small; the heap is large but you manage it. Knowing this distinction is the basis for understanding pointer and memory errors.

How the stack works

The stack grows and shrinks with function calls. When a function is called, its local variables are pushed onto the stack; when it returns, this space is released.

void f(void) {
    int x = 5;  /* on the stack */
}
/* when f returns, x is gone */

This is why returning the address of a local variable is dangerous; that address is no longer valid. The stack is fast but has limited capacity; deep recursion can cause a stack overflow.

Exercise Access a local variable via pointer

Declare int a = 10 (on the stack). Declare an int * pointer that points to a. Print the value the pointer points to with printf.

Show example solution

int *p = &a;
printf("%d\n", *p);

Test yourself

Where is the local variable int a = 10; stored?
- Stack
- Data segment
- Heap
- Register
Show answer

Stack — Local variables live on the stack.
Match region to property

Stack

Local variables
Heap

Allocated with malloc
free

Releases heap memory
automatic

Cleared on function return

Fill in the blank: pointer declaration

int a = 10;
____ *p = &a;

Show answer

int a = 10;
int *p = &a;

Next lesson: Automatic storage

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