Stack vs Heap

The stack: automatic and fast

Function locals live on the stack. When the function returns, that memory is reclaimed automatically:

void f(void) {
    int a[100];        // lives until f returns
}                      // gone — no cleanup code written by you

Fast, but two limits: the size must be known at compile time, and the memory dies with the function.

The heap: manual and flexible

The heap holds memory you request at runtime with malloc and release yourself with free:

#include <stdlib.h>

int *p = malloc(10 * sizeof(int));   // ask for room for 10 ints
// ... p stays valid across function boundaries ...
free(p);                             // YOU give it back

Heap memory survives until you free it — it can outlive the function that created it.

The one rule that follows you forever

If you allocated it, you own it: someone must free it exactly once.

• forget to free → memory leak
• free twice → undefined behavior (heap corruption)
• use after free → undefined behavior

Modules ahead (file I/O, data structures) build directly on this rule.