Mastering Pointers

Learning about pointers is easy and fun in C. Some C programming tasks are performed more easily with pointers. But dynamic memory allocation cannot be performed without using pointers. Pointer is a derived datatype and is considered as a system programming primitive.

What is Pointer?

Pointer is a variable that is basically used to store the address. The capability of that pointer is to fetch the content whose address stored in a pointer variable.
Pointer can be int*p , char*p, float*p, double*p. So,

char ch= ‘A’;
char *p= &ch;

double d= 9.6;
double *p=&d;

Operators in Pointers:-

1. &(Address-of operator): & is a unary operator used to store address of any type of variable.
2. *(Content-of operator): * is a unary operator which is used to fetch contents whose address is stored in a pointer variable.
   Note- Here, unary operator means only 1 operand.

The size of pointer depends on the architecture. However, in 32-bit architecture, the size of pointer is 2 bytes.

Source

As it can be seen from above, address of fff4 is stored by pointer variable.
50 is value of number variable. But aaa3 is address of pointer variable.

So basically,
Variable is a value stored in named storage or memory address.
Pointer is a variable that points to that storage or memory address of another variable.

Declaring a Pointer:-

We can declare a pointer in the following way,

data_type * pointer_variable_name;

1. ‘data_type’ indicates the type of variable to which the pointer is pointing.
2. * (asterisk) is basically an indirection operator that declares pointer.

#include <stdio.h>

int main() {
    int b = 10;       // Declaration of variable
    int *q;           // Declaration of pointer variable
    q = &b;           // Pointer q stores the address of variable b

    printf("Variable q stores address as: %x\n", q);   // Address is accessed
    printf("Variable q stores value as: %d\n", *q);    // Value is accessed

    return 0;
}

/* 
Output:
Variable q stores address as: 60ff08
Variable q stores value as: 10 
*/

Differentiating between * and &:-

#include <stdio.h>

int main() {
    int *ab, b;
    b = 25;

    printf("Address of b: %p\n", &b);
    printf("Value of b: %d\n\n", b);  // 25

    ab = &b;

    printf("Address of pointer ab: %p\n", ab);
    printf("Content of pointer ab: %d\n\n", *ab);  // 25

    b = 14;

    printf("Address of pointer ab: %p\n", ab);
    printf("Content of pointer ab: %d\n\n", *ab);  // 14

    *ab = 5;

    printf("Address of b: %p\n", &b);
    printf("Value of b: %d\n\n", b);  // 5

    return 0;
}

/* 
Output:
Address of b: 2686784
Value of b: 25
Address of pointer ab: 2686784
Content of pointer ab: 25
Address of pointer ab: 2686784
Content of pointer ab: 14
Address of b: 2686784
Value of b: 5
*/

1. int *ab, b;

Here, ab is a pointer and b is a normal variable (both are type of int) As they both are not initialised initially, so pointer ab points to either no address or a random address. While, b has an address but contains random garbage value.

2. b= 25;
25 is assigned to variable b. That is, 25 is stored in memory location of variable b.

3. ab = &b;
pointer ab is assigned with the address of variable b.

4. b = 14;
This assigns 14 to variable b.

5. *ab= 5;
This changes the value at the memory location pointed by the pointer ab to 5.

Types Of Pointers:-

1. Dangling Pointer
2. Far Pointer
3. Void Pointer
4. Complex Pointer
5. Huge Pointer
6. Null Pointer
7. Wild Pointer
8. Near Pointer

Advantages Of Pointers:-

1. Memory locations, elements of an array structure, etc. can be accessed through pointers.
2. They are used to form complex data structure such as linked list, graph, tree, etc.
3. Pointers are widely used in dynamic memory allocation and de-allocation.

Disadvantages Of Pointers:-

1. Pointers may lead to errors such as segmentation faults and are also responsible for memory leakage.
2. If an incorrect value is provided, they may cause memory corruption.
3. It is a bit difficult to work with pointers, so it is programmer’s responsibility to manipulate a pointer carefully.

Following important pointer concepts should be clear to any C programmer:-

Pointer program for swapping of 2 numbers without using the 3rd variable-

#include<stdio.h>

int main() {
    int p = 40, q = 50, *a1 = &p, *a2 = &q;

    printf("Before swap: *a1=%d *a2=%d", *a1, *a2);

    *a1 = *a1 + *a2;
    *a2 = *a1 - *a2;
    *a1 = *a1 - *a2;

    printf("\nAfter swap: *a1=%d *a2=%d", *a1, *a2);

    return 0;
}

/* 
Output:
Before swap: *a1=40 *a2=50
After swap: *a1=50 *a2=40
*/

Summary:-

1. A pointer is basically a memory location where data is stored.
2. They cab be used with array and string to access elements more efficiently.
3. Arithmetic operations can be done on a pointer which is known as pointer arithmetic.
4. Pointers also point to function making it easy to call those different functions in the case of defining an array of pointers.
5. Also, typecast void pointer are used when you want to deal different variable data type.