void pointer in C

Till now, we have studied that the address assigned to a pointer should be of the same type as specified in the pointer declaration. For example, if we declare the int pointer, then this int pointer cannot point to the float variable or some other type of variable, i.e., it can point to only int type variable. To overcome this problem, we use a pointer to void. A pointer to void means a generic pointer that can point to any data type. We can assign the address of any data type to the void pointer, and a void pointer can be assigned to any type of the pointer without performing any explicit typecasting.

Syntax of void pointer

    
        void *pointer name;

Declaration of the void pointer is given below:

    
        void *ptr;
In the above declaration, the void is the type of the pointer, and 'ptr' is the name of the pointer.

Let us consider some examples:

int i=9; // integer variable initialization.
int *p; // integer pointer declaration.
float *fp; // floating pointer declaration.
void *ptr; // void pointer declaration.
p=fp; // incorrect.
fp=&i; // incorrect
ptr=p; // correct
ptr=fp; // correct
ptr=&i; // correct

Size of the void pointer in C

The size of the void pointer in C is the same as the size of the pointer of character type. According to C perception, the representation of a pointer to void is the same as the pointer of character type. The size of the pointer will vary depending on the platform that you are using.

Let's look at the below example:

    
        #include   
            int main()  
            {  
                   void *ptr = NULL; //void pointer  
                   int *p  = NULL;// integer pointer  
                   char *cp = NULL;//character pointer  
                   float *fp = NULL;//float pointer  
                    //size of void pointer  
                    printf("size of void pointer = %dnn",sizeof(ptr));  
                    //size of integer pointer  
                    printf("size of integer pointer = %dnn",sizeof(p));  
                    //size of character pointer  
                    printf("size of character pointer = %dnn",sizeof(cp));  
                    //size of float pointer  
                    printf("size of float pointer = %dnn",sizeof(fp));  
                    return 0;  
            }

Output

Advantages of void pointer

Following are the advantages of a void pointer:

The malloc() and calloc() function return the void pointer, so these functions can be used to allocate the memory of any data type. 
    
        #include   
            #include  
            int main()  
            {  
                   int a=90;  
           
                   int *x = (int*)malloc(sizeof(int)) ;  
                   x=&a;  
                   printf("Value which is pointed by x pointer : %d",*x);  
                   return 0;  
            }

Output

The void pointer in C can also be used to implement the generic functions in C.

Some important points related to void pointer are:

Dereferencing a void pointer in C

The void pointer in C cannot be dereferenced directly. Let's see the below example. 
    
        #include   
            int main()  
            {  
                int a=90;  
                void *ptr;  
                ptr=&a;  
                printf("Value which is pointed by ptr pointer : %d",*ptr);  
                return 0;  
            }

Output

In the above code, *ptr is a void pointer which is pointing to the integer variable 'a'. As we already know that the void pointer cannot be dereferenced, so the above code will give the compile-time error because we are printing the value of the variable pointed by the pointer 'ptr' directly.

Output

Now, we rewrite the above code to remove the error. 
    
        #include   
            int main()  
            {  
                int a=90;  
                void *ptr;  
                ptr=&a;  
                printf("Value which is pointed by ptr pointer : %d",*(int*)ptr);  
                return 0;  
            }

Output

In the above code, we typecast the void pointer to the integer pointer by using the statement given below:

(int*)ptr;

Then, we print the value of the variable which is pointed by the void pointer 'ptr' by using the statement given below:

*(int*)ptr;

Output

Arithmetic operation on void pointers

We cannot apply the arithmetic operations on void pointers in C directly. We need to apply the proper typecasting so that we can perform the arithmetic operations on the void pointers.

Let's see the below example:

    
        #include   
            int main()   
            {   
               float a[4]={6.1,2.3,7.8,9.0};  
               void *ptr;  
               ptr=a;  
               for(int i=0;i<4;i++)  
              {  
                  printf("%f,",*ptr);  
                  ptr=ptr+1;         // Incorrect.  
                 
            }}

Output

The above code shows the compile-time error that "invalid use of void expression" as we cannot apply the arithmetic operations on void pointer directly, i.e., ptr=ptr+1.

Let's rewrite the above code to remove the error.

        
            #include   
                int main()   
                {   
                   float a[4]={6.1,2.3,7.8,9.0};  
                   void *ptr;  
                   ptr=a;  
                   for(int i=0;i<4;i++)  
                  {  
                      printf("%f,",*((float*)ptr+i));  
                   }}
The above code runs successfully as we applied the proper casting to the void pointer, i.e., (float*)ptr and then we apply the arithmetic operation, i.e., *((float*)ptr+i).

Output

Why we use void pointers?

We use void pointers because of its reusability. Void pointers can store the object of any type, and we can retrieve the object of any type by using the indirection operator with proper typecasting.

Let's understand through an example.


    #include   
        int main()   
        {   
          int a=56; // initialization of a integer variable 'a'.  
          float b=4.5; // initialization of a float variable 'b'.  
          char c='k'; // initialization of a char variable 'c'.  
           void *ptr; // declaration of void pointer.  
           // assigning the address of variable 'a'.  
           ptr=&a;  
           printf("value of 'a' is : %d",*((int*)ptr));  
           // assigning the address of variable 'b'.  
           ptr=&b;  
           printf("nvalue of 'b' is : %f",*((float*)ptr));  
           // assigning the address of variable 'c'.  
           ptr=&c;  
            printf("nvalue of 'c' is : %c",*((char*)ptr));  
            return 0;  
        }

Output