Introduction to Pointers

Understanding Memory and Pointers

A pointer is one of C's most powerful features. To understand it, first picture memory as a long street of numbered houses. Every variable lives at a numbered location called its address.

The Address-of Operator

The & operator gives you the memory address of a variable.

int age = 19;
printf("Value: %d\n", age);
printf("Address: %p\n", &age);

Declaring a Pointer

A pointer variable stores an address rather than an ordinary value. You declare one by adding a * to the type.

int age = 19;
int *ptr = &age;   // ptr now holds the address of age

Read int *ptr as “ptr is a pointer to an int”.

The Dereference Operator

Putting * in front of a pointer dereferences it — it follows the address to reach the actual value stored there.

int age = 19;
int *ptr = &age;

printf("%d\n", *ptr);   // 19  the value ptr points to

*ptr = 25;                // change age through the pointer
printf("%d\n", age);    // 25  age was modified

Notice the two uses of *. In a declaration it creates a pointer. In an expression it dereferences a pointer. They look the same but mean different things.

Why Pointers Matter

• They let a function change a caller's variable, not just a copy.
• They make it possible to work with arrays and strings efficiently.
• They are the foundation of dynamic memory and data structures like linked lists.

Pointers in Functions

Normally C passes a copy of a value to a function. By passing an address instead, the function can modify the original variable.

void setToTen(int *p) {
    *p = 10;
}

int main(void) {
    int x = 0;
    setToTen(&x);
    printf("%d\n", x);   // 10
    return 0;
}

Never dereference a pointer that does not point to valid memory. An uninitialised or NULL pointer dereference typically crashes the program.