Pointers

int x = 10;
int *p = &x;
printf("%d", *p); // 10

🔥 Dynamic allocation

#include <stdlib.h>

int *p = (int *) malloc(sizeof(int));

*p = 42;

allocates memory on heap
returns void* (generic pointer)
you cast it (optional in C, but people still do it)

⚠️ VERY IMPORTANT:

if (p == NULL) {
    // allocation failed
}

🧼 `free`

free(p);

free(p);
free(p); // 💀 undefined behavior

🧨 `malloc` does NOT initialize memory

int *p = malloc(sizeof(int));
printf("%d", *p); // garbage value

int *p = (int *) malloc(5 * sizeof(int));

Allocates memory
Does NOT initialize it
Memory contains garbage values

🧊 `calloc`

int *p = (int *) calloc(5, sizeof(int));

Allocates memory for 5 integers
Initializes everything to 0

🔄 `realloc` (resizing memory)

p = realloc(p, 10 * sizeof(int));

resizes previously allocated memory
may move memory to a new location

So always:

int *temp = realloc(p, new_size);
if (temp != NULL) {
    p = temp;
}

Because if realloc fails, your original pointer is still valid…

📦 Arrays using pointers

int *arr = malloc(5 * sizeof(int));

arr[0] = 10;
arr[1] = 20;
// OR
*(arr + 1) = 20;

🧠 Pointer arithmetic

int *p = arr;
p++; // moves to next int (not +1 byte)

C automatically adjusts by sizeof(type).

🏗️ Struct allocation

struct Person {
    int age;
};

struct Person *p = malloc(sizeof(struct Person));
p->age = 25;

🧵 Strings (char pointers)

char *str = malloc(6 * sizeof(char));
strcpy(str, "Hello");

Important: allocate space for \0

"Hello" = 5 chars + 1 null = 6

🧨 Dangling pointers

int *p = malloc(sizeof(int));
free(p);

printf("%d", *p); // 💀 Pointer still exists… memory doesn’t.

Fix:

free(p);
p = NULL;

Now at least it fails safely.

⚡ Stack vs Heap

Stack

int x = 10;

automatic
fast
destroyed when function ends

int *p = malloc(sizeof(int));

manual
slower
YOU must free it

🧪 Common mistakes

// ❌ Forgetting `sizeof`
int *p = malloc(5); // wrong

// ✅ Correct
int *p = malloc(5 * sizeof(int));

// ❌ Using uninitialized pointer
int *p;
*p = 10; // 💀

// ❌ Memory leak
p = malloc(...);
p = malloc(...); // lost reference to old memory

Last modified: 25 March 2026