C – Pointers

A C pointers is a variable that holds the memory address of another variable. Pointer variables are declared using the * operator, and they can point to various data types, including integers, characters, arrays, structures, and even other pointers. Manipulating pointers requires careful handling to avoid issues like memory leaks, dangling pointers, and segmentation faults.

• int a; // An integer
• int *a; //A pointer to an integer
• int **a; //A pointer to a pointer to an integer
• int a[10]; //An array of 10 integers
• int *a[10]; //An array of 10 pointers to integers
• int (a)[10]; //A pointer to an array of 1 integers
• int (a)(int); //A pointer to a function a that take an integer argument and return an integer
• int (*a[10]){int); //An array of 10 pointers to functions that take an integer argument and return an integer

Pointer

A pointer is a variable capable of holding the address of another variable. Dereferencing, accomplished with the (*) operator, allows accessing the value of the variable.

• Syntax : Datatype * variable name ;
• Syntax example : int *p;
• Rule : That two variables should be same data type.

Example

Int x=10;

Int *p=&x;

(Or)

Int *p;

p=&x;

• To access the value of x : printf(“%d ”,p); —> value of x is 10 Or printf(“%d”,(&x)) —->value of x is 10
• To access addres of x: printf(“%d”,p); —–> address of x is printed Or printf(“%d”,&x);——> address of x

• Access the address of the variable syntax : &variable_name
• Declaring pointers syntax: datatype *var;  Example : int *a;
• Initializing pointer syntax : pointer_name =&variable_name;  Example : int *a; int x; a =&x;
• Pointer expressions :
  1. For addition +,
  2. For subtraction -,
  3. For increment ++,
  4. For decrement --.

Advantage

• Use pointer to increase the speed of program execution
• Reduce program complexity
• To use pointer to access the inside function’s variable value.
• To use pointer to transfer the data from outside or inside the functions.

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Void pointer

Void pointer is a pointer which can point to any datatype of memory block
But to access the complete memory one has to do type casting

• Syntax: void *p;

Example:

int x=10;
Char t =’u’;
void *p;
p=&x;
printf(“ %d”,(int)p); // casting is done
p=&t;
printf(“ %c”,(char)p); // casting is done

• What is the advantage of declaring void pointers? When we do not know what type of the memory address the pointer variable is going to hold, then we declare a void pointer for such.

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Function Pointer

A function pointer stores the address of a function, allowing it to be called later. This is useful as functions encapsulate behavior.

• syntax : int(*fun)(int, int, int);

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NULL Pointers in C

A pointer pointing to nothing is called so.

• The NULL pointer always initializes.
• This pointer is a pointer that is not pointing to any valid memory in the heap.
• Null pointer holds the value a NULL value as its content.

Syntax :

• Int *p=NULL; (address is 0 or null here);
• Int *p=0; is also called as null. but for our reference, we are using the NULL term

NULL pointer assignment:

*q=0;

(or)

<p>char *q=(char*)0;

(or)

char *p=NULL;

• What is (void*)0 ? //Representation of NULL pointer
• NULL Macro is defined in locale.h , stddef.h , stdio.h , stdlib.h , string.h , time.h , and wchar.h

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Dangling Pointer

A pointer begins with a valid address but becomes a dangling pointer once that address is released or freed.

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Near pointer

Near pointer occupies 2 bytes in DOS

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Far Pointer

far pointer occupies 4 bytes in DOS in C Pointers

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Huge Pointer

huge pointer occupies 4 bytes in DOS

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Pointer to a Function

A pointer that holds the reference of a function is called a pointer to a function.

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Pointer to Pointer

one pointer refers to the address of another pointer

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pointer on pointer

It’s a pointer variable which can hold the address of another pointer variable. It de-refers twice to point to the data held by the designated pointer variable.
Example: int x = 5, *p=&x, **q=&p; Therefore x  can be accessed by *q .

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Pointer and Array

• int (ptr)[10]; //  ptr is a pointer to an array of 10 integers.

Example-1: POINTER TO AN ONE DIMENSIONAL ARRAY

#include<stdio.h>
int main ()
{
	int a[3]={1,2,3};
	
	int *p=(int*)&a;
	// or
	// int *p=a;
	
	/*here a and &a RETURNS is same starting address only*/
	printf("%d %d \n",p[0],p[1]);
	
	printf("%d %d %d",*p,*(p+1),&a);
	// here p[0] or *p is same only;
	// p[1] or *(p+1) is same only
	
	return 0;
}

Example-2: POINTER TO AN ONE DIMENSIONAL ARRAY

.#include<stdio.h>
 int main ()
 {
 int a[3]={1,2,3};
 int(*p)[3];
 p=a;
 printf("% d%d",(*p)[0],p[0][0]);
 return 0;
 }

Example-3: POINTER TO AN TWO DIMENSIONAL ARRAY

#include<stdio.h>
int main()
{
	int a[2][2] = {{1,2},{3,4}}; //
	
	int (*p)[2][2] = &a;

	printf("%d\t",*p[0][0]);// it will print 1
	
	printf("%d\t",p[0][0][0]); //HERE it means p[0][0]+0 it will print 1
	
	printf("%d\t",(*p)[0][1]); //HERE it means p[0][0]+1 it will print 2
	
	printf("%d\t",p[0][1][0]); //HERE it means p[0][1]+0 it will print 3
	
	printf("%d\n",p[0][1][1]); // here it means p[0][1]+1 it will print 4
}

Example-4: POINTER TO AN TWO DIMENSIONAL ARRAY

#include<stdio.h>
int main()
{
	int arr[2][2]={{1,2},{3,4}};

	int (*p)[2]=arr;

	printf("%d%d%d%d",*(*p),*(*p+1),**(p+1),*(*(p+1)+1));

	return 0;
}

Example-5: POINTER TO STRINGS

STRING is an one dimensional array of char data type

#include<stdio.h>
int main ()
{
	char p[]="muthu"; /* here p[] and *p are same */
	
	printf("%s\n",p); /*it prints muthu */
	
	printf("%d\n",p); /* here p holds starting address 0th location*/
	
	printf("%d\n",p+1);/* here p+1 holds address of 1st location */
	
	printf("%d\n",p+2);/* here p+2 holds address of 2nd lcn*/
	
	printf("%d\n",p+3);/* here p+3 holds address of 3rd lcn*/
	
	printf("%d\n",p+4);/* here p+4 holds address of 4th lcn*/
	
	printf("%c",*p); /* *p holds value at 1st addres*/
	
	printf("%c",*(p+1));/* *(p+1) holds value at 1st addres*/
	
	printf("%c",*(p+2));/* *(p+2) holds value at 1st addres*/
	
	printf("%c",*(p+3));/* *(p+3) holds value at 1st addres*/
	
	printf("%c\n",*(p+4));/* *(p+4) holds value at 1st addres*/
	
	return 0;
}

Example-6: POINTER TO STRINGS

#include<stdio.h>
int main ()
{
	char p[]="muthu";
	
	printf("%d\n",p); /* prints starting addres*/
	
	char *t=p; /* valid initialization */
	
	/* char t[]=p; error invalid initialization WE CAN ONLY USE *t to
	initial string value from another variable but not using t[] only * operator is used for
	dereferencing */
	
	/* u can also initialize the new pointer variable using the existing variable */
	printf("%d\n",t); /* prints starting addres*/
	
	char *s=t;
	
	printf("%d\n",p); /* prints starting addres*/
	
	printf("%s",s);
	
	return 0;
}

Example-7: WHAT HAPPENS IF YOU PRINT FUNCTION NAME WHAT VALUE IT PRINTS

#include<stdio.h>

void muhtu()
{

}

main()
{
	printf(“%d”,muhtu); // it prints the address of the function
}

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Call by reference in C Pointers

Call by reference allows passing the address of a variable as an argument, enabling modifications to the variable within the called function. This means changes to the formal parameter value can affect the actual arguments.

Example swapping of two numbers

#include<stdio.h>
void main( )
{
	int a = 10, b = 20 ;
	printf ( "value before swap\na = %d b = %d\n", a, b ) ;
	swapr ( &a, &b ) ;// calling function
	printf ( "value after swap\na = %d b = %d", a, b ) ;
}
swapr( int *x, int *y )
{
	int t ;
	t = *x ;
	*x = *y ;
	*y = t ;
}

• The calling function invokes or calls the function, while the called function executes due to the call.

Example of PASSING ARRAY TO FUNCTION USING CALL BY VALUE

#include<stdio.h>
#include<stdlib.h>

struct Array
{
	int arr[5];
};

void passbyvalue(struct Array temp)
{
	temp.arr;
	
	int i;
	
	for (i = 0; i <5; ++i)
		printf("%d ", temp.arr[i]);
	
	printf("\n");
}

int main()
{
	int i;
	
	struct Array obj=
	{
		{1,2,3,4,5}
	};
	
	for (i=0; i<5; i++)
		obj.arr[i];
	
	for (i = 0; i <5; ++i)
		printf("%d ", obj.arr[i]);
	
	passbyvalue(obj);
	
	return 0;
}

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Reference Video