C - Pointer in C

1. What is a Pointer?

A pointer in C is a variable that stores the memory address of another variable.

Normally:

But a pointer stores where in memory another value is located.

Think of it like:

A variable is a house with a number inside.
A pointer is a note saying, “The number you want is in house number 1234.”

You declare a pointer by putting an asterisk * before its name:

int *p;    // p is a pointer to an int
char *c;   // c is a pointer to a char
float *f;  // f is a pointer to a float

The * in the declaration means:

“This variable will hold the address of a value of this type.”

To get the memory address of a variable, use the address-of operator &:

int x = 10;
printf("%p", &x); // prints the address of x

int x = 10;
int *p = &x; // p now stores the address of x

Now:

Dereferencing means accessing the value stored at the address a pointer is holding:

printf("%d", *p); // prints 10

Here, *p reads as “the value at the address stored in p”.

Example:

int x = 10;
int *p = &x;

Memory might look like this:

Variable	Address	Value
`x`	0x1000	10
`p`	0x2000	0x1000

*p = 20; // changes the value of x to 20
printf("%d", x); // prints 20

Because p points to x, dereferencing it lets you modify x directly.

If p is an int *, it must point to an int:

int a = 5;
int *p = &a; // OK
float f = 3.14;
p = &f;      // ERROR: types don’t match

A pointer can store the address of another pointer:

int x = 10;
int *p = &x;
int **pp = &p;

printf("%d", **pp); // prints 10

Here:

If a pointer doesn’t point to anything, set it to NULL:

int *p = NULL;
if (p == NULL) {
    printf("Pointer is empty.\n");
}

A NULL pointer is safer than leaving it uninitialized.

When you increment or decrement a pointer, it moves to the next element of its type.
Example:

int arr[3] = {10, 20, 30};
int *p = arr; // points to arr[0]

p++; // now points to arr[1]
printf("%d", *p); // prints 20

Important: The pointer moves by sizeof(type), not by 1 byte.