Pointers

What is a pointer?

A pointer is a variable that stores the memory address of another object, such as an integer variable.

Imagine every house has an address. The address tells you where the house is located. Similarly, a pointer stores the memory address where a variable is located.

Definition: A pointer is a special variable that stores the memory address of another variable.

Declaring a pointer

Syntax:

data_type *pointer_name;

Examples:

int *p;
float *fp;
char *cp;

• int *p; declares p as a pointer to an integer.
• float *fp; declares fp as a pointer to a float.
• char *cp; declares cp as a pointer to a character.

The * in a declaration indicates a pointer type. It does not mean multiplication in this context.

Initializing a pointer

Pointer initialization means assigning a valid address to a pointer before using it to access an object.

#include 

int main(void)
{
    int x = 25;
    int *p = &x;

    printf("Value of x = %d\n", x);
    printf("Address of x = %p\n", (void *)&x);
    printf("Value of p = %p\n", (void *)p);

    return 0;
}

Example output (the address varies between executions):

Value of x = 25
Address of x = 0x7ffd...
Value of p = 0x7ffd...

The address printed for x matches the value stored in p.

The indirection operator (*)

The indirection operator, also called the dereference operator, is used to access or modify the value stored at the memory address held by a pointer.

Suppose:

int x = 25;
int *p = &x;

Then:

Expression	Meaning
x	Value of the variable x
&x	Address of x
p	Address stored in pointer p
*p	Value of the object pointed to by p

Here, *p evaluates to 25. The pointer variable p stores &x, which is the address of the object x (with value 25). Dereferencing p using *p accesses the value stored in x.

Example: C Program demonstrating indirection

#include 

int main(void)
{
    int x = 25;
    int *p = &x;

    printf("x = %d\n", x);
    printf("*p = %d\n", *p);

    *p = 50;

    printf("New value of x = %d\n", x);

    return 0;
}

Output:

x = 25
*p = 25
New value of x = 50

Why did x change to 50?

The statement *p = 50; changes the object pointed to by p. Since p points to x, the value of x becomes 50.

Difference between & and *

Operator	Name	Purpose	Example
&	Address-of	Obtains an object's address	p = &x;
*	Indirection / dereference	Accesses the object through a pointer	y = *p;

Memory trick: & finds the address; * follows the address to access the object.

Remember that * has different meanings depending on its context:

int *p;    // Declaration: p is a pointer
y = *p;    // Expression: access the pointed-to object

Size of a pointer in C

The size of a pointer is the amount of memory required to store a memory address, generally it takes the size of int.

Size of pointers for different data types

#include 

int main()
{
    int *p1;
    char *p2;
    float *p3;
    double *p4;

    printf("Size of int pointer: %u bytes\n", sizeof(p1));
    printf("Size of char pointer: %u bytes\n", sizeof(p2));
    printf("Size of float pointer: %u bytes\n", sizeof(p3));
    printf("Size of double pointer: %u bytes\n", sizeof(p4));

    return 0;
}

Output:

Size of int pointer: 4 bytes
Size of char pointer: 4 bytes
Size of float pointer: 4 bytes
Size of double pointer: 4 bytes

Why is the pointer size the same for different data types?

A pointer stores the memory location of a variable, regardless of whether that variable is an int, char, float, or double.

Pointer to pointer or double pointer in C

Definition

A pointer to pointer, also called a double pointer, is a pointer that stores the address of another pointer. It is declared using two asterisks (**).

Syntax

data_type **pointer_name;

Example:

int **pp;

Here, pp is a double pointer that can store the address of an integer pointer.

Example: C Program using a double pointer

#include 

int main()
{
    int a = 10;
    int *p = &a;
    int **pp = &p;

    printf("Value of a = %d\n", a);
    printf("Value using p = %d\n", *p);
    printf("Value using pp = %d\n", **pp);

    return 0;
}

Output:

Value of a = 10
Value using p = 10
Value using pp = 10

Understanding p, *p, pp, *pp, and **pp

Expression	Meaning	Result
a	Value of variable a	10
p	Address of a	Address of a
*p	Value stored at the address in p	10
pp	Address of pointer p	Address of p
*pp	Value of p (address of a)	Address of a
**pp	Value of a	10

The actual address values depend on where the variables are stored in memory.