Stack vs Heap

The stack and the heap are two regions of memory a program uses. The stack stores function call frames, parameters and local variables in last-in, first-out order and is cleaned up automatically when a function returns. The heap stores dynamically allocated data whose size or lifetime is not known at compile time.

Why it matters

Stack allocation is very fast because it only moves a pointer, but the stack is small and fixed in size. Heap allocation is flexible and can hold large or long-lived data, but it is slower and must be freed, either manually or by a garbage collector. Understanding the difference explains errors such as stack overflow from runaway recursion, and it explains why returning a pointer to a local variable is a bug.

Example

void f(void) {
    int a = 5;                       // stack
    int *b = malloc(sizeof(int));    // b on stack, its target on heap
    *b = 7;
    free(b);                         // heap memory must be released
}

When f returns, a disappears automatically, but the allocated block would leak without free.

Common mistakes

• Assuming everything in managed languages lives on the heap or everything on the stack; it varies by language and runtime.
• Recursing without a base case, which exhausts the stack.
• Returning the address of a local variable.

Related

• Recursion
• Pointer
• Garbage collection
• Stack trace

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