Stack and Heap

Every running C program has two very different places to put data: the stack and the heap. Understanding the difference is essential once your programs get bigger than a single function.

The stack

Local variables live on the stack, memory is claimed automatically when a function is called and freed automatically the instant it returns:

#include <stdio.h>

void greet(void) {
    char name[32] = "Ada"; // lives on the stack
    printf("Hi, %s\n", name);
} // name's memory is reclaimed right here, automatically

int main(void) {
    greet();
    return 0;
}

The stack is extremely fast (allocating is just moving a pointer), but limited in size (usually a few MB) and strictly scoped, you can never return a pointer to a local stack variable and use it after the function returns, that memory is gone.

The heap

The heap is a much larger pool of memory that you manage manually, with malloc and free:

#include <stdlib.h>

int main(void) {
    int *numbers = malloc(10 * sizeof(int)); // ask for room for 10 ints
    if (numbers == NULL) {
        // malloc failed, out of memory
        return 1;
    }
    numbers[0] = 42;
    free(numbers); // you MUST free it yourself when done
    numbers = NULL; // avoid leaving a dangling pointer around
    return 0;
}

Heap memory survives until you explicitly free it, which is exactly what makes it useful for data that needs to outlive the function that created it, and exactly what makes it dangerous: forget to free and you leak memory, free twice or use it after freeing and you get undefined behavior.