C Pointers - Ravi R. Oza

Table of Contents

In this tutorial, we are going to learn pointers, what is pointer, when to use pointers, usage of pointers, pointer that point to a variable, pointer that points to an array or a structure, pointer in UDF etc.,

Introduction

A pointer is a variable that represents the location of a data item, such as a variable or an array element.
Pointers have a number of useful applications.
- pointer can be used to pass information back and forth between a function and a reference point.
- provides a way to return multiple data items from a function via arguments.
- closely associated with arrays and therefore provide an alternative way to access individual array elements.

Address of (&) operator

As we have seen in scanf() function, & is used just before the variable name, to read and assign the values to the address of the variable.

What is the use of `%p` in printf() in C?

In C we have seen different format specifiers.
Here we will see another format specifier called %p.
This is used to print the pointer type data.
%p prints memory address of a variable in hexadecimal form. Let us see the example to get a better idea.

example

void main()
{
	int age = 18;
	char name = 'r';
	float mark = 34.34;		
	printf ("\n address of age  variable =  %p",&age);
	printf ("\n address of name variable =  %p",&name);
	printf ("\n address of mark variable =  %p",&mark);
}

What is Pointer?

Pointer is variable that points to another variable. Lets assume that we have a variable age in our program, &age points to the address of that variable.

To scan a above age variable, we write like this scanf("%d",&age). Address operator & is use to refer an address in C language.

Benefits of Pointer

Pointers are more efficient in handling arrays and data tables.
Pointers can be used to return multiple values.
Use of Pointer arrays to characters strings is helpful in saving memory.
It is use to allocate dynamic memory (runtime).
Pointer increases the execution speed of a program.

Lets see how to get an address of a variable

#include <stdio.h>

// www.raviroza.com
// 9-Dec-2020, 7.57 pm

void main()
{
  int age = 5;
  printf("Value of age is : %d\n\n", age);

  // get the address of age variable
  printf("Address of age is : %p", &age);  
  getch();
}

How to declare Pointer?

The syntax of pointers is similar to the variable declaration in C, but we use the ( * ) dereferencing operator in the pointer declaration.

Syntax:

dataType *variableName;

where,

ptr is the name of the pointer.
datatype is the type of data it is pointing to.

The above syntax is used to define a pointer to a variable. We can also define pointers to functions, structures, etc.

Code snippet

int age = 100;

int *ptr;

ptr = &age;

// to alter/change the value of age by using *ptr;

*ptr = *ptr + 1;

Here, age is an integer variable, and *ptr is pointer variable that is referencing or pointing to the variable age.

Interestingly, it is possible to change the value of age variable by using *ptr as it is done in the last line of code.

Guidelines to declare pointers

Pointer variables ares declaring using the asterisk (*).
Pointer itself needs a memory location.
Memory Variable to refer and the pointer variable should of same type, means that a int pointer can refer only a int memory variable.

How to Use Pointers?

Pointer Declaration
- In pointer declaration, we only declare the pointer but do not initialize it.
- To declare a pointer, we use the ( * ) dereference operator before its name.
- The pointer declared here will point to some random memory address as it is not initialized.
- Such pointers are called wild pointers.
Pointer Initialization
- Pointer initialization is the process where we assign some initial value to the pointer variable.
- We generally use the ( & ) address of operator to get the memory address of a variable and then store it in the pointer variable.
Dereferencing
- A pointer is defined as a derived data type that can store the address of other C variables or a memory location.
- We can access and manipulate the data stored in that memory location using pointers.
- Dereferencing a pointer is the process of accessing the value stored in the memory address specified in the pointer.
- We use the same ( * ) dereferencing operator that we used in the pointer declaration.

Example:
	int var = 10; 
	int * ptr; 
	ptr = &var;
	// or direct pointer definition
	int *ptr = &var;

AND & OPERATOR

Suppose v is a variable that represents some particular data item.
The address location can be determined by the expression &v.
where & is a unary operator called the address operator.
That evaluates the address of its operand.

Types of Pointers

Pointers can be classified into many different types based on the parameter on which we are defining their types.
If we consider the type of variable stored in the memory location pointed by the pointer, then the pointers can be classified into the following types:

Integer Pointer
- As the name suggests, these are the pointers that point to the integer values.
- These pointers are pronounced as Pointer to Integer.
- Similarly, a pointer can point to any primitive data type. It can point also point to derived data types such as arrays and user-defined data types such as structures.
- example

void main()
{	int age = 10; // int type variable 	
	int*  ptr;    // declare int type pointer
	// data type of ptr and var must be similar
	
	ptr = &age; // assign address of age to pointer
	printf ("\n value   of age = %d",age);	
	printf ("\n address of age = %p",age);	
	printf ("\n value   of ptr = %p",*ptr);
}

Array Pointer
- Pointers and Array are closely related to each other. Even the array name is the pointer to its first element.
- They are also known as Pointer to Arrays.
- Pointer to an array using the given syntax.
- example

int main()
{
   int arr[5] = {1,2,3,4,5};
   int i, *ptr;
   ptr=&arr[0];   
   //clrscr();
   printf ("\n\n POINTERS/ADDRESS OF ARRAY ELEMENTS \n");   
   for(i = 0; i<5; ++i)
   {
      //printf(" &arr[%d] = %p\n", i, &arr[i]);
      printf(" %d \n", *ptr++);      
   }
   printf("\n Base Address of Array is : %p", arr);

   //getch();
}

Structure Pointer
- The pointer pointing to the structure type is called Structure Pointer or Pointer to Structure.
- It can be declared in the same way as we declare the other primitive data types.
- In C, structure pointers are used in data structures such as linked lists, trees, etc.
Function Pointer
Double Pointer
- In C language, we can define a pointer that stores the memory address of another pointer.
- Such pointers are called double-pointers or pointers-to-pointer.
- Instead of pointing to a data value, they point to another pointer.
- Syntax:
  - datatype ** ptr_name;

Pointer as function argument

Pointers are often passed to a function as arguments.
When an argument is passed by reference, however (i.e. when a pointer is passed to a function) the address of a data item is passed to the function.
Pointer as a function argument permits the corresponding data item to be altered globally from the called function.
The contents of that address can be accessed freely, either within the function or within the calling routine..
When pointers are used as argument to a function the formal arguments that are pointers must each be preceded by an asterisk (*).

example

void main()
{
	int n = 1, v = 3;
	func (&n, &v) ;
	printf (“\n after calling func : n=%d v=%d“ ,n, v);
}
void func (int *pu, int *pv)
{
	*pu = 0;
	*pv = 0;
	printf(“\n within func pu=%d pv=%d”, *pu,*pv);
}

Example – Pointer of `int, float` and `char` data type

# include <stdio.h>
# include <conio.h>

// www.raviroza.com
// 11-Dec-2021, 8.00 am

void main()
{
    printf("\n Pointer Demo \n");

    int i = 10;
    int *iptr;
    
    float f = 10.34;
    float *fptr;
    
    char c = 'r';
    char *cptr;
    
    iptr = &i;
    fptr = &f;
    cptr = &c;
    
    printf ("int   : %d is at %p\n",i,iptr);
    printf ("float : %f is at %p\n",f,fptr);
    printf ("char  : %c is at %p\n",c,cptr);
    
    getch();
}

Pointer and Array

The array is also known group of sequential data in memory. Let’s see with an example to get the address of each array element

Example 1 : Address of Array elements

# include <stdio.h>
# include <conio.h>

// www.raviroza.com
// 11-Dec-2021, 9.00 am

void main() 
{
   int arr[5];
   int i;

   printf ("\n\n POINTERS/ADDRESS OF ARRAY ELEMENTS \n");

   for(i = 0; i < 5; ++i)
   {
      printf("&arr[%d] = %p\n", i, &arr[i]);
   }

   printf("Base Address of Array is : %p", arr);

   getch();
}

Example 2 : Pointer to an Array

# include <stdio.h>
# include <conio.h>

// www.raviroza.com
// 11-Dec-2021, 9.00 am

void main() 
{
     int arr[5] = {1, 2, 3, 4, 5};
     int* ptr;

     // ptr is assigned the address of the 1st element
     ptr = &arr[0]; 

     printf("*ptr     : %d\n", *ptr);      // 1
     printf("*(ptr+1) : %d\n", *(ptr+1));  // 2
     printf("*(ptr+2) : %d",   *(ptr+2));  // 3
     getch();
}

Example 3 : Array of Pointers

# include <stdio.h>
# include <conio.h>

// www.raviroza.com
// 08-Oct-2022, 12.00 pm

void main() 
{
    
    int x=100, y=200;
    // array of integers
    int arr[5] = {1, 2, 3};
     
    // array of integer pointers
    int *ptr[5];
         
    int i;
     
    // assign address of each array elements to pointers
    ptr[0] = &arr[0];
    ptr[1] = &arr[1];
    ptr[2] = &arr[2];
    // assign integer x and y to pointers
    ptr[3] = &x;
    ptr[4] = &y;

    for(i=0; i<5; i++)
    {
        // modify the value of variable x, y and array elements using array of pointers
        *ptr[i] = *ptr[i] + 2;
        printf ("%d  ",*ptr[i]);
    }
    getch();
}

Pointers and Structure

Example 1 : Pointer to Structure

# include <stdio.h>
# include <conio.h>

// www.raviroza.com
// 7-Oct-2022, 8.25 am

struct student
{
   int age;
   float weight;
};

void main()
{
    struct student *studentPtr, student1;
    studentPtr = &student1;   
    clrscr();

    printf("Enter age : ");
    scanf("%d", &studentPtr->age);

    printf("Enter weight : ");
    scanf("%f", &studentPtr->weight);

    printf("Displaying Student Details:\n");
    printf("Age: %d\n", studentPtr->age);
    printf("weight: %f", studentPtr->weight);

    getch();
}

Example 2 : Dynamic memory allocation of structs

# include <stdio.h>
# include <conio.h>

// www.raviroza.com
// 7-Oct-2022, 8.25 am

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

struct student 
{
   int age;
   char name[30];
};

void main()
{
   struct student *ptr;
   int i, n;

   printf("Enter the number of students: ");
   scanf("%d", &n);

   // allocating memory for N numbers of struct student
   ptr = (struct student*) malloc(n * sizeof(struct student));

   for(i = 0; i < n; ++i)
   {
       printf("Enter first name and age respectively: ");

       // To access members of 1st struct student,
       // ptr->name and ptr->age is used

       // To access members of 2nd struct student,
       // (ptr+1)->name and (ptr+1)->age is used
       scanf("%s %d", (ptr+i)->name, &(ptr+i)->age);
   }

   printf("Displaying Student Information:\n");
   for(i = 0; i < n; ++i)

   {
       printf("Name: %s\tAge: %d\n", (ptr+i)->name, (ptr+i)->age);

   }
   getch();
}

Pointer and Function

When a pointer is passed as an argument to a function, it is also known as function with reference parameter. When a function called by reference is, the address (pointer or address) of a variable is passed to the function. Following is an example of pointer as an argument to a function

# include <stdio.h>
# include <conio.h>

// www.raviroza.com
// 8-Dec-2021, 10.30 am

void byref(int*);

void byref(int *i)
{
	*i = *i + 1;
}

void main()
{
	int temp=10;
	
	byref(&temp);
	printf ("\n\ntemp = %d",temp);
	getch();
}

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