Please refer to the introduction to pointers, pointer declarations, and initializations for a better understanding of the below topics:

Constant Pointer:

The constant pointer means that the address is constant that is it cannot be changed but the value to which it is pointing to can be changed.

Syntax:

data_type *const ptr_name;

Example:

The address which is fetched upon doing malloc() should not be lost or changed, hence constant pointers can be used to obtain such address.

Example:

Since the pointer is not const, the address can be changed (to which it is pointing).

#include <stdio.h>

 int main()
 {
   int b = 10;
   int c = 11;
   int *ptr = &b;
   printf ("\n The address is: %x value is: %d", ptr, *ptr);
   ptr = &c;
   printf ("\n The address is: %x value is: %d", ptr, *ptr);
   return 0;
 }  
 
 Output:
 The address is: ccb31044 value is: 10
 The address is: ccb31040 value is: 11

Current address cannot be changed:

 #include <stdio.h>

 int main()
 {
   int b =10;
   int c =11;
   int *const ptr = &b;
   printf("\n The address is: %x value is: %d", ptr, *ptr);
   ptr= &c;
   printf("\n The address is: %x value is: %d", ptr, *ptr);
   return 0;
 }

Output:

constant_pointer.c:11:4: error: assignment of read-only variable 'ptr'
 ptr= &c

Pointer to constant:

Pointer to constant means the address can be changed but value cannot be changed.

Syntax:

data_type const *ptr_name
          or
const data_type *ptr_name

Example:

Passing array or string to a function which should not modify the content of the array or the string.

  #include <stdio.h>
  
  int main()
  {
    int b = 10;
    int c = 20;
    int *ptr = &b;
    printf("\n Address of pointer is: %p\n", ptr);
    printf ("\n The value of old pointer is: %d\n",*ptr)
    ptr = &c;
    printf("\n Address of modified pointer is: %p\n", ptr);
    *ptr = 30;
    printf("\n Value of modified is pointer is: %d\n", *ptr);
    return 0;
    }

Output:

 Address of pointer is: 0x7fff65524244
 The value of old pointer is: 10
 Address of modified pointer is: 0x7fff65524240
 Value of modified is pointer is: 30

Now with pointers to constant:

#include <stdio.h>
  
  int main()
  {
    int b =10;
    int c =20;
    int const *ptr =&b;
    printf("\n Address of pointer is %p\n", ptr);
    ptr = &c;
    printf("\n Address of modified pointer is %p\n", ptr);
    *ptr = 30;
    printf("\n Value of modified is pointer is %d\n", *ptr);
    return 0;
    }

    Output:
    pointer_constant.c:16:5: error: assignment of read-only location 
    '*ptr' *ptr =30

There is a pointer for which neither the address nor the value can be changed

Syntax:

data_type const *const ptr_name

Void Pointer:

It is a special pointer that can hold the address of any data type. The type of pointer must be known before typecasting accordingly for dereferencing.

Example:

#include <stdio.h>

int main()
{
  int a =10;
  void *b = &a;
  printf("\n The value is: %d\n", *(int*)b)
  return 0;
}  

Output:
The value is: 10

The addresses that are returned from malloc(), calloc() and realloc() are void pointers that are later typecasted.

Categories: C Language